text:
Paramecium

Paramecium (, , plural "paramecia" only when used as a vernacular name) is a genus of eukaryotic, unicellular ciliates, widespread in freshwater, brackish, and marine environments. Paramecia are often abundant in stagnant basins and ponds. Because some species are readily cultivated and easily induced to conjugate and divide, they have been widely used in classrooms and laboratories to study biological processes. Paramecium species are commonly studied as model organisms of the ciliate group and have been characterized as the "white rats" of the phylum Ciliophora.

Historical background
Paramecium were among the first ciliates to be observed by microscopists, in the late 17th century. They were most likely known to the Dutch pioneer of protozoology, Antonie van Leeuwenhoek, and were clearly described by his contemporary Christiaan Huygens in a letter from 1678. The earliest known illustration of a Paramecium species was published anonymously in Philosophical Transactions of the Royal Society in 1703.

In 1718, the French mathematics teacher and microscopist Louis Joblot published a description and illustration of a microscopic poisson (fish), which he discovered in an infusion of oak bark in water. Joblot gave this creature the name "Chausson", or "slipper", and the phrase "slipper animalcule" remained in use as a colloquial epithet for Paramecium, throughout the 18th and 19th centuries.

The name "Paramecium" – constructed from the Greek παραμήκης (paramēkēs, "oblong") – was coined in 1752 by the English microscopist John Hill, who applied the name generally to "Animalcules which have no visible limbs or tails, and are of an irregularly oblong figure." In 1773, O. F. Müller, the first researcher to place the genus within the Linnaean system of taxonomy, adopted the name Paramecium but changed the spelling to Paramæcium. In 1783, Johann Hermann changed the spelling once more, to Paramœcium. C. G. Ehrenberg, in a major study of the infusoria published in 1838, restored Hill's original spelling for the name, and most researchers have followed his lead.

Description
Species of Paramecium range in size from 0.06 mm to 0.3 mm in length. Cells are typically ovoid, elongate, or foot- or cigar-shaped.

The body of the cell is enclosed by a stiff but elastic structure called the pellicle. The pellicle consists of an outer cell membrane (plasma membrane), a layer of flattened membrane-bound sacs called alveoli, and an inner membrane called the epiplasm. The pellicle is not smooth, but textured with hexagonal or rectangular depressions. Each of these polygons is perforated by a central aperture through which a single cilium projects. Between the alveolar sacs of the pellicle, most species of Paramecium have closely spaced spindle-shaped trichocysts, explosive organelles that discharge thin, non-toxic filaments, often used for defensive purposes.

Typically, an anal pore (cytoproct) is located on the ventral surface, in the posterior half of the cell. In all species, there is a deep oral groove running from the anterior of the cell to its midpoint. This is lined with inconspicuous cilia which beat continuously, drawing food into the cell. Paramecium are primarily heterotrophic, feeding on bacteria and other small organisms. A few species are mixotrophs, deriving some nutrients from endosymbiotic algae (chlorella) carried in the cytoplasm of the cell.

Osmoregulation is carried out by contractile vacuoles, which actively expel water from the cell to compensate for fluid absorbed by osmosis from its surroundings. The number of contractile vacuoles varies depending on the species.

Movement
A Paramecium propels itself by whip-like movements of the cilia, which are arranged in tightly spaced rows around the outside of the body. The beat of each cilium has two phases: a fast "effective stroke," during which the cilium is relatively stiff, followed by a slow "recovery stroke," during which the cilium curls loosely to one side and sweeps forward in a counter-clockwise fashion. The densely arrayed cilia move in a coordinated fashion, with waves of activity moving across the "ciliary carpet," creating an effect sometimes likened to that of the wind blowing across a field of grain.

The Paramecium spirals through the water as it progresses. When it happens to encounter an obstacle, the "effective stroke" of its cilia is reversed and the organism swims backward for a brief time, before resuming its forward progress. This is called the avoidance reaction. If it runs into the solid object again, it repeats this process, until it can get past the object.

It has been calculated that a Paramecium expends more than half of its energy in propelling itself through the water. This ciliary method of locomotion has been found to be less than 1% efficient. This low percentage is nevertheless close to the maximum theoretical efficiency that can be achieved by an organism equipped with cilia as short as those of the members of Paramecium.

Gathering food
Paramecium feed on microorganisms such as bacteria, algae, and yeasts. To gather food, the Paramecium makes movements with cilia to sweep prey organisms, along with some water, through the oral groove (vestibulum, or vestibule), and into the cell. The food passes from the cilia-lined oral groove into a narrower structure known as the buccal cavity (gullet). From there, food particles pass through a small opening called the cytostome, or cell mouth, and move into the interior of the cell. As food enters the cell, it is gathered into food vacuoles, which are periodically closed off and released into the cytoplasm, where they begin circulating through the cell body by the streaming movement of the cell contents, a process called cyclosis or cytoplasmic streaming. As a food vacuole moves along, enzymes from the cytoplasm enter it, to digest the contents. As enzymatic digestion proceeds, the vacuole contents become more acidic. Within five minutes of a vacuole's formation, the pH of its contents drops from 7 to 3. As digested nutrients pass into the cytoplasm, the vacuole shrinks. When the fully digested vacuole reaches the anal pore, it ruptures, expelling its waste contents outside the cell.

Symbiosis
Some species of Paramecium form mutualistic relationships with other organisms. Paramecium bursaria and Paramecium chlorelligerum harbour endosymbiotic green algae, from which they derive nutrients and a degree of protection from predators such as Didinium nasutum. Numerous bacterial endosymbionts have been identified in species of Paramecium. Some intracellular bacteria, known as kappa particles, give Paramecium the ability to kill other strains of Paramecium that lack kappa particles.

Genome
The genome of the species Paramecium tetraurelia has been sequenced, providing evidence for three whole-genome duplications.

In some ciliates, like Stylonychia and Paramecium, only UGA is decoded as a stop codon, while UAG and UAA are reassigned as sense codons (that is, codons that code for standard amino acids), coding for the amino acid glutamic acid.

Learning
The question of whether Paramecium exhibit learning has been the object of a great deal of experimentation, yielding equivocal results. However, a study published in 2006 seems to show that Paramecium caudatum may be trained, through the application of a 6.5 volt electric current, to discriminate between brightness levels. This experiment has been cited as a possible instance of cell memory, or epigenetic learning in organisms with no nervous system.

Reproduction
Like all ciliates, Paramecium have a dual nuclear apparatus, consisting of a polyploid macronucleus, and one or more diploid micronuclei. The macronucleus controls non-reproductive cell functions, expressing the genes needed for daily functioning. The micronucleus is the generative, or germline nucleus, containing the genetic material that is passed along from one generation to the next.

Paramecium reproduction is asexual, by binary fission, which has been characterized as "the sole mode of reproduction in ciliates" (conjugation being a sexual phenomenon, not directly resulting in increase of numbers). During fission, the macronucleus splits by a type of amitosis, and the micronuclei undergo mitosis. The cell then divides transversally, and each new cell obtains a copy of the micronucleus and the macronucleus.

Fission may occur spontaneously, in the course of the vegetative cell cycle. Under certain conditions, it may be preceded by self-fertilization (autogamy), or it may immediately follow conjugation, in which Paramecium of compatible mating types fuse temporarily and exchange genetic material.

Conjugation
In ciliates such as Paramecium, conjugation is a sexual phenomenon that results in genetic recombination and nuclear reorganization within the cell. During conjugation, two Paramecium of a compatible mating type come together and form a bridge between their cytoplasms. Their respective micronuclei undergo meiosis, and haploid micronuclei are exchanged over the bridge. Following conjugation, the cells separate. The old macronuclei are destroyed, and both post-conjugants form new macronuclei, by amplification of DNA in their micronuclei. Conjugation is followed by one or more "exconjugant divisions."

 * Stages of conjugationStages of ciliate conjugation.svg

In Paramecium caudatum, the stages of conjugation are as follows (see diagram at right):
 * 1) Compatible mating strains meet and partly fuse
 * 2) The micronuclei undergo meiosis, producing four haploid micronuclei per cell.
 * 3) Three of these micronuclei disintegrate. The fourth undergoes mitosis.
 * 4) The two cells exchange a micronucleus.
 * 5) The cells then separate.
 * 6) The micronuclei in each cell fuse, forming a diploid micronucleus.
 * 7) Mitosis occurs three times, giving rise to eight micronuclei.
 * 8) Four of the new micronuclei transform into macronuclei, and the old macronucleus disintegrates.
 * 9) Binary fission occurs twice, yielding four identical daughter cells.

Aging
In the asexual fission phase of growth, during which cell divisions occur by mitosis rather than meiosis, clonal aging occurs leading to a gradual loss of vitality. In some species, such as the well studied Paramecium tetraurelia, the asexual line of clonally aging Paramecium loses vitality and expires after about 200 fissions if the cells fail to undergo autogamy or conjugation. The basis for clonal aging was clarified by transplantation experiments of Aufderheide in 1986. When macronuclei of clonally young Paramecium were injected into Paramecium of standard clonal age, the lifespan (clonal fissions) of the recipient was prolonged. In contrast, transfer of cytoplasm from clonally young Paramecium did not prolong the lifespan of the recipient. These experiments indicated that the macronucleus, rather than the cytoplasm, is responsible for clonal aging. Other experiments by Smith-Sonneborn, Holmes and Holmes, and Gilley and Blackburn demonstrated that, during clonal aging, DNA damage increases dramatically. Thus, DNA damage in the macronucleus appears to be the cause of aging in P. tetraurelia. In this single-celled protist, aging appears to proceed as it does in multicellular eukaryotes, as described in DNA damage theory of aging.

Meiosis and rejuvenation
When clonally aged P. tetraurelia are stimulated to undergo meiosis in association with either conjugation or automixis, the genetic descendants are rejuvenated, and are able to have many more mitotic binary fission divisions. During either of these processes, the micronuclei of the cell(s) undergo meiosis, the old macronucleus disintegrates, and a new macronucleus is formed by replication of the micronuclear DNA that had recently undergone meiosis. There is apparently little, if any, DNA damage in the new macronucleus. These findings further solidify that clonal aging is due, in large part, to a progressive accumulation of DNA damage; and that rejuvenation is due to the repair of this damage in the micronucleus during meiosis. Meiosis appears to be an adaptation for DNA repair and rejuvenation in P. tetraurelia. In P. tetraurelia, CtlP protein is a key factor needed for the completion of meiosis during sexual reproduction and recovery of viable sexual progeny. The CtlP and Mre11 nuclease complex are essential for accurate processing and repair of double-strand breaks during homologous recombination.

List of species
Paramecium aurelia species complex: Other species:
 * Paramecium primaurelia
 * Paramecium biaurelia
 * Paramecium triaurelia
 * Paramecium tetraurelia
 * Paramecium pentaurelia
 * Paramecium sexaurelia
 * Paramecium septaurelia
 * Paramecium octaurelia
 * Paramecium novaurelia
 * Paramecium decaurelia
 * Paramecium undecaurelia
 * Paramecium dodecaurelia
 * Paramecium tredecaurelia
 * Paramecium quadecaurelia
 * Paramecium sonneborni
 * Paramecium buetschlii
 * Paramecium bursaria
 * Paramecium calkinsi
 * Paramecium caudatum
 * Paramecium chlorelligerum
 * Paramecium duboscqui
 * Paramecium grohmannae
 * Paramecium jenningsi
 * Paramecium multimicronucleatum
 * Paramecium nephridiatum
 * Paramecium polycaryum
 * Paramecium putrinum
 * Paramecium schewiakoffi
 * Paramecium woodruffi

title: Paramecium


text:
Talk:Glossary of American football terms

Glossary entries as articles
Not every entry in the glossary needs it's own article. Some can be explained by a single sentence or less. Bluelion 00:30 Mar 3, 2003 (UTC)

Bolding
Is there any significance to the fact that some terms are in bold, and others are not? - Molinari 08:06 12 Jul 2003 (UTC)
 * No, just sloppy styling. It's a lot better now.

Laterals
The entry for backward pass says that it's different than a lateral, but the entry for lateral just points back to backward pass. I'd fix this, but I'm not familiar with the terms. 157.127.155.214 (talk) —Preceding undated comment added 00:24, 23 December 2010 (UTC).
 * Technically speaking, according to the rules, there are only forward passes and backward passes, the later including those passes thrown exactly parallel to the line of scrimmage. Wschart (talk) 04:58, 27 December 2012 (UTC)

True or false??
True or false: this is a list of dictionary definitions. 66.245.124.71 23:38, 21 Aug 2004 (UTC)

 * That's why I deleted some excess material under "neutral zone", which material was also describing the wrong thing. Some entries are linked to articles.  You probably COULD have an article for just about every entry, but no more than defining info should be in the glossary itself.
 * The X, Y, and Z entries should be reconsidered, as the jargon is far from universal. Y.A. Tittle's booklet "How to Watch Pro Football on TV" that I read 35+ years ago said that in the huddle, those letters would describe the primary, secondary, and tertiary pass receivers, respectively, the signal caller pointing to them as the letters were given.
 * robgood@bestweb.net 12/13/04

 * That's actually what you do, or these days (7+ years later!) it will just be a list of WP:DICDEFs, will be transwikied to Wiktionary]], and deleted. Encyclopedic glossaries have to have in-depth, encyclopedic information. See MOS:GLOSS for guidance, and Glossary of cue sports terms for a well-developed, well-sourced example. — SMcCandlish    Talk⇒〈°⌊°〉 Contribs. 19:52, 14 February 2012 (UTC)

Link suggestions
An automated Wikipedia link suggester has some possible wiki link suggestions for the Glossary_of_American_football article, and they have been placed on this page for your convenience. Tip: Some people find it helpful if these suggestions are shown on this talk page, rather than on another page. To do this, just add to this page. — LinkBot 10:24, 17 Dec 2004 (UTC)

The box/pocket
What exactly is "the box" or "the pocket"? Ground 01:40, 18 Jan 2005 (UTC)
 * I added them, but don't think it's my best work. Gentgeen 05:20, 18 Jan 2005 (UTC)

"Bye" entry isn't football-specific
Do we really need this entry for "bye"? That's a general sports/games term not specific to football. Unless someone posts an objection by, say, 4/15/05, I'll delete it. - robgood@bestweb.net

46 defense
The 46 defense has 4 linemen and 4 linebackers, not 4 linemen and 6 linebackers.

Nope, the 46 defense has 4 linemen and 3 linebackers plus a strong safety lined up where one might mistake him for a 4th linebacker. Adembroski 11:40, 10 December 2006 (UTC)

Orphan page
Amazingly, this page isn't link to from any of the other AF articles, which seems quite an omission. Are the terms glossed not much used on those articles, or are they just not link? I noticed that, say, the term "blocking" was badly linked on one page, and could possibly link here (though a short consolidated section or article on blocking techniques might be a preferable target, if one exists). If all else fails, perhaps this should be linked to in the main article on a "see also" basis... Alai
 * Actually, lots of other pages link to the glossary. Please go back to the Article tab and then click on "What links here" to see a list.  If you tried it from this page you would get the idea it is an orphan, because no other pages link to the Talk page. Johntex\talk 02:47, 31 March 2007 (UTC)

Technical vs. Layman terms
There are a few technical inaccuracies that perhaps a layman football fan would not quite understand. The generalities as applied by the media and fans are accurate, but a coach or player knows the difference.

For example, from a technical, play calling, perspective, a "blitz" occurs when a defensive back rushes the quarterback. The same action by a linebacker is not a blitz; it is a "dog" or a "dogging" linebacker.

Play Action is a combination of two separate types of passes: Play Passes, and Action Passes. A Play Pass occurs when the offense attempts to convince the defense that a run is occurring. An Action Pass occurs when the quarterback moves out of the pocket, either on a roll out, sprint out, or waggle. Play Action, technically, applies to plays in which both of those occur, and has come to be used by the media to refer only to Play Passes. Adembroski 04:05 October 23, 2005

2007-02-1 Automated pywikipediabot message
--CopyToWiktionaryBot 14:05, 1 February 2007 (UTC)

Leg-whipping?
The explanation of crackback block mentions leg-whipping, but that term isn't defined elsewhere. Can someone who knows what it is please add it? China wants to know! ;-) 218.25.32.210 (talk) 06:27, 1 September 2009 (UTC)

Template-structured glossary
I converted the list from an unstructured glossary to a structured glossary. It should help people who had accessibility issues with the unstructured list. See MOS:Glossaries. If consensus is to change it back, I can deal. TheTito  Discuss  01:29, 15 November 2009 (UTC)
 * Note that if you link the term, e.g. to an external article, you have to repeat the term without the linking in the second parameter, or incoming links will fail:
 * Same goes for if you do wikimarkup or add anchors:
 * — SMcCandlish   Talk⇒〈°⌊°〉 Contribs. 19:43, 14 February 2012 (UTC)
 * Same goes for if you do wikimarkup or add anchors:
 * — SMcCandlish   Talk⇒〈°⌊°〉 Contribs. 19:43, 14 February 2012 (UTC)
 * — SMcCandlish   Talk⇒〈°⌊°〉 Contribs. 19:43, 14 February 2012 (UTC)
 * — SMcCandlish   Talk⇒〈°⌊°〉 Contribs. 19:43, 14 February 2012 (UTC)
 * — SMcCandlish   Talk⇒〈°⌊°〉 Contribs. 19:43, 14 February 2012 (UTC)

Spike
This item needs to better explained. Spiking the ball is said to be "Technically an incomplete pass, it stops the clock." However, a little farther in the paragraph, a description of a play says "the spike would run the clock out". I don't understand. Doesn't spiking STOP the clock? So then how could the clock run out???

Reply: I put in the item about "the spike would run the clock out". Thanks for pointing out the apparent inconsistency; yes, the spike STOPS the clock, but it takes time to line up to get that play started, and that's where the situation can happen that there is so little time left (say, 1 second?) that the clock would run out before the ball can be spiked. In the case of Ryan Leaf in the Rose Bowl, his team got a 1st down and (under college rules) the clock stopped (2 seconds left on it) while the chains were moved, and then he tried to spike the ball, and the ruling was that time ran out before that play was finished. The clock stopped all right, but there was no time left on it so there could be no next play. Do you understand what I am saying? Maybe we need to consider rewording that comment about "would run the clock out". — Preceding unsigned comment added by 128.63.16.82 (talk) 15:59, 2 September 2011 (UTC)

Oct. 11, 2011: Last Sunday's NFL game where Oakland defeated Houston, 25-20, is an example of late-game spike. Looking in nfl.com play-by-play, we see that the Houston quarterback completed a pass; his team then hurried to line up (the clock was still running) and with 0:08 left he started a spike play, leaving 0:06 on the clock. The next play was the last play; that quarterback threw a pass that was intercepted in the end zone. — Preceding unsigned comment added by 128.63.16.82 (talk) 21:18, 11 October 2011 (UTC)

Terms to add
I thought it might be helpful to have a section for people to suggest a term to add since I've seen a few sections started for certain terms. HotshotCleaner (talk) 01:37, 17 October 2011 (UTC)

Chop block
Might it be possible for someone to add a definition for chopping or a chop block? — Preceding unsigned comment added by 68.80.167.99 (talk) 01:56 Dec 23, 2006

Go in motion
How 'bout "go in motion"? Mauvila 07:15, 4 Feb 2005 (UTC)

Stuff
Does anyone know what the "stuff" statistic on defense is? I've seen it recorded on espn.com, yahoo.com and more sites.

Hand count
What is the "hard count"? How does it work and who uses it often in the nfl? 69.127.1.115 (talk) 01:51, 13 October 2008 (UTC)

Quarterback rating and rushing average
Shouldn't terms like quarterback rating (with link to main article) and rushing average be part of this glossary? --ChaChaFut 02:31, 31 March 2007 (UTC)
 * Sounds great, please add them. Johntex\talk 02:45, 31 March 2007 (UTC)

Horse collar
I didn't see horse-collar on the list and thought that it could be added. Thoughts? HotshotCleaner (talk) 01:37, 17 October 2011 (UTC)

Scrimmage kick
The term "scrimmage kick" is used three times in the glossary, and was redlinked. No such article, no entry here. — SMcCandlish   Talk⇒〈°⌊°〉 Contribs. 19:41, 14 February 2012 (UTC)

Clutch
I came to this page hoping to get a definition of clutch. Please add. Thanks, Richard. 50.47.243.206 (talk) 11:55, 24 November 2013 (UTC)

please clarify the intentional-grounding definition
I found this (we are not here considering the foul occurring in the end zone):

"This foul costs the offense a loss of down and 10 yards. If it occurs 10 yards behind the line of scrimmage, then the 10 yards is taken from the spot of the foul."

I'm not sure what this means. Perhaps it should say "If it occurs 10 or more yards behind the line of scrimmage", but I don't know enough to be confident in changing this myself. I did, however, check the NFL rules digest; if the spot of the foul is more than 10 yards behind line of scrimmage, the ball is placed there (and there is still loss of down). I don't know how this gets shortened when the offending team is near its own goal line.

out of bounds and out of the end zone
Has anyone considered defining those terms? Out of bounds means across the sidelines between the goal lines. The sidelines continue along the end zones, but crossing the sideline from the end zone, as well as crossing the end line at back of end zone, is referred to as going out of the end zone.

I did, however, hear "out of bounds" when I saw a video of Dan Orlovsky's "you are an idiot" safety. He stepped across the end line, not sufficiently aware of where of he was. — Preceding unsigned comment added by 128.63.16.82 (talk) 20:25, 5 March 2012 (UTC)

Quarterback Controversy
'Quarterback Controversy' is linked as a glossary term on some pages and leads here but there is no entry. Examples: KC Quarterbacks,  Rich Gannon -- It would seem the term needs to be re-added or the links need to be removed. - zeroality (talk) 03:41, 24 January 2013 (UTC)

Spring football game
I just came across the term "spring football game" at the Jack Hoffman article and had trouble figuring out what it means. As I understand it is an annual intersquad football training game played by many university teams, correct? Is this a common enough term to add it here and link to? I'm guessing most non-North-Americans don't know what that is. :) Amalthea  14:13, 9 June 2014 (UTC)

External links modified
Hello fellow Wikipedians,

I have just added archive links to 2 one external links on Glossary of American football. Please take a moment to review my edit. If necessary, add after the link to keep me from modifying it. Alternatively, you can add to keep me off the page altogether. I made the following changes:
 * Added archive https://web.archive.org/20040823190402/http://www.cowboysplus.com:80/classic/webspecials/landry/landrybuild.htm to http://www.cowboysplus.com/classic/webspecials/landry/landrybuild.htm
 * Added archive https://web.archive.org/20051201115219/http://www.sportingnews.com:80/archives/landry/quotes.html to http://www.sportingnews.com/archives/landry/quotes.html

When you have finished reviewing my changes, please set the checked parameter below to true to let others know.

Cheers.—cyberbot II  Talk to my owner :Online 01:49, 10 February 2016 (UTC)

External links modified
Hello fellow Wikipedians,

I have just modified 4 external links on Glossary of American football. Please take a moment to review my edit. If you have any questions, or need the bot to ignore the links, or the page altogether, please visit this simple FaQ for additional information. I made the following changes:
 * Corrected formatting/usage for http://www.draftsharks.com/article/1933/schauf-s-still-celebrating-black-monday-firing-of-mike-shanahan
 * Added archive https://web.archive.org/web/20120220084300/http://www.cyclonefanatic.com/forum/pro-sports/19008-nfls-black-monday.html to http://www.cyclonefanatic.com/forum/pro-sports/19008-nfls-black-monday.html
 * Added archive https://web.archive.org/web/20061022114800/http://www.colts.com/sub.cfm?page=article7&news_id=4062 to http://www.colts.com/sub.cfm?page=article7&news_id=4062
 * Added tag to http://nflhs.org/tipsdrills/coachbass/coachbass_09192001_sim.asp
 * Corrected formatting/usage for http://www.nj.com/giants/index.ssf/2017/08/what_does_it_mean_when_an_nfl_player_is_waivedinju_1.html

When you have finished reviewing my changes, you may follow the instructions on the template below to fix any issues with the URLs.

Cheers.— InternetArchiveBot  (Report bug) 00:46, 20 October 2017 (UTC)

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


text:
Talk:Transfinite induction

Redundancy?
A needless redundancy may be here. Suppose we show that for any ordinal b, for all ordinals a < b, P(a) is true. Then it is not necessary to prove separately that P(the first ordinal) is true, because a special case of the induction step is that if P(a) is true for all ordinals a less than the smallest ordinal, then P(the smallest ordinal) is true. And it is vacuously true that P(a) is true for all ordinals a less than the smallest ordinal! Michael Hardy 00:41 Mar 3, 2003 (UTC)

So it now reads "If P(a) follows from the truth of P(b) for all a < b, then it is simply a special case to say that P(0) is true, since it is vacuously true that P(b) holds for all b < 0."... Don't we mean to say "b < a" in the first part of the sentence? A5 20:45 8 Jun 2003 (UTC)

I've now reversed the order of a and b, so that it says "If P(b) follows from the truth of P(a) for all a < b, then it is simply a special case to say that P(0) is true, since it is vacuously true that P(b) holds for all b < 0." Michael Hardy 21:34 8 Jun 2003 (UTC)

 * This is not the case. Suppose you have the sequence {1, 0, 0, 0, ...} and the proposition P(n) that the series sum after n is S(n) = 0. Assuming S(k)=0 gives S(k+1)=0, but S(0) = 1 so P(0) is false. This is still a valid example if you correct the S(k) and S(k+1) terms for transfinite scenarios.
 * The redundancy you're seeing is that the induction step assumes P(0) is true as well as all other b < a. You still have to prove it to be so.
 * As such I've deleted the paragraph. Mark Hurd 20:11, 3 Nov 2004 (UTC)
 * Sorry.Michael has correctly reverted my edit, because I was reading this explanation as P(0) is proved by assuming P(b) for all b < 0. IOW I saw it saying P(0) was vacuously true. Mark Hurd 10:03, 4 Nov 2004 (UTC)

This may seem silly to others, but upon reading, I noticed in the proof that the indexing set is integers. Perhaps this condition on α,β should be stated earlier. Based on the name of the page, I was complacent assuming that those were elements of an arbitrary indexing set, but if so, P(β+1) is useless. ub3rm4th 00:37, 3 May 2006 (UTC)
 * Not quite; see Ordinal arithmetic. The article does say "ordinal" all over the place; do you think it should be emphasized? Melchoir 00:42, 3 May 2006 (UTC)
 * "Suppose whenever for all α < β, P (α) is true, then P (β) is also true." Should this instead read "Suppose that for all ordinals α and β satisfying α < β, then P (α) is true implies P (β) is also true." ?  It's standard to note where variables are taken from before their first use, not after.
 * Well, fit it! Melchoir 05:26, 7 May 2006 (UTC)

Isn't there some redundancy in the first two paragraphs of Transfinite Induction? The same thing being said ie. chimpionspeak 02:26, 9 March 2010

 * To chimpionspeak: Yes, the redundancy is clearly indicated by the use of "That is,". It is intentional. The concept is hard to render in English and we felt that expressing it in two different ways would give the reader a better chance of understanding it. JRSpriggs (talk) 08:34, 10 March 2010 (UTC)

 * Okay. I thought that it didn't add anything because the first sentence of the second paragraph and the second sentence of the first paragraph are the same. chimpionspeak 00:23, 12 March 2010

Proof
We should add a proof for the transfinite induction principle here. indirect proof: Suppose we have proved the crucial step P(m P(n) as well an P(x) for all minimal elements (for ordinals it would be 1). Now suppose there existed an N for which P(N) does not hold. Since we have proved P(mP(N) there must exist an M<N for which P does not hold. The same argument can be applied to M and so on; we have an infinite descending chain (it is infinite because it never reaches the minimal elements- P is true for them [P(0) for the ordinals]). And that contradicts our well-founding. —Preceding unsigned comment added by 217.232.29.149 (talk • contribs)

Transfinite recursion
Currently, transfinite recursion is a redirect here. That's not necessarily bad. But if it's to stay that way, then the page really ought to say something about transfinite recursion (a method of definition, not of proof). --Trovatore 01:51, 16 July 2005 (UTC)
 * First cut in place. Could use an example or two, and possibly some a more rigorous treatment in addition to (not instead of) the informal descriptions given. --Trovatore 07:03, 16 July 2005 (UTC)

Successor Case vs Limit Case
The page says "Notice that the second and third cases are identical except for the type of ordinal considered (...)", talking about the Successor Case and Limit Case of Transfinite induction. This doesn't seem right; the cases are different because in the Limit case, the limit ordinal is not in the form β+1. I think this comment is misleading (at best), and should be removed. Any objections? --RRM 16:39, 4 April 2007 (UTC)

 * The similarity lies in the fact that one is allowed, in both cases, to use the property for all previous ordinals in showing it for the current ordinal. So a common proof is often possible. JRSpriggs 07:47, 5 April 2007 (UTC)

Sets of ordinals???
Ordinals form class, not set. —The preceding unsigned comment was added by 83.5.233.94 (talk) 11:05, 14 April 2007 (UTC).
 * The class of all ordinals is not a set. That doesn't mean there's no such thing as a set of ordinals. --Trovatore 18:33, 15 April 2007 (UTC)

" "
What?--68.161.161.206 (talk) 11:52, 18 April 2008 (UTC)

 * The first and last order signs should actually be angle brackets, meaning the sequence of elements rα indexed by α < c. — EJ (talk) 12:25, 18 April 2008 (UTC)

Regarding Relationship to the axiom of choice
I think I understand the section. However, I have two questions.

1.) Does one need ordinals at all to define transfinite recursion/induction in ZF? Well-ordering should be enough. Iv'e seen it done without ordinals. (Yes, in that horrible Halmos book since you ask;) Is that enough to make the article belong to the Set Theory category as well. Perhaps not, looks like double work unless ordinals are allowed in Set Theory Category?

2.) If one lives completely i ZF (and ZF only) suddenly want the luxory of having transfinite recursion/induction available for ALL sets, the one would have to add to ZF AC making it ZFC so that within ZFC (only) AC actually is required if transfinite recursion/induction available for all sets? I. e. one must have those magic words "Well-order!"? YohanN7 (talk) 20:24, 16 July 2009 (UTC)

 * 1) One needs neither ordinals nor well-ordering; well-founded relations are enough, as described in that article. You seem to misunderstand how Wikipedia categories work. Since Category:Ordinal numbers is a subcategory of Category:Set theory, any article in the former is implicitly also in the latter. An article should not be explicitly included simultaneously in a category and one of its ancestors. I have no idea what do you mean by "looks like double work unless ordinals are allowed in Set Theory Category".

 * 2) I have no idea what do you mean by "transfinite recursion/induction available for all sets" either. Transfinite induction has nothing to do with the axiom of choice, whether you do it for ordinals or for well-founded relations. For example, ∈-induction applies to all sets, and it does not need the axiom of choice, only the axiom of foundation. You cannot do induction (in the form discussed here) for anything other than well-founded relations, as transfinite induction over a relation R implies that R is well-founded. — Emil J. 10:28, 17 July 2009 (UTC)

 * Transfinite induction has nothing to do, formally speaking, with the axiom of choice. In practice, though, the arguments for which you want to use transfinite induction, will generally require AC.  In particular, one frequently wants to exhaust all the elements of some set, one at a time, in an inductive process.  I think that's what Yohan was getting at with the "available for all sets" phrase. --Trovatore (talk) 21:34, 17 July 2009 (UTC)

 * Thanks for the replies. They were accurate albiet Emils was a bit ironic in places. Both the present article and well-founded relation do however seem to reserve the phrase transfinite induction for induction on well-ordered sets, in particular ordinals or some set indexed by ordinals. In fact, the present article states in the very first scentence that Transfinite induction is an extension of mathematical induction to well-ordered sets.... YohanN7 (talk) 06:18, 19 July 2009 (UTC)
 * Umm, right, that's a point. Emil is certainly correct that you can do induction on any wellfounded relation, and you're correct that it isn't described as transfinite induction in this article.
 * The question is, should it be? This is a point of usage rather than mathematics.  My linguistic intuition here is that induction on a wellfounded relation other than a linear order is not standardly described as transfinite induction. --Trovatore (talk) 07:39, 19 July 2009 (UTC)

 * I did a quick "Google" on transfinite induction. It seems that either ordinals or well-ordering IS assumed. Of course it's technically correct that a well-founded relation is sufficient to do induction as Emil and you pointed out. In well-founded relation the name noetherian induction is mentioned for the general case of a well-founded relation. This seems to be the case when I search the net as well. Best thing might be to create a new page called Noetherian Induction and move the induction/recursion material from well-founded relation to the new page. At any rate, if one does not know (like I didn't) that induction/recursion works if [and only if?] one has a well-founded relation then that page isn't the most natural place to search for an answer. YohanN7 (talk) 08:06, 19 July 2009 (UTC)

 * What I meant is that the concept of transfinite induction generalizes to well-founded relations. The name does not, "transfinite induction" is indeed only used for ordinals. The general concept is known as "well-founded induction", see e.g. . Note that we already have well-founded induction as a redirect to well-founded relation; I do not see any need to create a separate article, it's adequately covered there. Some folk call well-founded relations "Noetherian", but a quick search confirms that "Noetherian induction" is by an order of magnitude less common than "well-founded induction", and I frankly despise the name. Anyway, Noetherian induction also redirects to well-founded relation. For good measure, I created redirects well-founded recursion and Noetherian recursion, too. — Emil J. 10:27, 20 July 2009 (UTC)

How about this for a start: The passage

"... More generally, one can define objects by transfinite recursion on any well-founded relation R. (R need not even be a set; it can be a proper class, provided it is a set-like relation; that is, for any x, the collection of all y such that y R x must be a set.)

Relationship to the axiom of choice

There is a popular misconception that transfinite induction, or transfinite recursion, or both, require the axiom of choice (AC). This is incorrect. Transfinite induction can be applied to any wellordered set. However, frequently proofs or constructions using transfinite induction also use the axiom of choice to wellorder a set. ..."

is changed to

"... Relationship to the axiom of choice

There is a popular misconception that induction, or recursion, or both, require the axiom of choice (AC). This is incorrect. One can use induction to prove things and one can define objects by recursion on any well-founded relation R. (R need not even be a set; it can be a proper class, provided it is a set-like relation; that is, for any x, the collection of all y such that y R x must be a set.) However, frequently proofs or constructions using induction and recursion also use the axiom of choice to wellorder a set in order to be able to use the simple prescriptions given in this article. ..."

That puts the spotlight a bit more on what the issue is. Also, the question of what the standard terminology is is temporarily avoided. YohanN7 (talk) 11:05, 19 July 2009 (UTC)

 * Now wait a minute. I may be halfway off the wall. Take the case of recursion. Why the heck shouldn't definition by transfinite recursion - as described in the article - yield a perfectly valid function f:O->X where O is (a subset of) the ordinals - the point being that O has a well-founded relation (the wellorder in this case) and X is quite arbitrary. An example is the standard definition of the Borel sets which yield an ordinal sequence into the powerset of the reals. Proof by induction seems to be a different matter since there we must exhaust the set X in a one-to-one manner. That cannot be done with the prescription in the article unless one in fact wellorders X - or X is wellordered as comes. I need to sleep. YohanN7 (talk) 15:31, 19 July 2009 (UTC)

OkOk redirects are fine with me, though not perfect. I STILL argue that the present article is misleading. —Preceding unsigned comment added by YohanN7 (talk • contribs) 21:01, 21 July 2009 (UTC)

 * The tone of that section of the article is not very good anyway, which may be why you see it as "misleading". I will replace it with part of your text. — Carl (CBM · talk) 04:01, 22 July 2009 (UTC)

How about minimal element?
Let P(α) be a property defined for all ordinals α. Suppose that whenever P(β) is true for all β < α, then P(α) is also true. Then transfinite induction tells us that P is true for all ordinals.

That is, if P(α) is true whenever P(β) is true for all β < α, then P(α) is true for all α. Or, more practically: in order to prove a property P for all ordinals α, one can assume that it is already known for all smaller β < α. But induction is valid only if the property P(α) is true for minimal element: α=0. I quess it should be mentioned. Eugepros (talk) 06:58, 10 August 2010 (UTC)

 * If there are no smaller elements, then they all satisfy P vacuously. Thus you get P(0) from nothing and the induction begins. JRSpriggs (talk) 07:20, 10 August 2010 (UTC)

Cases
All of the three cases—the zero case, the successor case, and the limit case—are cases of proving that if something is true of every ordinal in some initial segment of the ordered class of ordinals, then it is true of the smallest strict upper bound of the set.

I see an anonymous editor saying the zero case is a special case of the limit case. If the limit case is regarded as the case where the smallest strict upper bound is the same as the smallest (weak) upper bound, then the anonymous editor is right. (Of course, there may still be practical reasons why the zero case must be treated separately from other limit cases in particular proofs.) Michael Hardy (talk) 19:05, 15 January 2011 (UTC)

 * OK, so first of all, in the state in which the anon left the article, it claimed that the induction could be done at limit ordinals only. That was clearly nonsense.  It's possible that it was an editing mistake.
 * If we take the view that it was an editing mistake, then what we have is someone who has noticed that we can formally state transfinite induction in the form, if
 * $$\forall \alpha (\forall \beta<\alpha P(\beta)) \implies P(\alpha)$$
 * then
 * $$\forall \alpha P(\alpha)$$
 * This is true, formally, we can indeed state it like that. But it's not the way proofs by transfinite induction actually go, basically ever.  In virtually all nontrivial cases we end up distinguishing at least the three cases mentioned (base, successor ordinal, limit ordinal).  Many times those break up into further cases (a common one is to need to distinguish between limit ordinals of cofinality ω and all other limit ordinals). --Trovatore (talk) 20:45, 15 January 2011 (UTC)

I think maybe some serious concrete examples would help. Michael Hardy (talk) 05:08, 16 January 2011 (UTC)
 * Yes, I have one in mind. It'll take a bit of energy to write it up though.  I'll see if I can get motivated. --Trovatore (talk) 05:12, 16 January 2011 (UTC)

Maybe the article should explicitly comment on the form seemingly suggested by the bare statement that if
 * $$\forall \alpha (\forall \beta<\alpha P(\beta)) \implies P(\alpha)$$

then
 * $$\forall \alpha P(\alpha)$$

and the form of proofs in actual practice. Michael Hardy (talk) 19:24, 16 January 2011 (UTC)

 * I think that the better placement of the parentheses is
 * $$\forall \alpha (\forall \beta<\alpha \, P(\beta) \implies P(\alpha) ) $$
 * because it makes clear that the quantifier over alpha should be applied to the whole implication, not just its hypothesis. That the quantifier over beta is limited to the hypothesis I would assume due to unary operators (negation and quantifiers) taking priority over binary operators (implication, conjunction, disjunction, and equivalence). JRSpriggs (talk) 21:50, 16 January 2011 (UTC)

The difficulty of translating verbal statements of Transfinite Induction into symbolic logic indicates how readers can easily be confused by versions of Transfinite Induction that do not treat the $$ P(0) $$ case separately. I take that in above discussion the expression $$ \forall \beta < \alpha P(\beta) $$ is regarded as being equivalent to $$ \forall \beta ( (\beta < \alpha) \implies P(\beta)) $$. For the sake if the general reader, it would be useful to explain that a statement of Transfinite Induction that does not include the $$ P(0) $$ case explcitly intends the "induction hypothesis" be ::$$ \forall \beta ( ( \beta < \alpha \implies P(\beta) ) \implies P(\alpha) $$.  216.223.227.93 (talk) 17:30, 26 April 2013 (UTC)

(I edited my above comment) Tashiro (talk) 17:50, 26 April 2013 (UTC)

well-ordered vs. ordinals
The lead section talks about well-ordered sets, the rest of the article about ordinals. That is not nice. — Preceding unsigned comment added by 24.85.86.45 (talk) 04:55, 23 October 2011 (UTC)
 * Ordinals are the archetypical well-ordered sets. Every well-ordered set is order-isomorphic to an ordinal, and ordinals are well-ordered sets. So there should be no problem. JRSpriggs (talk) 10:23, 23 October 2011 (UTC)

0 is a limit ordinal
Trovatore: This is a true statement, and it's something people might want to know. You also agreed with my claim that 0 can almost always be treated the same way as limit ordinals in proofs. So why did you remove from the page? Thehotelambush (talk) 00:39, 8 December 2011 (UTC)

 * 0 is not a limit ordinal. JRSpriggs (talk) 00:48, 8 December 2011 (UTC)

 * A limit ordinal is an ordinal such that it is the supremum of all ordinals preceding it. The article on limit ordinals mentions that this definition is sometimes used, and this is the more elegant definition. Anyways, I really don't care if you disagree; my point was only to mention that 0 can usually be treated exactly the same way as limit ordinals in proofs using transfinite induction (which incidentally demonstrates why this definition is more elegant). You agreed with this. So why did you remove it? Thehotelambush (talk) 00:58, 8 December 2011 (UTC)
 * Whether 0 is or is not a limit ordinal (like whether 0 is a natural number or whether finite sets are countable) is one of these points of terminology that people tend to get very exercised about, but on which there is no general agreement.
 * It is true that, formally, you can always treat 0 with the limit ordinals, and for that matter you can always treat all the ordinals together. That's kind of trivial, because all it really means is that, when you have a finite number of fixed cases, you can always combine them into a single function or predicate.  Instead of saying I start with X_0, apply f to X_alpha to get X_{alpha+1}, and apply g to the sequence of all X's below lambda when lambda is limit to get X_lambda, I can always say you apply h to the sequence of all X's below X_alpha to get X_alpha, where h of the empty sequence is X_0, h of a sequence with a last element is f of the last element, and h of a sequence without a last element is g of that sequence.
 * But this is usually a fairly silly thing to do, which is why in practice no one ever does it, for nontrivial arguments. It's misleading to plop it in neutrally as though it were a reasonable thing to do, which it almost never is. --Trovatore (talk) 00:58, 8 December 2011 (UTC)
 * It is not silly, it makes proofs more concise and more elegant, and is almost always a reasonable thing to do. So much so that I can't even recall whether any proofs have necessitated treating it separately. Some readers may be interested in the benefits of this approach, even if you are not.
 * "for that matter you can always treat all the ordinals together." Now you're just being pedantic, you understand perfectly well what I'm getting at. Thehotelambush (talk) 01:10, 8 December 2011 (UTC)
 * Let's take a concrete example. Probably the simplest nontrivial proof that goes by transfinite induction is the proof of the Cantor–Bendixson theorem.  You have a closed set of reals X, and you want to show that it's the disjoint union of a perfect set and a countable set.
 * In the inductive proof, you iteratively apply the operation of removing all isolated points. So X0 is X, X1 is X0 with all isolated points deleted, and so on.  At limit ordinals you take intersections.
 * You argue that this process must terminate at some countable ordinal, and then recover the countable set as the union of the points deleted along the way.
 * Now, can you unify the treatment of 0 with the treatment of the limit ordinals? Sure.  Instead of "take intersections", you say "take the intersection of everything that has gone before, and intersect that with X".
 * But this is artificial. Why intersect with X?  The only reason is to make the base case look formally like the limit step.  But the base case is different from the limit step, as anyone can see. --Trovatore (talk) 01:17, 8 December 2011 (UTC)
 * "as anyone can see" -- weasel words that I don't agree with. I don't care whether you think it's artificial, the point of WP:NPOV is to represent all viewpoints fairly, not to force one particular one on readers. I don't ask that you accept my POV, only that it is represented in the article and not simply removed because someone dislikes it. (And by the way, the concept of empty intersection is a clear, natural, and necessary one when taken from the point of view of limits in category theory.) Thehotelambush (talk) 01:27, 8 December 2011 (UTC)
 * I'm not arguing with the concept of empty intersection. The artificialness comes in intersecting with X.  Why would you do that?  There is no reason, except to make the cases look formally the same.
 * It is simply not correct to say that it is "usually not necessary" to treat 0 differently, unless you mean in the trivial sense I alluded to earlier, which is already dealt with in the text. --Trovatore (talk) 01:31, 8 December 2011 (UTC)
 * It is not arbitrary; X is the ambient space and one is considering subobjects (subsets) of X. Intersection is a limit in the powerset of X.
 * 2nd point - Since that's a statement that can't really be cited in either direction, how about we say instead "0 is sometimes considered a limit ordinal and is then treated in proofs similarly as limit ordinals when possible". And, I would be VERY interested to see a proof where it cannot be treated the same way as a limit ordinal (in a suitable elegant way, by using empty intersections or whatnot). Thehotelambush (talk) 01:46, 8 December 2011 (UTC)

Please see Wikipedia talk:WikiProject Mathematics/Archive/2011/Sep. JRSpriggs (talk) 02:01, 8 December 2011 (UTC)

 * Given that the other article at least mentions the alternative definition, I will add the more neutral sentence into the article. Thehotelambush (talk) 02:13, 8 December 2011 (UTC)
 * I object strongly to the assertion that 0 is "treated similarly when possible". No one strains to treat 0 the same as nonzero limit ordinals in actual practice, and there is no good reason to.  I think you're trying to impose a normative aesthetic that you would like to see followed, but where actual workers in the field simply don't agree with you. --Trovatore (talk) 03:06, 8 December 2011 (UTC)
 * YOU are imposing a normative aesthetic when you say "no one" does this, and when you claim that your way is the only way worth describing in the article. If one treats zero as a limit ordinal (as many do), then there is no reason to treat it differently, as I have demonstrated above. I do not claim that this is even common practice, but it's ridiculous to exclude it simply because it's a minority viewpoint. Thehotelambush (talk) 04:12, 8 December 2011 (UTC)
 * I am not trying to add to the article a claim that no one does it in practice, although it is the truth in nontrivial cases. You have demonstrated no such thing.  What you have done is make an ad-hoc argument that, in retrospect, you can fit the form of the proof into your Procrustean mold.  But this proof is still pretty trivial.  I'd like to see what you'd do to, say, the proof of the comparison lemma, where your base step starts with two mice.
 * In practice, most arguments start with some arbitrary, complicated object as the base case. It's a given thing, coming from the hypotheses assumed for the theorem.  It's usually only in the base step that it's useful to mention it, not in nonzero limit steps, and this makes the base step different. --Trovatore (talk) 04:21, 8 December 2011 (UTC)

I'm not familiar with the comparison lemma, if you can provide a proof for it I will gladly examine it. "I am not trying to add to the article a claim that no one does it in practice, although it is the truth in nontrivial cases." ok, so you admit your argument is based on a false claim (while dismissing mathematicians with whom you are unfamiliar, such as myself, as "trivial".). I am not making an ad hoc argument as, in my experience, virtually all proofs by transfinite induction/recursion are simpler and more concise given this treatment, but clearly your experience is different (given a limited understanding of empty intersections). Thehotelambush (talk) 04:39, 8 December 2011 (UTC)
 * What "limited experience of empty intersections"? I never objected to empty intersections.
 * The text you added was, Zero is sometimes considered a limit ordinal and is then treated in proofs in the same case as limit ordinals when possible.. The first part of that is true; zero is indeed sometimes considered a limit ordinal.  The second part is just false.  Yes, some arguments are presented this way, but it is not the usual treatment, even when "possible".  --Trovatore (talk) 04:50, 8 December 2011 (UTC)
 * I've changed the wording to be more neutral. Thehotelambush (talk) 04:57, 8 December 2011 (UTC)
 * OK, that's acceptable. --Trovatore (talk) 05:06, 8 December 2011 (UTC)
 * Thanks, I'm glad we worked this out. Thehotelambush (talk) 05:10, 8 December 2011 (UTC)

Unlike all limit ordinals, zero belongs to an open set, namely {0}, which contains no other ordinal. JRSpriggs (talk) 14:18, 8 December 2011 (UTC)

Vitali set example problematic?
The example states that choice is only used to well-order the reals. But the actual "construction" doesn't seem a real "construction". Specifically, the claim to be able to just "countinue; at each step" to chose uncountably often does not seem well-defined at all. Of course it is possible to clearly state this process in a formally correct way, but then it won't demonstrate what is meant to be demonstrated here. 217.84.235.60 (talk) 22:50, 11 January 2012 (UTC)
 * I am not sure I understand the objection. As far as I can tell, once the original wellorder is chosen, after that everything is completely well-specified and constructive; at each step you choose the least possible element of the equivalence class, "least" being understood in the sense of the chosen wellorder.  Are you claiming that the iteration as stated can proceed in two different ways, even given the same wellorder?  I think that that is not true. --Trovatore (talk) 23:28, 11 January 2012 (UTC)
 * To clarify a bit: The reason this example is in the article about transfinite induction is, presumably, to demonstrate it in use. But precisely this use is totally obscured in "chose at each step". For the reader who is not clear on what transfinite induction is, its use in that example is not at all obvious. More dangerously, these matters need very precise attention especially when in a learning stage. But looking anywhere in the article, nowhere is "chose something" uncountably often mentioned - instead, either a restriction of a class function (no strict classes mentioned here), or a version with precise steps to follow regarding the treatment of successor and limit ordinals. Neither of those appear anywhere either. I would think that any example on this page should take great care to be exact and rigorous in its use of mathematical terms, and this section is anything but.217.84.235.60 (talk) 05:03, 12 January 2012 (UTC)
 * I still don't follow you. Is your objection that what happens at each step is not actually "choosing", because it's pre-determined by the chosen wellorder?  Would you be happier if the word choose were replaced by take or some such?  As for proper classes, there are none in sight, so I don't see the point of mentioning them. --Trovatore (talk) 05:11, 12 January 2012 (UTC)
 * Maybe what you're really looking for is some sort of explanation as to how to turn the argument into a formal deduction from ZFC or something? In that case I completely disagree.  The important thing to get across here is what's really going on, "naively" or "Platonistically" if you will, and what's really going on is an uncountable sequence of discrete steps.  There is nothing ill-specified about it; it's completely canonical.  The details of how to create a formal ZFC argument are less canonical, and also less interesting. --Trovatore (talk) 06:30, 12 January 2012 (UTC)
 * This last bit is a bit closer, but not fully. I don't mean a formal deduction from ZFC - that'd certainly be far over the top. But a somewhat more formal deduction from the principles actually laid out in the paragraph above is what I'd expect to see, or at least hinting at it. The transition from having functions G as used in the above explanation to "doing something in steps" is what is needed to really "understand" transfinite induction, yet it is - for the most likely reader of this page, i.e. someone not yet familiar with how these things work - not trivial. Adding something like "so we have G defined as ... and then we apply the principles outlayed above" would help. To be clear: I'm not disputing the correctness of the statements there. I just don't think the example is particularly serving its purpose of showing where AoC is used and where the transfinite induction happens (and that AoC is not used there any more).217.84.242.179 (talk) 16:10, 12 January 2012 (UTC)
 * Actually, a fine example of what I'd expect to be explained is here: http://en.wikipedia.org/wiki/Ordinal_number#Transfinite_induction - that the main page has a less clear description of what's going on in the only example given is what I was trying to point out. — Preceding unsigned comment added by 217.84.234.165 (talk) 21:53, 14 January 2012 (UTC)

Too : missing prerequisites in section on transfinite recursion
This section is not self-contained. I'm marking it as.

 * What's a class function? I don't guess it's class function, since the latter has to do with group theory (though of course one never knows).
 * What's $$\upharpoonright$$? From https://en.wikipedia.org/wiki/Transfinite_sequence#Successor_and_limit_ordinals, I'd guess it's some limit operation. However, V is not restricted to be ordered. (In http://planetmath.org/transfiniterecursion, V is the class of all sets, but then this article should mention it as well). From http://www.proofwiki.org/wiki/Transfinite_Recursion/Theorem_1, I see it's a restriction.
 * I can guess what's "transfinite sequence", but the link points to the following definition:
 * If α is a limit ordinal and X is a set, an α-indexed sequence of elements of X is a function from α to X. This concept, a transfinite sequence or ordinal-indexed sequence, is a generalization of the concept of a sequence. An ordinary sequence corresponds to the case α = ω.

But this definition can't be used here (I think), since the index set is the whole Ord.

(BTW, this seems to overlap with https://en.wikipedia.org/wiki/Transfinite_sequence#Transfinite_induction, which has a better discussion of the concepts apparently, though it fails to provide enough definitions). --Blaisorblade (talk) 13:20, 16 October 2013 (UTC)

 * As in most parts of mathematics, there are certain things you have to know before you start. If you do not know them, then you will not be able to understand the subject at all.
 * Usually, a function is assumed to have a domain which is a set. A class function is something which is defined like a function but may have a class for its domain. So every function is a class function, but some class functions are not functions because their domains are proper classes.
 * In this context, the harpoon $$\upharpoonright$$ means that the domain of the function on the left of the harpoon is restricted to the class to the right of the harpoon. That is
 * $$F \upharpoonright C = \{ \langle x, y \rangle \, \vert \, \langle x , y \rangle \in F \land x \in C \} \,. $$
 * In this case the function may actually be a class function.
 * If you think you can explain things better, then please be my guest and edit the article. But please make small localized changes and wait for a few days to see whether others make corrections or reversion before continuing. JRSpriggs (talk) 15:19, 16 October 2013 (UTC)

 * This seems not true. The symbol $$\upharpoonright$$ is not the sole standard denotation for the restriction of functions (cf. http://en.wikipedia.org/wiki/Restriction_%28mathematics%29 ), so its meaning should be precised. Moreover, the type of G is wrong. Without fixing it V stands for any class. Thus the type V → V says that G may be even an empty function, what is surely wrong. Shouldn't V be, for example, the class of all sets? — K. 2014-02-25 — Preceding unsigned comment added by 79.184.244.27 (talk) 13:14, 25 February 2014 (UTC)

 * That some people use other symbols for restriction of a function does not change the fact that the harpoon is used for that purpose in this article (and in many papers on set theory).
 * "V" is used for the class of all sets here. There are no types in ZFC set theory, so I do not understand what you mean when you say "the type of G is wrong". JRSpriggs (talk) 08:25, 26 February 2014 (UTC)

 * Where is written that "V" is used for the class of all sets here? If you do not write this, the standard convention says that V is quantified universally. In other words the standard semantics of "Given a class function G: V → V" is "Let V be any class and G by any class function of the type V → V".

 * Going back to the harpoon. You probably haven't written science papers, so let me explain. If you write a paper, you do it for others, not for your self. So you should care about making it understandable for all readers. In a situation when you are using some notation that is not a total standard, it is in good taste to give a note about what this notation means. (cf. Theorem 1 at this page http://planetmath.org/transfiniterecursion ) K. 2014-02-27 — Preceding unsigned comment added by 79.186.221.235 (talk) 09:45, 27 February 2014 (UTC)

 * This is not a paper by me. It is the result of contributions by dozens of people. If you see something wrong, YOU should fix it. Do not tell me to fix it. JRSpriggs (talk) 10:50, 27 February 2014 (UTC)

I defined V and the harpoon. Even though it's apparently becoming more common, I'd never seen the harpoon. And being explicitly told that G is a function on sets (by defining V) would've saved me some puzzlement.

173.25.54.191 (talk) 06:00, 5 March 2014 (UTC)

 * Thank you for preventing some other people from experiencing the same puzzlement. JRSpriggs (talk) 07:53, 5 March 2014 (UTC)

As a fresh eye on this, it is definitely awkward, to read this section. The term class function appears out of the blue without having it defined anywhere on the page. There is no link, no reference, nothing. What to do about this? Another question is, how important is this bit of technical information to the overall point of the article? The article is titled "Transfinite Induction" and this section isn't even about that. So now we've detoured into a related subject, and we're expecting the reader to know jargon that he apparently didn't have to know in order to understand the main topic. All of this, and it's not even the topic that brought him here... In addition to making it more accessible, perhaps we also need to better justify this section.

I am trying to think of the audience and I'm trying to be reasonable about what technical terms we should allow or not... I'm not sure the general public even thinks of functions as sets anyway... So from that perspective it seems as tho we're muddying up the article for the sake of pedantic accuracy. On the one hand, I think the topic is technical enough, that it is reasonable to expect the reader to have some interest in set theory. And they may appreciate the precision of our language later, once our meaning is clear. On the other hand, even the highly motivated and interested reader who isn't already accustomed to the jargon is going to find this presentation jarring.

Consider whether this section should be moved to its own page, which can then be linked to in the "See also" section. It would be less jarring i think in that fashion. 24.110.50.184 (talk) 07:18, 6 May 2014 (UTC)

Problems (2015)
I'm not happy with the footnotes. The second one is not entirely bad, but the first one assumes ZF set theory. In NBG or MK set theories (among others), a "class function" is just a function which is not necessarily a "set". Also, transfinite recursion doesn't quite work in ZF. You can correct the conclusion as follows:
 * 1) (Traditional) for any ordinal λ there is a function F: λ → V such that for all α < λ, ....
 * 2) (Advanced) There is a "class function" F: Ord → V such that ....

This article needs specific references. I don't have a copy of Suppes's book. I do have a copy of my late mother's book, Set Theory for the Mathematician, which has most of the relevant theorems, but uses NBG or MK as the underlying set theory. (I also have Kelley's book, General Topology, but I don't think it actual uses transfinite induction. It also uses, of course, Morse-Kelley set theory, rather than ZF.) — Arthur Rubin  (talk) 07:50, 23 August 2015 (UTC)

Sections "Transfinite induction" and "Transfinite recursion"
I just noticed that some time ago a section header titled "Transfinite induction" was removed by a bot, effectively moving the contents of the first section into the lede section, see. I think the reason for changes like this is that the section title was equal to the page's title, leading to conflicting and IDs, see MOS:HEAD: "Section and subsection headings should preferably be unique within a page; otherwise section links may lead to the wrong place, and automatic edit summaries can be ambiguous." Anyway, I think I actually prefer the flow of the page before this change, with two sections "Transfinite induction" and "Transfinite recursion", see. What do the other wikipedians say? Should something like the section title be reinstated? – Tobias Bergemann (talk) 10:37, 8 October 2015 (UTC)

The final inference: that P(all ordinals) is true. Verdana ♥ '''Bold 15:25, 24 August 2020 (UTC)

question about P
It says that if P(alpha) is true, then P(all ordinals) is true.

The set of new ordinals that are inferred to be true after transfinite induction: is that set the compliment of alpha? Verdana ♥ '''Bold 15:25, 24 August 2020 (UTC)

 * You are misinterpreting the line "Let $$P(\alpha)$$ be a property defined for all ordinals $$\alpha$$. Suppose that whenever $$P(\beta)$$ is true for all $$\beta < \alpha$$, then $$P(\alpha)$$ is also true.  Then transfinite induction tells us that $$P$$ is true for all ordinals.".
 * The conclusion is $$ \forall \alpha P (\alpha) $$. JRSpriggs (talk) 19:18, 24 August 2020 (UTC)

title: Talk:Transfinite induction


text:
Monazite

Monazite is a primarily reddish-brown phosphate mineral that contains rare-earth elements. Due to variability in composition, monazite is considered a group of minerals. The most common species of the group is monazite-(Ce), that is, the cerium-dominant member of the group. It occurs usually in small isolated crystals. It has a hardness of 5.0 to 5.5 on the Mohs scale of mineral hardness and is relatively dense, about 4.6 to 5.7 g/cm3. There are five different most common species of monazite, depending on the relative amounts of the rare earth elements in the mineral:
 * monazite-(Ce), (Ce,La,Nd,Th)PO4 (the most common member),
 * monazite-(La), (La,Ce,Nd)PO4,
 * monazite-(Nd), (Nd,La,Ce)PO4,
 * monazite-(Sm), (Sm,Gd,Ce,Th)PO4,
 * monazite-(Pr), (Pr,Ce,Nd,Th)PO4.

The elements in parentheses are listed in the order of their relative proportion within the mineral: lanthanum is the most common rare-earth element in monazite-(La), and so forth. Silica (SiO2) is present in trace amounts, as well as small amounts of uranium and thorium. Due to the alpha decay of thorium and uranium, monazite contains a significant amount of helium, which can be extracted by heating.

The following analyses are of monazite from: (I.) Burke County, North Carolina, US; (II.) Arendal, Norway; (III.) Emmaville, New South Wales, Australia.

Monazite is an important ore for thorium, lanthanum, and cerium. It is often found in placer deposits. India, Madagascar, and South Africa have large deposits of monazite sands. The deposits in India are particularly rich in monazite.

Monazite is radioactive due to the presence of thorium and, less commonly, uranium. The radiogenic decay of uranium and thorium to lead enables monazite to be dated through monazite geochronology. Monazite crystals often have multiple distinct zones that formed through successive geologic events that lead to monazite crystallization. These domains can be dated to gain insight into the geologic history of its host rocks.

The name monazite comes from the μονάζειν (to be solitary), via German Monazit, in allusion to its isolated crystals.

Structure
All monazites adopt the same structure, meaning that the connectivity of the atoms is very similar to other compounds of the type M(III)PO4. The M(III) centers have a distorted coordination sphere being surrounded by eight oxides with M–O distances around 2.6 Å in length. The phosphate anion is tetrahedral, as usual. The same structural motif is observed for lead chromate (PbCrO4). Monazite also shares many structural similarities with; zircon, xenotime, scheelite, anhydrite, barite, and rhabdophane.

Mining history
Monazite sand from Brazil was first noticed in sand carried in ship's ballast by Carl Auer von Welsbach in the 1880s. Von Welsbach was looking for thorium for his newly invented incandescent mantles. Monazite sand was quickly adopted as the thorium source and became the foundation of the rare-earth industry.

Monazite sand was also briefly mined in North Carolina, but, shortly thereafter, extensive deposits in southern India were found. Brazilian and Indian monazite dominated the industry before World War II, after which major mining activity transferred to South Africa. There are also large monazite deposits in Australia.

Monazite was the only significant source of commercial lanthanides, but because of concern over the disposal of the radioactive daughter products of thorium, bastnäsite came to displace monazite in the production of lanthanides in the 1960s due to its much lower thorium content. Increased interest in thorium for nuclear energy may bring monazite back into commercial use.

Mineralization and extraction
Because of their high density, monazite minerals concentrate in alluvial sands when released by the weathering of pegmatites. These so-called placer deposits are often beach or fossil beach sands and contain other heavy minerals of commercial interest such as zircon and ilmenite. Monazite can be isolated as a nearly pure concentrate by the use of gravity, magnetic, and electrostatic separation.

Monazite sand deposits are prevalently of the monazite-(Ce) composition. Typically, the lanthanides in such monazites contain about 45–48% cerium, about 24% lanthanum, about 17% neodymium, about 5% praseodymium, and minor quantities of samarium, gadolinium, and yttrium. Europium concentrations tend to be low, about 0.05%. South African "rock" monazite, from Steenkampskraal, was processed in the 1950s and early 1960s by the Lindsay Chemical Division of American Potash and Chemical Corporation, at the time the largest producer of lanthanides in the world. Steenkampskraal monazite provided a supply of the complete set of lanthanides. Very low concentrations of the heaviest lanthanides in monazite justified the term "rare" earth for these elements, with prices to match. Thorium content of monazite is variable and sometimes can be up to 20–30%. Monazite from certain carbonatites or from Bolivian tin ore veins is essentially thorium-free. However, commercial monazite sands typically contain between 6 and 12% thorium oxide.

Acid cracking
The original process for "cracking" monazite so as to extract the thorium and lanthanide content was to heat it with concentrated sulfuric acid to temperatures between 120 and 150 °C for several hours. Variations in the ratio of acid to ore, the extent of heating, and the extent to which water was added afterwards led to several different processes to separate thorium from the lanthanides. One of the processes caused the thorium to precipitate out as a phosphate or pyrophosphate in crude form, leaving a solution of lanthanide sulfates, from which the lanthanides could be easily precipitated as a double sodium sulfate. The acid methods led to the generation of considerable acid waste, and loss of the phosphate content of the ore.

Alkaline cracking
A more recent process uses hot sodium hydroxide solution (73%) at about 140 °C. This process allows the valuable phosphate content of the ore to be recovered as crystalline trisodium phosphate. The lanthanide/thorium hydroxide mixture can be treated with hydrochloric acid to provide a solution of lanthanide chlorides, and an insoluble sludge of the less-basic thorium hydroxide.

Extraction of rare-earth metals from monazite ore

The extraction of rare-earth metals from monazite ore begins with digestion with sulfuric acid followed by aqueous extraction. The process requires many neutralizations and filtrations.

The final products yielded for this process are thorium-phosphate concentrate, RE hydroxides, and uranium concentrate. Depending on the relative market prices of uranium, thorium, and rare earth elements as well as the availability of customers and the logistics of delivering to them, some or all of those products may be economical to sell or further process into a marketable form, while others constitute tailings for disposal. Products of the uranium and thorium decay series, particularly radium will be present in trace amounts and form a radiotoxic hazard. While radium-228 (a product of thorium decay) will be present only in extremely minute amounts (less than one milligram per metric ton of thorium), and will decay away with a half life of roughly 5.75 years, radium-226 will be present at a ratio above 300 milligrams per metric ton of uranium and due to its long half life (~1600 years) will essentially remain with the residue. As radium forms the least soluble alkaline earth metal sulfate known, radium sulfate will be present among the solid filtration products after sulfuric acid has been added.

Containing nuclear waste
In two studies, one testing synthetic monazites radioactive waste storage capabilities by submerging it in a contaminated wastewater system for an extended period of time, and the other comparing the durability of the crystalline structures of multiple minerals, they investigate the ability of monazite to act as a host for nuclear byproducts from high-grade plutonium in decommissioned nuclear weapons and spent fuel from nuclear reactors. Results from both investigations show that monazite is one of the better options for storage in comparison to the previously used borosilicate glass.

One study done at Oak Ridge National Laboratory in Tennessee the performance of synthetic monazite to borosilicate glass in radioactive waste management is compared. This experiment involved synthetic monazite and borosilicate glass being soaked in a contaminated simulated Savannah River defense wastes for 28 days, during the time period the leaching rates from both materials were measured. The results show that the synthetic monazite is a far more effective material for containing radioactive waste due to its low leaching rates and slow corrosion rate.

In a second study natural monazite is found to have an enhanced ability to deal with radiation byproducts due the property of radiation "resistance" as it is able to remain crystalline after being subjected to high amounts of alpha-decay radiation and becoming amorphized. Due to this high durability, it is seen as a better alternative for hosting materials such as radioactive strontium than other tested minerals. Synthetic monazite is also shown to have similar durability to that of the natural crystalline samples after it becomes fully amorphized.

title: Monazite