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Talk:Cellular automaton

Von Neumann Neighborhood
The figure showing neighborhood types is incorrect; the Von Neumann neighborhood of radius 2 includes all cells within Manhattan distance 2, ie, includes the immediately diagonally adjacent cells. — Preceding unsigned comment added by 71.236.131.179 (talk) 17:48, 11 November 2018 (UTC)

Edit war over link
A cease-fire is now in place for three days, please discuss the link here, as you all should have been doing already. Pursue dispute resolution if needed. Beeblebrox (talk) 19:47, 11 March 2010 (UTC)

The link in dispute was added by someone with a blatantly apparent conflict of interest, apparently in ignorance of wikipedia policies regarding conflict of interest. Its not a link to a cellular automata site, so to me it doesn't seem to add anything to the article nor does it seem notable enough for inclusion. Unless some rationale is provided for its inclusion I say remove the link.

On the other hand, the very first external link in the external links section is indeed link spam, as indicated by the fact that someone keeps adding it at the very very top of the external links list despite the fact that any reasonable person would realize that most wikipedia users couldn't care less about an open source C++ library. The link was identified as link spam and removed on 5 March 2010. If whois is run on the IP address of the contributer of the link, one finds what looks to be yet another blatantly apparent conflict of interest. 76.125.44.38 (talk) 16:13, 14 March 2010 (UTC)

Looking over the external links I see other problems. The Monash University link looks to be of questionable notability, since no description is provided to say why it is notable. The link to CAPOW should be removed unless a suitable description saying why it is notable is provided for wikipedia users (wikipedia is not a link farm). The link to CAPIG should be removed or at least moved to the wikipedia "Pre Image" article. The link to "Random Cellular Automata by ..." should be removed based upon lack of notability and blatantly apparent conflict of interest. For the same reasons the same link should be removed from the "Multi-agent systams" article. 76.125.44.38 (talk) 16:32, 14 March 2010 (UTC)

 * In general I'm in favor of cutting back link farms. So while I haven't taken the trouble of looking carefully at all the links, the only one I'd argue strongly for keeping is Golly. (The Wolfram looks like it should be informative, but actually isn't, while MCell is also quite popular but platform-restricted.) —David Eppstein (talk) 16:44, 14 March 2010 (UTC)

More importantly, this article needs proper references and clean-up. Shouldn't be too hard for a scientific topic. Blatantly missing is for example: Wiener & Rosenblueth (1946!). Those authors studied spiral waves in an excitable cellular automaton (to model heart tissue) which shows dynamics very similar to those in the chemical BZ (and other excitable) reaction/systems. In this context, I must raise some doubts that Dewdney (sorry, Prof. Dewdney) was the first to do this (unfortunately, I don't have/know the Sci. Am. article). Also what is the meaning of "So far, no naturally occurring chemical cellular automata have been observed"? This probably refers to the idea that snails use bio-automata to pattern their shells. Such processes, however, are much better described by PDE models (such as the Meinhardt-Gierer models). I would suggest that CAs are fine models to reproduce (some of) those patterns but from a biophysical perspective, PDEs are superior ("more realistic") since they are based in fundamental theories (kinetics, stat. mech.) and allow for the measurement of physical quantities such as diffusion coeffs. and rate constants. Also what is this ref. note 10 (and others)? Looks to me less notable than the disputed link, which at least tells its story in a few lines that wikipedia users might appreciate. I will try to do some changes next week once the lock is gone.Agora2010 (talk) 19:25, 14 March 2010 (UTC)

I have started to remove some of the external links. Agora2010 (talk) 01:04, 16 March 2010 (UTC)

This C++ link seems to be rather persistent. I personally feel that a C++ CA library is too specialized for wikipedia as already stated above by someone else. How can one react to this? How do others feel about this link? Agora2010 (talk) 12:24, 17 March 2010 (UTC)

 * Agreed it's too specific. It looks like a nice free C++ library for macroscopic CA modelling that has been cited in a few papers but that's not enough. It doesn't fit WP:ELYES or WP:ELMAYBE. Can the person suggesting the link explain why they think it matches WP:EL? Ferkel (talk) 10:24, 18 March 2010 (UTC)

I would argue that it make sense to have a link to the CAPOW page, as this is, so far as I know, the only general purpose CA package which is designed to work with continuous-valued CAs. In particular, CAPOW has CA models of the Wave Equation, as well as Reaction-Diffusion. I don't really understand the relevance of the term of "link farm" in this context. CAPOW was developed at SJSU under research grants with student teams over a number of years, and is freeware meant to stimulate futher study. The two papers on the Introductory page explain more about the purpose of continous-valued CAs. Rudyrucker (talk) 03:23, 25 April 2010 (UTC)

 * Rudy: It is good to see that you take an interest in the quality of Wikipedia. Good show, man!!!  William R. Buckley (talk) 03:56, 25 April 2010 (UTC)

Changes to the "History" Section
First of my apologies for not being logged in when I edited the section as 67.237.114.167. Here are some explanations of my changes:

I removed the Rucker book as it is not a major part of the field's history.

I removed the Wolfram background. If more biographical background information should be given, then for von Neumann, Wiener, or Ulam.

I added a paragraph on Wiener and Rosenblueth, which I consider to be among the founding fathers of CA research.

It would be very interesting to research the links between Wiener and von Neumann. I read on non-wikipedia pages that the two were part of the "Harvard-Navy-IBM Mark I project" and had formed "The Teleological Society". Who knows more about this?

More references are needed to back the Wiener and Ulam statements. —Preceding unsigned comment added by Agora2010 (talk • contribs) 04:09, 15 March 2010 (UTC)

Also the last statement worries me: "Despite much confusion in the press and academia, the book did not argue for a fundamental theory of physics based on cellular automata, and although it did describe a few specific physical models based on cellular automata, it also provided models based on qualitatively different abstract systems." It sounds like some odd justification rather than an objective statement. Agora2010 (talk) 04:15, 15 March 2010 (UTC)

I modified the statement about Wiener-Rosenblueth. It is reasonable to call them among the founding fathers of CA research, but their model is not a CA, only "CA - like", because it is not totally discrete. The medium is continuous in their model. From around 1990 (as far as I can tell), many authors called the Wiener-Rosenblueth model a CA, but see the 1974 article I added in my edit for an earlier statement that it is not. Consulting the original article also makes this clear. I also added a reference to another Wikipedia article about a true CA model of excitable media from 1978, for which it is possible to prove a theorem characterizing those initial conditions for which nontrivial patterns persist. Sph110 (talk) 02:31, 15 June 2013 (UTC)

A Thought
I'm not apprised of the proper etiquette; please accept this edit as a question. Do the curators of this site read the forum at forum.wolframscience.com? I would think you would not want to add minute variations on cellular automata to this Wikipedia page; however, some developments since the publication of Dr. Wolfram's ANKOS book, that have been posted there, are significant, central ideas (as central as the generalizations you discuss on this page, in terms of hexagonal lattices). Specifically, the cells-with-memory idea. A. M. Nash anna.m.nash@gmail.com —Preceding unsigned comment added by 70.15.168.211 (talk) 16:27, 11 April 2010 (UTC)

CA and fundamental physical reality
Hi, I am a Ph.D. Candidate in philosophy and cognitive science: it's me who added the section in the first place - I am writing my dissertation on emergent computation and I found that the CA entry was missing something very important; CA have been studied by some among the most brilliant minds of our time and in recent year there is growing interest for their use and application. Unfortunately, a user deleted completely the paragraph (together with useful references and citations) but I couldn't find the reasons behind his actions, so I undo the change and start this discussion to better understand how to convey the proposed content within the existing entry. —Preceding unsigned comment added by 83.103.119.124 (talk) 07:56, 6 September 2010 (UTC)

 * I agree that this section is relevant, and should be included, but only if you make it neutral and encyclopaedic. For example, "Andrew Ilachinski points out in his monumental monography" contravenes WP:PEACOCK and WP:NPOV. I would vote to keep all except the first paragraph of your section.

 * Furthermore, I wouldn't consider Konrad Zuse to be the father of the modern PC. I would attribute that, undoubtedly, to John von Neumann -- he invented both parallel processing, and the architecture where data and program are stored in the same place. Zuse did, admittedly, create the first working implementation of an electro-mechanical computer, but the design of the modern computer exclusively belongs to von Neumann.

 * Regards, Calcyman (talk) 12:30, 6 September 2010 (UTC)

 * Thanks a lot for your comments. I also agree with both your observations: "monumental monography" is not neutral and I am gonna change it immediately - thanks for the remark. I also aknowledge that "father of modern PC" is somewhat vague and maybe an overstatement: yes, there is von Neumann and yes (of course!) there is Turing. The original meaning I had in mind was something along the lines of: "first to actually build a modern PC", but it went out wrong. Best Regards JT —Preceding unsigned comment added by 83.103.119.124 (talk) 16:01, 6 September 2010 (UTC)

author of the original article
Seeing as there are so many references to melanie mitchell in the original article, one can only assume melanie mitchell wrote it. —Preceding unsigned comment added by 131.252.222.216 (talk) 02:44, 23 September 2010 (UTC)

 * And how is this important? Calcyman (talk) 11:57, 24 September 2010 (UTC)

 * It isn't important who wrote the first version. Yet, for what it's worth, when I first read the article, and largely owing to the specific content and structure, I thought it had been written by either Stephen Wolfram, or somebody  associated with him (even if only as a fan).  William R. Buckley (talk) 20:38, 25 September 2010 (UTC)

Reversible
The setting used in the paper by J. Binghama, B. Binghamb is completely different from the setting used in the section about reversibility. They consider only finite systems with a finite lattice (using either periodic boundary conditions or null boundary conditions). The algorithm they give is linear in the size of the lattice of the finite system. In their setting reversibility is decidable in any dimension. I've removed the reference to this paper since it introduces too much confusion. Gthen (talk) 22:14, 22 October 2010 (UTC)

Is Rule 30 "chaotic"?
In Chaos theory, the word "chaotic" is given a very specific meaning, and a distinction is drawn between this meaning and the "common usage" meaning. Now in this article (Cellular automaton), I believe that the context of the word where it is first used: "... lead to chaotic, seemingly random histories ... " clearly implies that specific meaning. Now recognizing that I am totally out of my depth here, I ask: does Rule 30 really display "chaotic" behaviour using that word in the Chaos theory context? Can that be "proved", one way or the other? I have also asked this same question on the Rule 30 page. Old_Wombat (talk) 08:56, 10 May 2011 (UTC)
 * Answered there. —David Eppstein (talk) 16:15, 10 May 2011 (UTC)

Removed paragraph concerning Wolfram's classification
I recently removed the following paragraph:
 * However, it has recently been proven that Kolmogorov complexity can formalize Wolfram's classification, characterize these computing systems by their qualitative behavior and extend the scope of the dynamical investigation to the evolution of cellular automata with different initial configurations successfully classifying (and giving answer to previous concerns) all elementary cellular automata into Wolfram's classes. [Hector Zenil, Compression-based investigation of the dynamical properties of cellular automata and other systems journal of Complex Systems 19:1, 2010]

Since there has been some disagreement over the removal, let me explain my reasoning. First, this paper is not really "formalizing Wolfram's classification": it is proposing its own different classification. I've that too, in a different paper (which I also don't think should be added here), 10.1007/978-1-84996-217-9_6; there is a huge literature on this problem, in which neither my paper nor Zenil's paper stand out (in Google scholar, his has 3 citations, mine 5, neither very impressive yet). So I think citing this one and ignoring all the others gives it undue weight. Additionally, by credulously repeating the claims from the paper and not using an unbiased review of the source, we risk repeating the author's hype rather than covering its content neutrally. And finally, I simply do not believe the claims in the removed paragraph. Kolmogorov complexity cannot be used in anything resembling a successful classification system, because to be successful a classification system has to be usable to classify specific rules, and the undecidability of Kolmogorov complexity makes that problematic at best. —David Eppstein (talk) 21:23, 19 May 2011 (UTC)
 * Approximating Kolmogorov complexity has been largely shown to be of great use for classifying objects, see e.g. the work of Li and Vitanyi based on compression techniques. The procedure in Zenil's paper classifies all Wolfram's elementary cellular automata (ECA) in exactly the same classes proposed by Wolfram himself even if I may agree that at the end the classification turns out to be different because the paper pragmatically addresses the problem of taking into account the evolution of a CA for different initial configurations, something that was overlooked initially in Wolfram's first classification. I agree on not adding more info until more reviews appear. 90.46.37.131 (talk) 23:16, 20 May 2011 (UTC)
 * It seems the paper in question has been badly summarized by the IP from Spain. It wasn't formalizing anything. The DOI for your paper doesn't work for me (Springer gives an error), can your write the title? Tijfo098 (talk) 22:28, 19 May 2011 (UTC)
 * Being capable of classifying ECA with an objective measure is in my opinion a formalization of the specific classification of ECA into Wolfram's classes and something worth to eventually mention. 90.46.37.131 (talk) 23:16, 20 May 2011 (UTC)
 * "Growth and decay in life-like cellular automata", Game of Life Cellular Automata (Andrew Adamatzky, ed.), Springer-Verlag, 2010, pp. 71-98. There's a free version online at . By the way, I definitely support your addition of the Culik-Yu material here. —David Eppstein (talk) 00:52, 20 May 2011 (UTC)
 * I don't see the secondary source that you were asking for for any new addition into this section, could you explain why this one is different? is it the number of citations? Thanks 90.46.37.131 (talk) 23:16, 20 May 2011 (UTC)
 * David, I find interesting your paper. Thanks for the reference. 90.46.37.131 (talk) 23:20, 20 May 2011 (UTC)

 * I read the paper and as far as I can say it does not support the claims made in the article, it is interesting that one can get a classification that way but people have been doing that sort of thing for ages, it doesn't say anything particular new, and I don't see that it has even been refereed. There is no guarantee the classification will continue on it is some heuristic classification. In my opinion it should not go into the article and I don't see why anyone would want to try and stick it in. Dmcq (talk) 00:37, 21 May 2011 (UTC)
 * Notice nobody is trying to stick in... nothing else has been inserted to the section in question other than what David Eppstein *allowed* to be inserted (not sure what the criterion was though or why Eppstein thinks he can authorize or not what is inserted). But your claims that a published paper in a journal (Complex Systems 19:1, 2010) is not refereed is outrageous and only shows once more the bad faith of the friends of Eppstein and Kieffer to crush by any means whatever or whoever they may dislike. I cannot even discuss the content with you given that you cannot even see that the paper is obviously refereed (obviously you haven't read anything). You even go further saying that everybody has done the same for ages... A very scientific claim I guess. I think Eppstein, Kieffer and friends are too enamored of beating straw men and engaged in too much incredibly slipshod reasoning. 90.46.37.131 (talk) 01:04, 21 May 2011 (UTC)
 * I've struck out that bit about refereed, yes of course it would have been. I did not claim it had not been refereed, I made a silly mistake based where I got the paper from. That I agree with Eppstein that the paper does not say anything worthwhile as far as this article is concerned is my own opinion, I am not a friend or enemy or anything of them. The techniques are quite commonly used in lexical analysis, how do you think people use computers to authenticate paintings for instance nowadays? It is even something I've done a bit of myself and am still developing some extra twiddles to analyze a database of over 4000 scottish country dances. Dmcq (talk) 10:26, 21 May 2011 (UTC)
 * With all the allegation of bad faith I decided to check where your ip was located and got a hub saying amontsouris in Paris. Now Parc Montsouris is actually in Paris and the University of Paris is where I see this Hector Zenil's email points to. So It sounds to me that you are either that person or a close friend to be going to so much trouble to stick in stuff like this. Please read WP:COI? Dmcq (talk) 10:54, 21 May 2011 (UTC)
 * I see the ip has been blocked for a week and can't answer. Anyway if they can say something about why the publication should be in when they come back that would be far more constructive than what's gone before. Dmcq (talk) 11:31, 21 May 2011 (UTC)
 * I'm in Paris and any Whois would tell you so (nothing I'm trying to hide as you seem to suggest), yet I don't see how you got to the Parc Montsouris but it is a nice park in the South of Paris as far as I know, not where the university is though. I have already written that the author of this paper gave a talk recently in the university from where I got to know about it and thought it was nice to see that a procedure with no human intervention could perfectly classify all cellular automata into exactly the classes that Wolfram originally suggested, something that you seem to diminish, and that's fine if it does not convince you. Your reasoning about my IP is as sound as, for example, saying that you are Elvis Presley because  your IP is from Mississippi. There are 4 million inhabitants in the proper city of Paris and about 11 million in the greater area. I still don't understand, however, why you want me to make me seen as if I were obsessed by the paper in question as if I had been editing this Wikipedia article other than the original two consecutive times I did before (see the history if you want). Had I wanted to pursuit an edit war I would have done so. I understand you are not convinced by the paper  (or my summary) and that is just fine, period. I think it is more constructive, however, to write a summary if not of this paper in particular, of the important results of the papers on this subject (Wolfram's classification) rather than deleting the first contribution I do to Wikipedia, contributions which obviously you are now discouraging with such a suffocating attitude. I was unfairly blocked a week by insane allegations of personal attacks, but if I had not come before is because I have better things to do, like continue studying in my university. Should I wanted to continue arguing I would just do so as you can see that my IP is dynamic (not by personal choice) but again, it is not my battle, nor I consider this is my article. So to you to decide what to accept or reject for this article that cries for improvement. 82.123.116.194 (talk) 19:00, 15 June 2011 (UTC) Greg
 * I had a look and have not found where you said about a talk at a university. I raised the matter of where you were because it has been experience that people who start accusing others of bad faith are the one who are engaged in it. And yes I thought 11 million out of 6 billion i.e. 1 in 600 was very significant odds. You did not answer any points about disagreements with including the bit you put in but simply engaged in personal attacks which indicated to me you had an emotional attachment to the subject. Dmcq (talk) 22:26, 15 June 2011 (UTC)
 * That is not significant at all for many reasons, e.g. accounting that someone would give a talk and someone else in the same place would be interested about it. But this conversation takes us nowhere. I don't see what you say I left unanswered. I would be happy to answer specific questions but if you don't convince yourselves of the relevance of the results of something that's perfectly fine for me. I have already explained (and I just did again) why I think it was relevant. 82.123.116.194 (talk) 23:25, 15 June 2011 (UTC)
 * As a sign of good faith and peace flag I would like to propose someone to do a summary of some papers I think would be relevant to mention. There are, for example, attempts using Lyapunov exponents to classify CA, I could look for some specific ones. Also some topological attempts and Israeli and Goldenfeld's work showing that some prediction of coarse-grained structures in Wolfram's class 3 and 4 may be reducible. I also wanted to make a further clarification concerning Zenil's paper, one objection is that apparently the conclusion says that it is proving something. I think it is proving that algorithmic complexity classifies ECA into Wolfram's classes, that doesn't mean that it proves the general problem in general, specially because as you say, the are both undecidable sides: unreachability and eventually the halting problem and on the other hand the uncomputability of algorithmic complexity. I am not, however, an expert on CA, so I leave wording and summarizing the papers to others. The current summary of the section looks fine for me. 82.123.116.194 (talk) 19:39, 15 June 2011 (UTC)
 * I don't go in for peace flags or anything like that. If you just deal straightforwardly with improving the article that's all anybody could want on Wikipedia. It is important to not claim more than sources say, WP:5P is I think a good guide to the main principles behind Wikipedia. Dmcq (talk) 22:48, 15 June 2011 (UTC)
 * Notice that my last intervention was on specific contributions to the discussion of the improvement to the section, yet you focus on my shallow opening about the peace flag that wasn't but an indication that I was trying to leave the previous discussion behind. I have nothing more to contribute. 82.123.116.194 (talk) 23:25, 15 June 2011 (UTC)

Fuzzy
"Fuzzy cellular automata" is a hot topic in research, but there is nothing about it in the article. — Preceding unsigned comment added by 150.214.75.141 (talk) 10:35, 28 May 2012 (UTC)

Page lacks reference to Holland and Holland machines
It does have genetic algorithms and Mitchell's work, but it completely lacks John Holland's work. 17:43, 27 June 2012 (UTC) — Preceding unsigned comment added by 143.232.210.38 (talk)

Connection between fractals and cellular atomata?
Is there any connections between fractals and cellular automata? That is, can a cellular automaton (sometimes) be considered to be a fractal? —Kri (talk) 19:32, 20 August 2012 (UTC)
 * Not a super duper expert on the most technical aspects, but I don't think the usual rules of something like Conway's are scalable in the same way fractals are. —Torchiest talkedits 19:35, 20 August 2012 (UTC)
 * Some fractals (notably the Sierpinski triangle can be generated as patterns from simple seeds by some cellular automata. That said, the cellular automaton itself is not a fractal. —David Eppstein (talk) 19:54, 20 August 2012 (UTC)
 * There's a lot of literature about the fractal structure of space-time diagrams of some cellular automata. I recommend this recent paper which includes a survey of many previous works. —Gthen (talk) 11:35, 27 August 2012 (UTC)

Yes, some cellular automata are fractals, as the famous quote by Benoit Mandelbrot begins "Fractals are everywhere in nature ...".

Dimension of a further fractal example. The Sierpinski triangle, an object with Hausdorff dimension of log(3)/log(2)≈1.58.

For example, the Cantor set, a zero-dimensional topological space, is a union of two copies of itself, each copy shrunk by a factor 1/3; hence, it can be shown that its Hausdorff dimension is ln(2)/ln(3) ≈ 0.63.

There are a lot of good reads, googling fractals and cellular automata, however, one of them I can recommend, other than by Mandelbrot, is Wolfram's.

https://www.wolframscience.com/nks/notes-2-1--mathematical-interpretation-of-cellular-automata/#:~:text=In%20the%20context%20of%20pure,the%20locality%20of%20the%20rule).

Signed - Henry John Steven Smith, 1874 — Preceding unsigned comment added by 49.184.24.3 (talk) 18:23, 25 April 2022 (UTC)

Robots and latttices vs. motion of liquids
Does anyone know anything about the claims in the article about Neumann's self-replicating robots and Ulam's lattices being the inspiration for cellular automata? The books I'm looking through don't mention that aspect of the history. —Torchiest talkedits 15:52, 25 August 2012 (UTC)
 * I have, since this initial post, found plenty about von Neumann. I still haven't found a clear description of Ulam's supposed lattice work.  I am planning on removing that and rewriting it this weekend unless someone objects. —Torchiest talkedits 18:21, 12 October 2012 (UTC)
 * Never mind, finally found one. —Torchiest talkedits 19:20, 12 October 2012 (UTC)

Article organization
I feel like this article has too many very small sections. Does anyone have any ideas for merging or reorganizing them? I am thinking one easy move would be to put the Totalistic section at the bottom of the Classifications section. Another possibility would be to combine a handful of other sections into an Applications section. I'm open to any other thoughts on this. —Torchiest talkedits 20:00, 12 October 2012 (UTC)

WP:SELFCITE issues, now resolved apparently
Were discussed at COIN here. Just making a note here so it is in the article history. Jytdog (talk) 13:46, 13 June 2015 (UTC)

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Request edit on 23 October 2017
Could you possibly add 'langtons ant wave equation' as a link ?

Thank you
 * No. See Articles for deletion/Extended Langton's ant for why not. —David Eppstein (talk) 17:07, 28 October 2017 (UTC)

David Eppstein has not bothered to look at my math nor validate my octave script, very sad state of affairs if you ask me. — Preceding unsigned comment added by Dakoder (talk • contribs) 19:31, 20 November 2018 (UTC)

Pascal's Triangle
Wouldn't Pascal's Triangle be the first ever discovered and studied cellular automata? https://www.sciencedirect.com/science/article/pii/016727898890019X https://www.maa.org/press/periodicals/loci/joma/patterns-in-pascals-triangle-with-a-twist-one-dimensional-cellular-automata

I'm sure the idea floated around before 20th century. And Wolfram mentions that too: https://www.wolframscience.com/reference/notes/876b >Additive cellular automata such as rule 90 had implicitly arisen in studies of binomial coefficient modulo primes in the 1800s (see page 870) — Preceding unsigned comment added by NikitaSadkov (talk • contribs) 18:11, 12 October 2018 (UTC)
 * Pascal's triangle is not a cellular automaton. It is not even the time-space diagram formed by running a cellular automaton (what you are probably thinking of it as), because it does not have a finite number of distinct values in its cells. —David Eppstein (talk) 18:16, 12 October 2018 (UTC)

Missing References
The following relevant references are missing:

1. E.F. Codd proved that cellular automata are computation-universal, i.e., they can simulate any Turing machine, in: Cellular Automata, Academic Press, New York, 1968.

2. D. Gordon proved that one-dimensional totalistic cellular automata are computation-universal, in: D. Gordon, on the computational power of totalistic cellular automata, Mathematical Systems Theory 20:43-52, 1987.

title: Talk:Cellular automaton


text:
Talk:Metropolis–Hastings algorithm

x prime or x_t?
Here are two conflicting comments on the case a>1, which I have cut and pasted for readability. --Rinconsoleao 15:12, 16 July 2007 (UTC)

 * There seems to be a typo in the last display. If a>1, we would accept the proposal, x', not x_t. --Youyifong 06:38, 28 July 2006 (UTC)

 * Hi, is there a mistake here? In the step-by-step instructions,

x^{t+1} = \left \{ \begin{matrix} x'(x(t)?)                           & \mbox{if }a > 1 \\ x'\mbox{ with probability }a, & \mbox{if }a < 1 \end{matrix} \right. $$
 * (Postdoc 02:30, 16 July 2007 (UTC))

The algorithm always accepts if a>1. That is, $$x^{t+1}=x'$$ when a>1. Notice that this is consistent with the statements about the uniform random variable $$u$$ in the previous paragraph. I have rewritten the "step-by-step instructions" to make this clearer. --Rinconsoleao 15:12, 16 July 2007 (UTC)
 * Please check the page again. The pages mis-typed x' as x_t, as Youyifong mentioned again. Also, should it be "a >= 1" ? (Postdoc 05:38, 30 July 2007 (UTC))
 * Thanks! --Rinconsoleao 15:44, 31 July 2007 (UTC)

Gibbs sampling
The article says that the Gibbs-sampler is a special case of the MH-algorithm. As far as I know it is a complementary MCMC-method which works quite differently. --Smeyen 01:07, 24 February 2007 (UTC)

 * In Gibbs sampler, its proposal distribution is the full conditional distribution of some part of parameter space conditional on the rest, which always resultes in a acceptance probability of 1. See [http:://www.stat.purdue.edu/~lebanon/notes/metropolis.pdf] (Postdoc 05:46, 30 July 2007 (UTC))

Gibbs sampling is a special case of the Metropolis-Hastings sampler, with the a proposal distribution that at each iteration, updates one element of the parameter vector from the distribution of that element conditional on the current value of all of the others: $$g(x'|x) = P(x_i | x_{-i})$$, resulting in an acceptance probability that is always 1, and hence is always accepted. Proof left as an exercise for the reader, ☺. Gregory R. Warnes (talk) 23:27, 27 October 2017 (UTC)

Complexity

 * The Markov chain is started from a random initial value \displaystyle x^0 and the algorithm is run for many iterations until this initial state is "forgotten"

no bounds are known on the number of needed iterations, e.g. the mixing time of the Markov chain? Can we really claim in the introduction that this algorithm allow to compute something if we don't known how long it should be iterate? Levochik (talk) 07:43, 26 August 2009 (UTC)

Example
First of all, thank you for this article. It's the best I could find in the web about Monte Carlo simualtion. But unfortunately, I still don't get it. How can I compute a=a_1a_2 if I don't know P(x)? Perhaps additionally, you could include an example. Something where one can see the step-by-step instruction applied to a simple example. Just like a record of a view steps from a real simulation of some simple case. —Preceding unsigned comment added by Askamaster (talk • contribs) 10:30, 25 March 2010 (UTC)
 * You don't need P(x), you only need a function proportional to P(x), as the normalizations cancel in the ratio. It is quite often the case that you can easily determine a function proportional to P(x), but the integral to normalize it is very difficult to perform. This comes up quite often in Bayesian computations of the posterior distribution. Rjljr2 (talk) 15:49, 11 April 2010 (UTC)

 * article is NOT good, is NOT clear, you want improvement?

best part is who created it then explain why, what, when, are no answered I will give you it tells how does it give algorithm in detail so one can code it? nope, does it provide example? noJuror1 (talk) 10:28, 30 November 2017 (UTC)

The article suffers a bit from the Phd-on-wikipedia-syndrome. It descibes a fairly simple idea, but it tries to be very general and that makes it hard to read for people that don't know the subject beforehand. Compare with the article on Simulated_annealing for an simpler article on the same subject. nielsle — Preceding unsigned comment added by 89.23.239.219 (talk) 08:17, 13 January 2018 (UTC)

Proposal density
Clearly the Markov chain doesn't converge for all proposal densities Q. Certainly Q > 0 is sufficient, but I don't believe it is necessary. What condition is required for the proposal density? — Preceding unsigned comment added by WuTheFWasThat (talk • contribs) 19:24, 16 May 2012 (UTC)

Overview
The section Overview tries to give a formal derivation of the Algorithm. The foregoing sections are so well written that it was very easy to understand for me, even though I am not an expert in the field. Bringing the section Overview to the same didadactic level would be an asset. The questions I would like to have answered are: How can I see that the equation is still redundant and what does the term redundant mean in this context? How does one come to the idea of defining $$\displaystyle A(x\rightarrow x')$$ as the specified minimum function?

Some other proposals for improvements: The meaning of the variable $$\displaystyle x' $$ seems to be different from the one in the section Intuition. Would it be an option to use another notation? Furthermore, the section Overview repeats the definition of the algorithm in the section Intuition. However, at the same time it lists the steps of skipping samples to mitigate correlations. I suggest to put the complete list of steps into the previous section. Then, a proper name for the section Overview would e.g. be Formal Derivation.

Sorry, I am new to wikipedia and do not want to just poke your text. Please let me know if you wish me to do some of the mentioned improvements. —Contribo (talk) 20:20, 3 December 2012 (UTC)

 * I agree with you, "overview" is not the proper name; maybe "formal derivation" as you suggested sounds better. Nevertheless, the intuition section does not present the modern view of metropolis-hastings algorithm in the sense that it does not uses a general proposal (it is always assumed symmetrical, which is not the general case. However, I think the intuition part is to understand the idea of the method; that is why it skips the fact that the proposal also has to be considered in the acceptance in the general case.

 * Redundant in the sense that there can be another choices for A which also fulfills the equation you mention (maybe under-determined). I don't recall my statistical physics lectures, but I know there is at least another choice of A which also fulfills it and yet, it does not use the "min".Jorgecarleitao (talk) 20:53, 3 December 2012 (UTC)

 * Thank you for your explanations, I did not expect to get an Answer so quickly. I bring these issues into discussion not just because I would like to understand the matter but also to motivate you to work towards improving this article together with me. In my role as the learner I offer to ask you questions and you as the author could modify the article, such as to answer these questions. Of course I offer to do modifications as well — as far as I am qualified. Does this sound attractive?

 * In this sense let me point out some further aspects. You write the proposal also has to be considered in the acceptance in the general case. I am not sure if I get you right. Isn't this considered in the section Step-by-step Instructions? This brings me to another point: the Step-by-step Instructions should contain the skipping of T states to make the resulting sequence uncorrelated.

 * I noticed some more points regarding the section Overview: What is the role of ergodicity in this explanation. The text says that it can be shown that ergodicity is a requirement for the process to converge asymptotically. A reference to the proof would be great. How is it ensured that the process is ergodic? Another Point: The markov matrices in the wiki-reference have finite dimensions. However, the markov matrix for the process under discussion is infinte dimensional, this difference should at least be mentioned. --Contribo (talk) 21:24, 3 December 2012 (UTC)

Easter egg link
The link which is labeled "in practice" and leads to Bayesian statistics should be rephrased so as to not surprise the reader, as per WP:EASTEREGG. There should probably be some brief remark explaining what Bayesian statistics are and how they relate, but I don't know how to write such a remark. — Preceding unsigned comment added by 132.65.153.93 (talk) 18:03, 12 November 2013 (UTC)

Interacting Metropolis-Hastings methodology
(this was in the main article, and even though has a lot of information, it does not belong to Metropolis-Hastings. It is about sequential Monte Carlo and Markov Chain Monte Carlo as the text below itself claims. I'm storing it here so it is not lost in history.)

The central idea is to consider an interpolating sequence of target distributions $$P_k(x)~, k=0,\ldots,n$$ with an increasing complexity of sampling and such that $$P_n(x)=P(x)$$. We let $$h_k(x)$$ be a function that is proportional to $$P_k(x)/P_{k-1}(x)$$, for each $$k\geq 1.$$. In this notation, for any k we clearly have the product formula$$P_k(x)=\frac{1}{\mathcal Z_k}~\left\{\prod_{1\leq l\leq k}h_l(x)\right\}~P_0(x)$$for some normalizing constants $$\mathcal Z_k.$$ When the state space $S$ is finite, for any $$k\geq 1$$  we have

$$P_k(x)=\displaystyle\frac{h_k(x)~P_{k-1}(x)}{\displaystyle\sum_{y\in S}h_k(y)~P_{k-1}(y)}\quad\mbox{and}\quad \frac{\mathcal Z_k}{\mathcal Z_{k-1}}=\sum_{y\in S}h_k(x)~P_{k-1}(x)$$ For probability densities w.r.t. the Lebesgue measure dx on $S=\mathbb R^d$, for some dimension $$d\geq 1$$, we have

$$P_k(x)=\displaystyle\frac{h_k(x)~P_{k-1}(x)}{\displaystyle\int_{y\mathbb R^d}h_k(y)~P_{k-1}(y)~dy}\quad\mbox{and}\quad \frac{\mathcal Z_k}{\mathcal Z_{k-1}}=\int_{y\in \mathbb R^d}h_k(x)~P_{k-1}(x)~dx$$We assume that it is easy to sample independent random variables with common probability$$P_0(x)$$, and the functions $$h_k(x)$$ take values in $$[0,1]$$ and they can be computed easily for any state x.

We illustrate these rather abstract models with a couple of examples: ~\left\{\prod_{1\leq l\leq k}h_l(x)\right\}~P_0(x)\quad\mbox{with}\quad h_l(x)=1_{A_l}(x)$$ For any $$k\geq 0$$ denote by $$P_k(x\mapsto x')$$ the transition probability of the Metropolis–Hastings algorithm with target invariant measure $P_k(x)$. To simplify the presentation, we assume that the state space s finite.
 * Boltzmann-Gibbs distributions associated with an energy function $$V~:~x\in S\mapsto V(x)\in[0,\infty]$$ on some finite set $$S$$ and some inverse temperature parameter $$\beta>0$$ are defined by$$P(x)=\frac{1}{\mathcal Z}~e^{-\beta V(x)}\quad\mbox{with}\quad \mathcal Z=\sum_{y\in S}~e^{-\beta V(y)}$$In this situation, we can choose the interpolating sequence $$P_k(x)=\frac{1}{\mathcal Z_k}~e^{-\beta_k V(x)}\quad\mbox{with}\quad \mathcal Z_k=\sum_{y\in S}~e^{-\beta_k V(y)}\quad\mbox{and some parameters}\quad \beta_0=0<\beta_1<\ldots<\beta_n=\beta$$By construction, we have $$P_k(x)=\frac{1}{\mathcal Z_k}~\left\{\prod_{1\leq l\leq k}h_l(x)\right\}~P_0(x)\quad\mbox{with}\quad h_l(x)=e^{-(\beta_{l}-\beta_{l-1})V(x)}$$
 * Let $P_0(x)$ be the probability density of some real valued random variable $$X$$. Let $A\subset \mathbb R$  be some subset such that $$\mathbb P(X\in A)>0$$ and set $$P(x)=\frac{1}{\mathcal Z}~1_A(x)~P_0(x)\quad\mbox{with}\quad \mathcal Z=\int_{y\in \mathbb R}~1_A(y)~P_0(y)~dy$$with the indicator function $$1_A(.)$$ of a set $A$ . In this situation, we can choose the interpolating sequence $$P_k(x)=\frac{1}{\mathcal Z_k}~1_{A_k}(x)~P_0(x)\quad\mbox{with}\quad \mathcal Z_k=\int_{y\in \mathbb R}~1_{A_k}(y)~P_0(y)~dy>0\quad\mbox{and some decreasing subsets}\quad A_0=\mathbb R\supset A_1\supset \ldots\supset A_n=A$$By construction, we have $$P_k(x)=\frac{1}{\mathcal Z_k}
 * Initially, we sample a large number $$N$$ of independent random variables $\xi_0=\left(\xi^i_0\right)_{1\leq i\leq N}$ with common probability distribution $$P_0(x)$$. By the law of large numbers, we have $$P_0^N(x):=\frac{1}{N}\sum_{1\leq i\leq N}1_{\xi^i_0}(x)~\approx_{N\uparrow\infty}~P_0(x)\qquad\mbox{and}\qquad \sum_{y\in S}h_1(x)~P^N_{0}(x)=\frac{1}{N}\sum_{1\leq i\leq N}h_1(\xi^i_0)\approx_{N\uparrow\infty}~\frac{\mathcal Z_1}{\mathcal Z_{0}}$$
 * Acceptance-rejection with recycling:   The objective is to design a new population of samples $\xi_0~\longrightarrow~ \widehat{\xi}_0=\left(\widehat{\xi}^i_0\right)_{1\leq i\leq N}$  with distribution $P_1(x)$ . With a probability $$h_1(\xi^i_0)$$  we accept the state $$\xi^i_0$$ and we set $$\widehat{\xi}^i_0=\xi^i_0$$.  Otherwise, we sample a new indidual $$\widetilde{\xi}^i_0$$ in the current population with the probability measure

$$Proba\left(\widetilde{\xi}^i_0=\xi^j_0~|~\xi_0\right)=\frac{h_1(\xi^j_0)}{\sum_{1\leq k\leq N}h_1(\xi^k_0)},~\mbox{for each}~j\in\{1,\ldots,N\}$$and we set $\widehat{\xi}^i_0=\widetilde{\xi}^i_0.$ Using the fact that$$\displaystyle\frac{h_1(x)~P^N_{0}(x)}{\displaystyle\sum_{y\in S}h_1(y)~P^N_{0}(y)}=\sum_{1\leq i\leq N}\frac{h_1(\xi^i_0)}{\sum_{1\leq j\leq N}h_1(\xi^j_0)}~1_{\xi^i_0}(x)~\approx_{N\uparrow\infty}P_1(x)$$  we have the estimates$$\widehat{P}_0^N(x):=\frac{1}{N}\sum_{1\leq i\leq N}1_{\widehat{\xi}^i_0}(x)~\approx_{N\uparrow\infty}~P_1(x)$$ The interacting Metropolis-Hastings defined above belong to the class of mean field interacting particle methods and Feynman-Kac particle models (a.k.a. Sequential Monte Carlo methodologies ). In contrast to traditional Markov Chain Monte Carlo methods relying on the stability of Markov chain, the precision interacting Metropolis-Hastings only depends on the size $$N$$ of the system, in the sense that for any time $$k$$ we have
 * Proposal-exploration: The objective is to design a new population $$\widehat{\xi}_0~\longrightarrow~\xi_1=\left(\xi^i_1\right)_{1\leq i\leq N}$$ with distribution $P_1(x)$  using the Metropolis-Hastings transitions $$P_1(x\mapsto x')$$. From each selected states $$\widehat{\xi}^i_0$$ we sample independently a transition $$\widehat{\xi}^i_0~\longrightarrow~\xi^i_1~\sim~P_1(\widehat{\xi}^i_0\mapsto x')$$ By the law of large numbers, we have$$P^N_1(x^{\prime})=\frac{1}{N}\sum_{1\leq i\leq N}1_{\xi^i_1}(x^{\prime})~\approx_{N\uparrow\infty}~\sum_{x\in S}P_1(x)~P_1(x\mapsto x^{\prime})=P_1(x^{\prime})\qquad\mbox{and}\qquad \sum_{y\in S}h_2(x)~P^N_{1}(x)=\frac{1}{N}\sum_{1\leq i\leq N}h_2(\xi^i_1)\approx_{N\uparrow\infty}~\frac{\mathcal Z_2}{\mathcal Z_{1}}$$The next acceptance-rejection transition with recycling $\xi_1~\longrightarrow~ \widehat{\xi}_1=\left(\widehat{\xi}^i_1\right)_{1\leq i\leq N}$  is defined as above by replacing $h_0(.)$  by $h_1(.)$ ; and the second proposal-exploration $$\widehat{\xi}_1~\longrightarrow~\xi_2=\left(\xi^i_2\right)_{1\leq i\leq N}$$ is defined by moving the selected states with the Metropolis-Hastings transition $$P_2(x\mapsto x')$$, and so on.

$$P^N_k(x)=\frac{1}{N}\sum_{1\leq i\leq N}1_{\xi^i_k}(x)~\approx_{N\uparrow\infty}~P_k(x)$$In addition, we have the unbiased estimates of the normalizing constants$$\forall 1\leq k\leq n\qquad \frac{\mathcal Z_k}{\mathcal Z_{0}}=\prod_{1\leq l\leq k}\frac{\mathcal Z_l}{\mathcal Z_{l-1}}~\approx_{N\uparrow\infty}~\prod_{1\leq l\leq k}\frac{1}{N}\sum_{1\leq i\leq N}h_l(\xi^i_{l-1})$$In addition, for any function f(.) on S we have the Feynman-Kac formulae

$$\sum_{x\in S}~f(x)~P_k(x)=\frac{E\left(f(X_k)~\prod_{1\leq l\leq k}h_l(X_{l-1})\right)}{E\left(\prod_{1\leq l\leq k}h_l(X_{l-1})\right)}~\qquad\mbox{and}\qquad~{\mathcal Z_k}/{\mathcal Z_0}=E\left(\prod_{1\leq l\leq k}h_l(X_{l-1})\right)$$where $$X_k$$ stands for a non homogenous Markov chain with initial distribution $$P_0(x)$$ and Markov transitions $$\mathbb P\left(X_k=x'~|~X_{k-1}=x\right):=P_k(x\mapsto x')$$.

The analysis of the convergence of Feynman-Kac particle methods has been started in 1996 and in 2000 in the book and the series of articles. More recent developments can be found in the books. Applications of these Feynman-Kac methodologies to interacting Metropolis–Hastings algorithms are discussed in the series of articles. The unbiased properties of the particle estimate of the ratio $$\mathcal Z_k/\mathcal Z_0$$ are proved in in the context of filtering problems. Online adaptive choices of the temperature parameters and the decreasing subsets in the above examples can also be found in. In Operation Research, the interacting MCMC algorithm associated with target distributions defined in terms of indicator functions is sometimes referred as subset simulation.

Thinning
The paragraph on thinning is very poor. In particular the statement "this means that if we want a set of independent samples, we have to throw away the majority of samples and only take every nth sample" is plain wrong.

There is no such guarantee of independence, regardless of the interval chosen. In addition, most authors advise against the practice, since while thinning reduces correlation between subsequent steps, it also greatly reduces the number of samples finally obtained. There are some exceptions, as Art Owen discusses here http://statweb.stanford.edu/~owen/reports/bestthinning.pdf.

Patrick1894 (talk) 14:42, 17 April 2019 (UTC)

Rename? (In Wikipedia parlance suggested Move)
In my experience, supported for example by the frequencies of the terms "Metropolis", "Metropolis-Hastings" and "Monte Carlo" recorded between 1953 and 2012 in Google's Ngram viewer, this algorithm is far more often called "Metropolis" than "Metropolis-Hastings." Therefore I suggest the article be renamed (moved to) "Metropolis algorithm" with Metropolis-Hastings redirecting to it, and with a subsection, now lacking, that specifically explains and compares Hastings' 1970 improvement and generalization to the original 1953 algorithm.CharlesHBennett (talk) 22:06, 22 January 2020 (UTC)
 * Neutral. In my experience, "Metropolis" is more common than "Metropolis-Hastings" as well. However, I don't think we get a lot by moving this. Metropolis algorithm already redirects to this article. BernardoSulzbach (talk) 10:59, 23 January 2020 (UTC)

Formal derivation of transition probability incomplete
It is stated that the transition probability $$P(x'\mid x)$$ can be separated into a proposal step and accept/reject step. Mathematically,

 * $$P(x'\mid x) = g(x' \mid x) A(x', x).$$

where $$g(x' \mid x)$$ is the proposal distribution and $$A(x', x)$$ the acceptance distribution.

This transition distribution does not take into account rejection however. The transition probability is complete with an additional rejection term

 * $$P(x'\mid x) = g(x' \mid x) A(x', x) + \delta(x' - x) (1 - A(x', x))$$

where $$\delta(x)$$ is understood as the Dirac delta function. It is only later in the derivation with the choice for $$A(x', x)$$ that the last term vanishes, since a transition where $$x' = x$$ will always be accepted ($$A(x, x) = 1$$). — Preceding unsigned comment added by 213.125.242.82 (talk) 19:46, 8 April 2020 (UTC)

Oughtn't the core procedure be made a tad clearer? Suggest revision.
Calculate the acceptance ratio $$\alpha = f(x')/f(x_t)$$, which will be used to decide whether to accept or reject the candidate. Because f is proportional to the density of P, we have that $$\alpha = f(x')/f(x_t) = P(x')/P(x_t)$$.

 * Accept or reject:
 * Generate a uniform random number $$u \in [0, 1]$$.
 * If $$u \le \alpha$$, then accept the candidate by setting $$x_{t+1} = x'$$,
 * If $$u > \alpha$$, then reject the candidate and set $$x_{t+1} = x_t$$ instead.

May I suggest a rewrite as follows which shows the Hastings step more clearly? The above is correct, but:
 * It forces a generation of $$u$$ unnecessarily when $$\alpha > 1$$, and
 * It requires thinking to see that it works, specifically in the case $$\alpha = 1$$.

Suggested revision:

 * Accept or reject:
 * If $$\alpha > 1$$, then accept the candidate by setting $$x_{t+1} = x'$$.
 * If $$\alpha \le 1$$, then
 * Generate a uniform random number $$u \in [0, 1]$$.
 * If $$u \le \alpha$$, then accept the candidate with probability $$\alpha$$ by setting $$x_{t+1} = x'$$.
 * If $$u > \alpha$$, then reject the candidate and set set $$x_{t+1} = x_t$$ instead.

bayesianlogic.1@gmail.com 19:28, 14 December 2020 (UTC)

The plot with the three chains needs clarification
The rosenbrock function is typically used as an example for minimization, not maximization. The Metropolis Hastings can be seen as a maximization algorithm though, and indeed the caption is written as if we were maximizing something. Perhaps the function being used is 1/rosenbrock, in such a way that the minimum becomes a maximum?

Lnz.Rossi (talk) 16:59, 22 May 2021 (UTC)

 * Maximization of some function f is the same as minimization of -f. Here, instead of minimizing Rosenbrock, the Metropolis-Hastings algorithm is maximizing -1×Rosenbrock. RFZYN SPY  talk 02:47, 21 March 2024 (UTC)

Acceptance ratio formula failing to parse
Under 'Intuition', I see the following parsing error instead of the acceptance ratio formula: "Calculate the acceptance ratio Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "/mathoid/local/v1/":): {\displaystyle \alpha = f(x')/f(x_t)}, which will be used to decide whether to accept or reject the candidate. Because f is proportional to the density of P, we have that..."

When I try to edit, this error doesn't show up in the preview, so not sure how to fix this. 128.12.123.150 (talk) 03:22, 17 February 2023 (UTC)

 * It looks fine from my end. Perhaps some sort of browser incompatibility? Qflib, aka KeeYou Flib (talk) 04:43, 18 February 2023 (UTC)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Talk:Metropolis–Hastings algorithm


text:
Empanada

An empanada is a type of baked or fried turnover consisting of pastry and filling, common in Spain, other Southern European countries, Latin American countries, and the Philippines. The name comes from the Spanish empanar (to bread, i.e., to coat with bread), and translates as 'breaded', that is, wrapped or coated in bread. They are made by folding dough over a filling, which may consist of meat, cheese, tomato, corn, or other ingredients, and then cooking the resulting turnover, either by baking or frying.

Origins
The origin of empanadas is unknown, but they are thought to have originated in 7th century Galicia, a region in northwest Spain.

An empanada (empãada) is mentioned in the Cantigas de Santa Maria 57:VI (c. 1282): Entr' esses roubadores / viu jazer um vilão / desses mais malfeitores, / ũa perna na mão / de galinha, freame / que sacara com fame / entom dũ' empãada | que so um seu çurame/ comer quisera, / mais nom podera, / ca Deus nom queria.

In the midst of these robbers he saw lying there one of the most vicious of the rascals with a chicken leg in his hand. He had taken the cold morsel out of a pasty and was about to eat it under the cover of his cape. However, he could not, for God prevented it. Rabbinic Jewish books from the same period, including the Novellae of Asher ben Jehiel (1250-1327), the Novellae of Yom Tov of Seville ( c. 1260–1320), the Orchot Chayyim of Aaron ben Jacob (c. 1250-1325) and the Arba'ah Turim of Jacob ben Asher (c. 1270-1340) mention "inpanada" and "panada" as bread products containing fat, meat or fish on the inside.

A cookbook published in Catalan in 1520, Llibre del Coch by Robert de Nola, mentions empanadas filled with seafood in the recipes for Catalan, Italian, French, and Arabian food.

Argentina
Argentine empanadas are often served during parties and festivals as a starter or main course. Shops specialize in freshly made empanadas, with many flavors and fillings.

Every region of Argentina has its own characteristic variant. Those of Salta (salteñas) are small, juicy and spicy, and contain potatoes, peppers and ground chili. These are also popular in neighbouring Bolivia.

The Jujuy variant adds peas and garlic. Its filling is called recado and the repulgue (method of closing the empanada) simbado. The La Rioja variant includes hard-boiled egg, red bell pepper, olives, and raisins. In Jujuy, there are two variants: criollas and arabes. Those of Santiago are considered especially juicy. Those of Catamarca are similar but smaller. Tucumán is known for the empanada creole; an annual National Empanada festival is held in Famaillá. Those of Famaillá are made with matambre and fried in good fat, competing with the entreveradas (mixed-grated), in which the matambre is mixed with chicken breast, garlic, ground chili, hard-boiled egg and cumin. Those of Mendoza are large and include olives and garlic. Those of San Juan have a higher proportion of onion, making them juicier and slightly sweet. Olives are also common and sometimes fat is also added to the recado or the dough. In San Luis they are big, seasoned with oregano and hot pepper, and kneaded with pork fat. In Córdoba, they were called "federal cake" or empanadas de Misia Manuelita, famous because pears boiled in wine with cloves were added to their filling. Today they are not so sweet but it is tradition to sprinkle them with sugar. In Traslasierra they add carrots and potatoes. In the Litoral, where immigrants from various parts of the world predominated, Santa Fe, Entre Ríos and Corrientes fill them with river fish, such as surubí (catfish) or dorado, or with white sauce and Goya cheese. In the Cordillera of Patagonia, they are made with lamb and on the coast with seafood. In Buenos Aires, the Creole empanada is so important that it has been declared a Cultural Heritage of Food and Gastronomy by the Argentine Ministry of Culture.

Belize
In Belize, empanadas are known as panades. They are made with masa (corn dough) and typically stuffed with fish, chicken, or beans. They are usually deep fried and served with a cabbage or salsa topping. Panades are frequently sold as street food.

Brazil
In Brazil, an empanada is called a pastel (: pastéis) and consists of half-circle or rectangle-shaped thin crust pies with assorted fillings, fried in vegetable oil. The result is a crispy, brownish fried pie. The most common fillings are ground meat, mozzarella, catupiry, heart of palm, codfish, cream cheese, chicken and small shrimp. Pastéis with sweet fillings such as guava paste with Minas cheese, banana and chocolate also exist.

Chile
Empanadas are a staple part of Chilean cuisine. Commonly consumed in large quantities during the country's national day celebrations, many Chileans consider this to be their most representative dish. The most iconic variety is the oven-baked empanada de pino, which is filled with ground beef, minced onion, half or a quarter of a hard-boiled egg, and a single unpitted black olive.

Empanadas in Chile are eaten year-round and are either oven-baked or deep-fried; the latter is a popular street food.

Costa Rica
In Costa Rica, an empanada is turnover usually made with nixtamalized (lime treated) masa (corn dough); nevertheless, it could be made with doughs of cassava (yuca), green or ripe plantain, in addition to wheat flour. The empanada can be pan fried, deep fried or baked.

The empanada can be filled with plenty of foods, although beef, pork or chicken meat, but also with cheese, palm heart or different kind of vegetable hashes (called picadillo) or refried beans are the most common ones. There are also sweet empanadas filled with tropical fruit marmalades (such as guava or pineapple), dulce de chiverre (a sweet jam of Cucurbita ficifolia, also known as chilacayote, alcayota, calabaza de cabello de ángel or cidra) or dulce de leche (manjar, arequipe or cajeta).

In the caribbean coast of Costa Rica, there are some kind of wheat flour empanadas similar to Jamaican beef patty, also found with chicken meat or vegetable filling (usually ackee); there are also sweet empanadas called "Plantain Tart" or "Plantintá" (made with ripe plantain jam filling) and "Pineapple Tart" (made with pineapple jam filling).

Cuba
In Cuba, empanadas are frequently enjoyed as snacks, appetizers, or main courses at parties and gatherings. In Havana, the most common variant is filled with picadillo or shredded chicken, often mixed with ingredients like cumin, garlic, onions, raisins, and green olives. Empanadas are also commonly eaten for dessert, including fillings such as guava paste and cream cheese. In Cuban cuisine, empanadas are almost always deep-fried.

Ecuador
Empanadas de viento or "windy" empanadas are fried, wheat-based empanadas stuffed with stringy cheese and sprinkled with sugar. They have been given this appellation for their inflated appearance as if they have been filled with air. Empanadas de viento can be made in cocktail size, appetiser size, and giant size, which is popular among the middle and working class. They are often eaten with coffee or with té de hierba luisa or lemon verbena tea. Their appearance is very similar to an Italian panzerotto.

Empanadas can be purchased from food stalls, markets, and restaurants. During religious holidays, women from the countryside fry empanadas at home and sell them in front of churches.

Empanadas de verde or plantain empanadas are plantain-based and filled with cheese and fried. These empanadas are most commonly found in the coastal regions of the country.

With the growth of Southern Cone and Colombian immigrants, wheat- and meat-based baked empanadas and corn-based empanadas have also become popular.

El Salvador
El Salvador is one of few countries where the empanada is made with plantain rather than a flour-based dough wrapping. A popular sweet variation, empanadas de platano are torpedo-shaped dumplings of dough made from very ripe plantains, filled with vanilla custard, fried, then rolled in sugar. They may alternatively have a filling made from refried beans rather than milk-based custard, but the flavour profile remains sweet rather than savoury.

France
In France, the traditional chaussons are made with a puff pastry dough filled with stew such as daube or confit, or a bechamel sauce mixed with ham and/or cheese. They also exist in sweet version (see chausson aux pommes). They are half-moon shaped. If the shape is rectangular they receive the name of friand. One regional version is the pâté lorrain, filled with pork meat cooked with wine and onions. All these versions are baked.

The fried versions can be made of puff pastry or shortcrust pastry and are called rissoles. The most famous is the rissole de Coucy, filled with meat or fish.

India
Gujhia is similar to a sweet empanada, in a half moon shape.

Samosa is a stuffed savory pastry.

Indonesia
In Indonesia, empanadas are known as panada. They are especially popular in Manado cuisine of North Sulawesi where their panada has a thick crust made from fried bread, filled with spicy cakalang fish (skipjack tuna) and chili, curry, potatoes or quail eggs. The panada in North Sulawesi was derived from Portuguese influence in the region. The dish is similar to karipap and pastel, although they have a thinner crust compared to panada.

Italy
The Sicilian 'mpanatigghi are stuffed, consisting of half-moon-shaped panzerotti filled with a mixture of almonds, walnuts, chocolate, sugar, cinnamon, cloves and minced beef. These are typical of Modica, in the province of Ragusa, Sicily. They are also known by the italianized word impanatiglie or dolce di carne (pasty of meat).

They were probably introduced by the Spaniards during their rule in Sicily which took place in the sixteenth century; this is suggested by the etymology of the name which comes from the Spanish empanadas or empanadillas, as well as the somewhat unusual combination of meat and chocolate, which occurs occasionally in Spanish cuisine. In previous centuries, game meat was used in 'mpanatigghi; today beef is used.

Philippines
Filipino empanadas usually contain ground beef, pork or chicken, potatoes, chopped onions, and raisins (picadillo-style), in a somewhat sweet, wheat flour bread. There are two kinds available: the baked sort and the flaky fried type. To lower costs, potatoes are often added as an extender, while another filling is kutsay (garlic chives).

Empanadas in the northern part of the Ilocos usually have savoury fillings of green papaya, mung beans, and sometimes chopped Ilocano sausage (chorizo) or longaniza and egg yolk. This particular variant is fried and uses rice flour for a crunchier shell. Empanadas can also be filled with mashed eggplant, scrambled eggs, and cabbage, which is called poqui poqui. In Bulacan, empanada de kaliskis (lit. 'fish scale empanada'), uniquely has a flaky multilayered crust resembling scales, hence the name. In Cebu, empanada Danao is a characteristically sweet-savory variant. It is filled with chopped chorizo and chayote, deep-fried, and dusted in white sugar before serving. In Zamboanga, empanada Zamboangueño is filled with chopped sweet potato, garbanzo beans, and served with a sweet vinegar dipping sauce.

Dessert versions of empanadas also exist, notably empanaditas, which commonly have a filling of latik (coconut caramel), honey and nuts, or peanut butter. Kapampangan versions of empanaditas have a yema (custard) and cashew nut filling. In Cebu, sinudlan empanada is a small deep-fried empanada with bukayo (sweetened coconut meat) filling.

Puerto Rico
In Puerto Rico, empanadas are made of a flour base and fried, known either as empanadillas or pastelillos- a hotly debated topic usually dependent on the region of the island and/or the type of border on the pastry.

Common fillings include meat such as ground beef, pork, chicken, pizza (tomato paste and cheese), guava and cheese, jueyes (crab), chapín (Spotted trunkfish), rabbit, octopus, conch, and much more depending on local cuisine.

United States
Empanadas, mainly based on South American recipes, are widely available in New York City, New Jersey, Baltimore, Philadelphia, Washington, D.C., and Miami from food carts, food trucks, and restaurants. Empanadas are usually found in U.S. areas with a large Hispanic population, such as San Antonio, Los Angeles, and San Francisco.

Venezuela
Traditional Venezuelan empanadas are made with ground corn dough, though modern versions are made with precooked corn. The dough may have a yellow color when toasted due to the addition of annatto. The fillings are very diverse, with the most conventional being cheese, shredded beef, chicken, cazón (school shark) in the Margaritan Island region especially, ham, black beans and cheese (commonly called dominó) and even combinations of mollusks. The empanadas have a half-moon shape and are fried in oil. Sometimes, they may have more than one filling, such as in empanadas de pabellón, which are made with a shredded beef filling (or cazón in the Margarita Island region), black beans, slices of fried plantain, and shredded white cheese.

Similar foods
The empanada resembles savory pastries found in many other cultures, such as the molote, pirozhki, calzone, samosa, knish, kreatopitakia, khuushuur, Jamaican patty and pasty.

In most Malay-speaking countries in Southeast Asia, the pastry is commonly called epok-epok or karipap (English: curry puff). Fried dumplings are found in Chinese cuisine (jiaozi) and in Vietnamese cuisine (bánh gối).

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Ripping

Ripping is the extraction of digital content from a container, such as a CD, onto a new digital location. Originally, the term meant to rip music from Commodore 64 games. Later, the term was applied to ripping WAV or MP3 files from digital audio CDs, and after that to the extraction of contents from any storage media, including DVD and Blu-ray discs, as well as the extraction of video game sprites.

Despite the name, neither the media nor the data is damaged after extraction. Ripping is often used to shift formats, and to edit, duplicate or back up media content. A rip is the extracted content, in its destination format, along with accompanying files, such as a cue sheet or log file from the ripping software.

To rip the contents out of a container is different from simply copying the whole container or a file. When creating a copy, nothing looks into the transferred file, nor checks if there is any encryption or not, and raw copy is also not aware of any file format. One can copy a DVD byte by byte via programs like the Linux dd command onto a hard disk, and play the resulting ISO file just as one would play the original DVD.

To rip contents is also different from grabbing an analog signal and re-encoding it, as it was done with early day CD-ROM drives not capable of digital audio extraction (DAE). Sometimes even encoding, i.e. digitizing audio and video originally stored on analog formats, such as vinyl records is incorrectly referred to as ripping.

Ripping software
A CD ripper, CD grabber or CD extractor is a piece of software designed to extract or "rip" raw digital audio (in format commonly called CDDA) from a compact disc to a file or other output. Some all-in-one ripping programs can simplify the entire process by ripping and burning the audio to disc in one step, possibly re-encoding the audio on-the-fly in the process.

For example, audio CDs contain 16-bit, 44.1 kHz LPCM-encoded audio samples interleaved with secondary data streams and synchronization and error correction info. The ripping software tells the CD drive's firmware to read this data and parse out just the LPCM samples. The software then dumps them into a WAV or AIFF file, or feeds them to another codec to produce, for example, a FLAC or MP3 file. Depending on the capabilities of the ripping software, ripping may be done on a track-by-track basis, or all tracks at once, or over a custom range. The ripping software may also have facilities for detecting and correcting errors during or after the rip, as the process is not always reliable, especially when the CD or the drive containing the CD itself is damaged or defective.

There are also DVD rippers which operate in a similar fashion. Unlike audio CDs, DVDs do contain data formatted in files for use in computers. However, commercial DVDs are often encrypted (for example, using Content Scramble System/ARccOS Protection), preventing access to the files without using the ripping software's decryption ability, which may not be legal to distribute or use. DVD files are often larger than is convenient to distribute or copy to CD-R or ordinary (not dual-layer) DVD-R, so DVD ripping software usually offers the ability to re-encode the content, with some quality loss, so that it fits in smaller files.

Legality
When the material being ripped is not in the public domain, and the person making the rip does not have the copyright owner's permission, then such ripping may be regarded as copyright infringement. However, some countries either explicitly allow it in certain circumstances, or at least don't forbid it. Some countries also have fair use-type laws which allow unauthorized copies to be made under certain conditions.

Europe
A directive of the European Union allows its member nations to instate in their legal framework a private copy exception to the authors and editors rights. If a member State chooses to do so, it must also introduce a compensation for the copyright holders. Most European countries, except for Norway, have introduced a private copying levy that compensates the owners directly from the country's budget. In 2009 the sum awarded to them was $55 million. In all but a few of these countries (exceptions include the UK and Malta), the levy is excised on all machines and blank materials capable of copying copyrighted works.

Under the directive, making copies for other people is forbidden, and if done for profit can lead to a jail sentence.

Netherlands
In the Netherlands, citizens are allowed to make copies of their legally bought audio and video. This includes CD, SACD, as well as DVD and Blu-ray. These copies are called "home copies" and may only be used by the ripper. Public distribution of ripped files is not allowed.

Spain
In Spain, anyone is allowed to make a private copy of a copyrighted material for oneself, providing that the copier has accessed the original material legally.

United Kingdom
Private copying of copyrighted material is illegal in the United Kingdom. According to a 2009 survey, 59% of British consumers believed ripping a CD to be legal, and 55% admitted to doing it.

In 2010, the UK government sought input on modernizing copyright exceptions for the digital age, and commissioned the Hargreaves Review of Intellectual Property and Growth. The review asserted that a private copying exception was overdue, citing that users were unaware of what was even legally allowed, and that a copyright law where "millions of citizens are in daily breach of copyright, simply for shifting a piece of music or video from one device to another" was not "fit for the digital age". The review recommended, among other things, that the government consider adopting the EU Copyright Directive's recommendation that member states enact an exception for private, noncommercial copying so long as the rights holders receive "fair compensation." Other EU member states chose to implement the exception paired with a tax on music purchases or widely varying levies on copying equipment and blank media. However, the Review reasoned that no such collections are necessary when implementing a copyright exception for format-shifting:

"The UK has a thriving market for personal media devices which rely on private copying. We see no economic argument for adding an extra charge to these devices in order to authorise reasonable private acts which are part of the normal use of devices. Indeed, without that copying, normal use of those devices would be largely restricted to playing music or films bought online. We are not aware of strong evidence of harm to rights holders done by this kind of private copying in the normal course of using digital equipment to play works. There is considerable evidence of overall public benefits from consumer use. [...] The Review favours a limited private copying exception which corresponds to what consumers are already doing. As rights holders are well aware of consumers’ behaviour in this respect, our view is that the benefit of being able to do this is already factored into the price that rights holders are charging. A limited private copying exception which corresponds to the expectations of buyers and sellers of copyright content, and is therefore already priced into the purchase, will by definition not entail a loss for right holders."

In August 2011, the government broadly accepted the recommendations of the Hargreaves Review. At the end of 2012, the government published "Modernising Copyright", a document outlining specific changes the government intends to make, including the Hargreaves-recommended exception for private, noncommercial copying.

Following each milestone in the reform process, press reports circulated that ripping non-DRM-protected CDs and DVDs was no longer illegal. However, the actual legislation to implement the changes is not yet in force; the Intellectual Property Office had only begun seeking review of draft legislation in June 2013, and the resulting Statutory Instruments (SIs) weren't laid out for Parliamentary approval until March 27, 2014, and weren't actually approved until July 14 (Commons) and July 27 (Lords); with an effective date of October 1, 2014. Anticipating approval, the Intellectual Property Office published a guide for consumers to explain the forthcoming changes and to clarify what would remain illegal. The private copying exception may seem to conflict with the existing Copyright Directive prohibition on overriding or removing any DRM or TPM (technical protection measures) that are sometimes used on optical media to protect the content from ripping. However, the "Modernising Copyright" report makes clear that any workarounds to allow access will not involve a relaxation of the prohibition.

On 17 July 2015, the private copying exemption was overturned by the High Court of Justice following a complaint by BASCA, Musicians' Union, and UK Music, making private copying once again illegal. The groups objected to the exclusion of a compensation scheme, presenting evidence contradicting an assertion that an exemption would cause "zero or insignificant harm" to copyright holders and thus did not require compensation.

United States
U.S. copyright law (Title 17 of the United States Code) generally says that making a copy of an original work, if conducted without the consent of the copyright owner, is infringement. The law makes no explicit grant or denial of a right to make a "personal use" copy of another's copyrighted content on one's own digital media and devices. For example, space shifting, by making a copy of a personally owned audio CD for transfer to an MP3 player for that person's personal use, is not explicitly allowed or forbidden.

Existing copyright statutes may apply to specific acts of personal copying, as determined in cases in the civil or criminal court systems, building up a body of case law. Consumer copyright infringement cases in this area, to date, have only focused on issues related to consumer rights and the applicability of the law to the sharing of ripped files, not to the act of ripping, per se.

Canada
The Copyright Act of Canada generally says that it is legal to make a backup copy of any copyrighted work if the user owns or has a licence to use a copy of the work or subject-matter as long as the user does not circumvent a technological protection measure or give any of the reproductions away. This means that in most cases, ripping DVDs in Canada is most likely illegal.

Australia and New Zealand
In Australia and New Zealand a copy of any legally purchased music may be made by its owner, as long as it is not distributed to others and its use remains personal. In Australia, this was extended in 2006 to also include photographs and films.

Opinions of ripping
Recording industry representatives have made conflicting statements about ripping.

Executives claimed (in the context of Atlantic v. Howell) that ripping may be regarded as copyright infringement. In oral arguments before the Supreme Court in MGM Studios, Inc. v. Grokster, Ltd.', MGM attorney Don Verrilli (later appointed United States Solicitor General by the Obama administration), stated: "The record companies, my clients, have said, for some time now, and it's been on their Website for some time now, that it's perfectly lawful to take a CD that you've purchased, upload it onto your computer, put it onto your iPod. There is a very, very significant lawful commercial use for that device, going forward."

Nevertheless, in lawsuits against individuals accused of copyright infringement for making files available via file-sharing networks, RIAA lawyers and PR officials have characterized CD ripping as "illegal" and "stealing".

When asked directly about the issue, RIAA president Cary Sherman asserted that the lawyers misspoke, and that the RIAA has never said whether it was legal or illegal, emphasizing that the RIAA had not yet taken anyone to court over that issue alone.

Fair use
Although certain types of infringement scenarios are allowed as fair use and thus are effectively considered non-infringing, "personal use" copying is not explicitly mentioned as a type of fair use, and case law has not yet established otherwise.

Personal copying acknowledgments
According to Congressional reports, part of the Audio Home Recording Act (AHRA) of 1992 was intended to resolve the debate over home taping. However, 17 USC 1008, the relevant text of the legislation, didn't fully indemnify consumers for noncommercial, private copying. Such copying is broadly permitted using analog devices and media, but digital copying is only permitted with certain technology like DAT, MiniDisc, and "audio" CD-R—not with computer hard drives, portable media players, and general-purpose CD-Rs.

The AHRA was partially tested in RIAA v. Diamond Multimedia, Inc., a late-1990s case which broached the subject of a consumer's right to copy and format-shift, but which ultimately only ascertained that one of the first portable MP3 players wasn't even a "digital recording device" covered by the law, so its maker wasn't required to pay royalties to the recording industry under other terms of the AHRA.

Statements made by the court in that case, and by both the House and Senate in committee reports about the AHRA, do interpret the legislation as being intended to permit private, noncommercial copying with any digital technology. However, these interpretations may not be binding.

In 2007, the Federal Trade Commission (FTC), a government office which requires business to engage in consumer-friendly trade practices, has acknowledged that consumers normally expect to be able to rip audio CDs. Specifically, in response to the Sony BMG copy protection rootkit scandal, the FTC declared that the marketing and sale of audio CDs which surreptitiously installed digital rights management (DRM) software constituted deceptive and unfair trade practices, in part because the record company "represented, expressly or by implication, that consumers will be able to use the CDs as they are commonly used on a computer: to listen to, transfer to playback devices, and copy the audio files contained on the CD for personal use."

A DVD ripper is a computer program that facilitates copying the content of a DVD to a hard disk drive. They are mainly used to transfer video on DVDs to different formats, to edit or back up DVD content, and to convert DVD video for playback on media players and mobile devices. Some DVD rippers include additional features such as Blu-ray support, DVD and Blu-ray Disc decryption, copy protection removal and the ability to make discs unrestricted and region-free. While most DVD rippers only convert video to highly compressed MP4 video files, there are other rippers that can convert DVDs to higher quality compressed video. These types of DVD rippers are used by the television and film industry to create broadcast quality video from DVD. Video ripped by these professional DVD rippers is an exact duplicate of the original DVD video.

Circumvention of DVD copy protection
In the case where media contents are protected using some effective copy protection scheme, the Digital Millennium Copyright Act (DMCA) of 1998 makes it illegal to manufacture or distribute circumvention tools and use those tools for infringing purposes. In the 2009 case RealNetworks v. DVD CCA, the final injunction reads, "while it may well be fair use for an individual consumer to store a backup copy of a personally owned DVD on that individual's computer, a federal law has nonetheless made it illegal to manufacture or traffic in a device or tool that permits a consumer to make such copies." This case made clear that manufacturing and distribution of circumvention tools was illegal, but use of those tools for non-infringing purposes, including fair use purposes, was not.

The Library of Congress periodically issues rulings to exempt certain classes of works from the DMCA's prohibition on the circumvention of copy protection for non-infringing purposes. One such ruling in 2010 declared, among other things, that the Content Scramble System (CSS) commonly employed on commercial DVDs could be circumvented to enable non-infringing uses of the DVD's content. The Electronic Frontier Foundation (EFF) hailed the ruling as enabling DVD excerpts to be used for the well-established fair-use activities of criticism and commentary, and for the creation of derivative works by video remix artists. However, the text of the ruling says the exemption can only be exercised by professional educators and their students, not the general public.

title: Ripping


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Newtonian

Newtonian refers to the work of Isaac Newton, in particular:
 * Newtonian mechanics, i.e. classical mechanics
 * Newtonian telescope, a type of reflecting telescope
 * Newtonian cosmology
 * Newtonian dynamics
 * Newtonianism, the philosophical principle of applying Newton's methods in a variety of fields
 * Newtonian fluid, a fluid that flows like water—its shear stress is linearly proportional to the velocity gradient in the direction perpendicular to the plane of shear
 * Non-Newtonian fluids, in which the viscosity changes with the applied shear force

Supplementary material

 * List of things named after Isaac Newton

title: Newtonian


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Atom (web standard)

The name Atom applies to a pair of related Web standards. The Atom Syndication Format is an XML language used for web feeds, while the Atom Publishing Protocol (AtomPub or APP) is a simple HTTP-based protocol for creating and updating web resources.

Web feeds allow software programs to check for updates published on a website. To provide a web feed, the site owner may use specialized software (such as a content management system) that publishes a list (or "feed") of recent articles or content in a standardized, machine-readable format. The feed can then be downloaded by programs that use it, like websites that syndicate content from the feed, or by feed reader programs that allow internet users to subscribe to feeds and view their content.

A feed contains entries, which may be headlines, full-text articles, excerpts, summaries or links to content on a website along with various metadata.

The Atom format was developed as an alternative to RSS. Ben Trott, an advocate of the new format that became Atom, believed that RSS had limitations and flaws—such as lack of on-going innovation and its necessity to remain backward compatible—and that there were advantages to a fresh design.

Proponents of the new format formed the IETF Atom Publishing Format and Protocol Workgroup. The Atom Syndication Format was published as an IETF proposed standard in (December 2005), and the Atom Publishing Protocol was published as  (October 2007).

Usage
The blogging community uses web feeds to share recent entries' headlines, full text, and even attached multimedia files. The providers allow other websites to incorporate a blog's "syndicated" headline or headline-and-short-summary feeds under various usage agreements. people use Atom and other web-syndication formats for many purposes, including journalism, marketing, bug-reports, or any other activity involving periodic updates or publications. Atom also provides a standard way to export an entire blog, or parts of it, for backup or for importing into other blogging systems.

It is common to find web feeds on major websites, as well as on many smaller ones. Some websites let people choose between RSS- or Atom-formatted web feeds; others offer only RSS or only Atom. In particular, many blog and wiki sites offer their web feeds in the Atom format.

A feed reader or "aggregator" program can be used to check feeds and to display new articles. Client-side readers may also be designed as standalone programs or as extensions to existing programs like web browsers.

Web-based feed readers and news aggregators require no software installation and make the user's "feeds" available on any computer with web access. Some aggregators syndicate (combine) web feeds into new feeds, e.g., taking all football-related items from several sports feeds and providing a new football feed.

Atom compared to RSS 2.0
When Atom emerged as a format intended to rival or replace RSS, CNET described the motivation of its creators as follows: "Winer's opponents are seeking a new format that would clarify RSS ambiguities, consolidate its multiple versions, expand its capabilities, and fall under the auspices of a traditional standards organization."

A brief description of some of the ways Atom 1.0 differs from RSS 2.0 has been given by Tim Bray, who played a major role in the creation of Atom:

Date formats
The RSS 2.0 specification relies on the use of formatted timestamps to communicate information about when items in the feed were created and last updated. The Atom working group chose instead to use timestamps formatted according to the rules specified by (which is a subset of ISO 8601; see Appendix A in RFC 3339 for differences).

Internationalization
While the RSS vocabulary has a mechanism to indicate a human language for the feed, there is no way to specify a language for individual items or text elements. Atom, on the other hand, uses the standard xml:lang attribute to make it possible to specify a language context for every piece of human-readable content in the feed.

Atom also differs from RSS in that it supports the use of Internationalized Resource Identifiers, which allow links to resources and unique identifiers to contain characters outside the US ASCII character set.

Modularity
The elements of the RSS vocabulary are not generally reusable in other XML vocabularies. The Atom syntax was specifically designed to allow elements to be reused outside the context of an Atom feed document. For instance, it is not uncommon to find atom:link elements being used within RSS 2.0 feeds.

Barriers to adoption
Despite the emergence of Atom as an IETF Proposed Standard and the decision by major companies such as Google to embrace Atom, use of the older and better-known RSS formats has continued. There are several reasons for this:

 * RSS 2.0 support for enclosures led directly to the development of podcasting. While many podcasting applications, such as iTunes, support the use of Atom 1.0, RSS 2.0 remains the preferred format.
 * Many sites choose to publish their feeds in only a single format. For example, CNN and The New York Times offer their web feeds only in RSS 2.0 format.
 * News articles about web syndication feeds have increasingly used the term "RSS" to refer generically to any of the several variants of the RSS format such as RSS 2.0 and RSS 1.0 as well as the Atom format.

Background
Before the creation of Atom the primary method of web content syndication was the RSS family of formats.

Members of the community who felt there were significant deficiencies with this family of formats were unable to make changes directly to RSS 2.0 because the official specification document stated that it was purposely frozen to ensure its stability.

Initial work
In June 2003, Sam Ruby set up a wiki to discuss what makes "a well-formed log entry". This initial posting acted as a rallying point. People quickly started using the wiki to discuss a new syndication format to address the shortcomings of RSS. It also became clear that the new format could form the basis of a more robust replacement for blog editing protocols such as the Blogger API and LiveJournal XML-RPC Client/Server Protocol as well.

The project aimed to develop a web syndication format that was:
 * "100% vendor neutral,"
 * "implemented by everybody,"
 * "freely extensible by anybody, and"
 * "cleanly and thoroughly specified."

In short order, a project road map was built. The effort quickly attracted more than 150 supporters, including David Sifry of Technorati, Mena Trott of Six Apart, Brad Fitzpatrick of LiveJournal, Jason Shellen of Blogger, Jeremy Zawodny of Yahoo, Timothy Appnel of the O'Reilly Network, Glenn Otis Brown of Creative Commons and Lawrence Lessig. Other notables supporting Atom include Mark Pilgrim, Tim Bray, Aaron Swartz, Joi Ito, and Jack Park. Also, Dave Winer, the key figure behind RSS 2.0, gave tentative support to the new endeavor.

After this point, discussion became chaotic, due to the lack of a decision-making process. The project also lacked a name, tentatively using "Pie," "Echo," "Atom," and "Whatever" (PEAW) before settling on Atom. After releasing a project snapshot known as Atom 0.2 in early July 2003, discussion was shifted off the wiki.

Atom 0.3 and adoption by Google
The discussion then moved to a newly set up mailing list. The next and final snapshot during this phase was Atom 0.3, released in December 2003. This version gained widespread adoption in syndication tools, and in particular it was added to several Google-related services, such as Blogger, Google News, and Gmail. Google's Data APIs (Beta) GData are based on Atom 1.0 and RSS 2.0.

Atom 1.0 and IETF standardization
In 2004, discussions began about moving the project to a standards body such as the World Wide Web Consortium or the Internet Engineering Task Force (IETF). The group eventually chose the IETF and the Atompub working group was formally set up in June 2004, finally giving the project a charter and process. The Atompub working group is co-chaired by Tim Bray (the co-editor of the XML specification) and Paul Hoffman. Initial development was focused on the syndication format.

The Atom Syndication Format was issued as a Proposed Standard in IETF in December 2005. The co-editors were Mark Nottingham and Robert Sayre. This document is known as atompub-format in IETF's terminology. The Atom Publishing Protocol was issued as a Proposed Standard in IETF in October 2007. Two other drafts have not been standardized.

Example of an Atom 1.0 feed
An example of a document in the Atom Syndication Format:

Including in HTML
The following tag should be placed into the head of an HTML document to provide a link to an Atom feed.

title: Atom (web standard)


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Kiwi (nickname)

"Kiwi" is a common self-reference used by New Zealanders, though it is also used internationally. Unlike many demographic labels, its usage is not considered offensive; rather, it is generally viewed as a symbol of pride and affection for most people of New Zealand.

The name derives from the kiwi, a native flightless bird, which is a national symbol of New Zealand. Until the First World War, the kiwi represented the country and not the people; however, by 1917, New Zealanders were also being called "Kiwis", supplanting other nicknames such as "Enzedder".

History
The kiwi has long had a special significance for the indigenous Māori people, who used its skin to make feather cloaks (kahu kiwi) for chiefs. The bird first came to European attention in 1811 when a skin ended up in the hands of a British Museum zoologist, George Shaw, who classified it as a type of penguin and portrayed it as standing upright. After early sightings by Europeans the kiwi was regarded as a curiosity; in 1835 the missionary William Yate described it as "the most remarkable and curious bird in New Zealand".

Representing the country
In the early 1900s cartoonists began to use the kiwi as a representation of New Zealand. For example, in a 1904 New Zealand Free Lance cartoon a plucky kiwi is shown growing to a moa after a rugby victory of 9–3 over a British team. The next year, The Westminster Gazette printed a cartoon of a kiwi and a kangaroo (representing Australia) going off to a colonial conference. Trevor Lloyd, who worked for The New Zealand Herald, also used a kiwi to represent the All Blacks rugby team, but he more often drew a moa. Other symbols for New Zealand at this time included the silver fern, a small boy, and a young lion cub. But until the First World War the kiwi was used as a symbol of the country rather than the people of New Zealand.

Representing the people
In the early-20th century, New Zealanders, especially soldiers and All Blacks players, were referred to internationally as "En Zed(der)s" (derived from the initials of the country's name, N.Z.) or "Maorilanders" (in reference to the Māori people and their historical contribution to the country). These terms were still being used near the end of the First World War of 1914–18. However, although New Zealand soldiers were often described as "Diggers" or as "Pig Islanders", by 1917 they were also being called "Kiwis".

The image of the kiwi had appeared on military badges since the South Canterbury Battalion used it in 1886, and several regiments took it up in the First World War. "Kiwis" came to mean the men of New Zealand regiments. The nickname is not thought to have originated as a reference to the physical attributes of the New Zealand servicemen (i.e. implying they were short and stocky or nocturnal like the bird). It was simply that the kiwi was distinct and unique to the country. Its prominent use on the New Zealand regiments' insignia also made for easy association. The nickname eventually became common usage in all war theatres. After the end of the First World War in November 1918, many New Zealand troops stayed in Europe for months or years awaiting transport home. At Sling Camp, near Bulford on Salisbury Plain in Wiltshire, New Zealand soldiers carved a chalk kiwi into the nearby hill in early 1919. The New Zealanders' presence popularised the nickname within Europe.

An Australian boot polish called Kiwi was widely used in the imperial forces. William Ramsay, a developer of the product, named the polish in honour of his wife's birthplace, New Zealand. Beginning in 1906, Kiwi Shoe Polish eventually became widely sold in the UK and the US, and the symbol became more widely known. The Australian National Dictionary also gives the first use of the term "Kiwi Kids" and "Kiwis" in 1917, to mean Australian army recruits who had kiwied up; in other words, they had highly-polished boots.

Following the Second World War of 1939–1945 the term gradually became attributed to all New Zealanders, and today throughout the world they are referred to as Kiwis, as well as often referring to themselves that way.

Current usage
Spelling of the word Kiwi, when used to describe the people, is often capitalised. The bird's name is spelled with a lower-case k and, being a word of Māori origin, normally stays as kiwi when pluralised. As an English word, the nickname normally takes the plural form Kiwis. Thus, "two Kiwis" refers to two people, whereas "two kiwi" refers to two birds. This linguistic nicety is exemplified by the conservation trust Save the Kiwi, which used the slogan "Kiwis for kiwi". In 2020, Kiwibank shifted to using the plural form Kiwi (capitalised but without the terminal -s) to refer to the people, most prominently changing its slogan from "Kiwis making Kiwis better off" to "Kiwi making Kiwi better off".

Kiwi is not generally considered to be a derogatory term, however there are New Zealanders, particularly some with Māori heritage, that find the appellation jarring and prefer not to identify with it. In 2018, a South Australian employment tribunal ruled that the nickname was not discriminatory, after a New Zealander unsuccessfully argued that she was a victim of racial discrimination after being labelled a "Kiwi".

In an official context, the term kiwi has been used in the name of government services and state-owned enterprises, such as Kiwibank, KiwiSaver, and KiwiRail, and is frequently used in government press releases to refer to everyone in or of New Zealand. In August 2022, Minister of Immigration Michael Wood referred to 85,000 holders of recently approved New Zealand 2021 resident visas as "new Kiwis".

title: Kiwi (nickname)