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Dov Ber of Mezeritch

Dov Ber ben Avraham of Mezeritch (דֹב בּער פֿון מעזעריטש; died December 4, 1772 O.S.), also known as the Maggid of Mezeritch or Mezeritcher Maggid, was a disciple of Rabbi Israel ben Eliezer (the Baal Shem Tov), the founder of Hasidic Judaism, and was chosen as his successor to lead the early movement. Dov Ber is regarded as the first systematic exponent of the mystical philosophy underlying the teachings of the Baal Shem Tov, and through his teaching and leadership, the main architect of the movement. He established his base in Mezhirichi (in Volhynia), which moved the centre of Hasidism from Medzhybizh (in Podolia), where he focused his attention on raising a close circle of disciples to spread the movement. After his death the third generation of leadership took their different interpretations and disseminated across appointed regions of Eastern Europe, rapidly spreading Hasidism beyond Ukraine, to Poland, Galicia and Russia.

His teachings appear in Magid Devarav L'Yaakov, Or Torah, Likutim Yekarim, Or Ha'emet, Kitvei Kodesh, Shemuah Tovah, and in the works authored by his disciples. His inner circle of disciples, known as the Chevraia Kadisha ("Holy Brotherhood"), included Rabbis Avraham HaMalach (his son), Nachum of Czernobyl, Elimelech of Lizhensk, Zusha of Hanipol, Levi Yitzchok of Berditchev, Boruch of Medzhybizh, Aharon (HaGadol) of Karlin, Chaim Chaykl of Amdur, Menachem Mendel of Vitebsk, Shmuel Shmelke of Nikolsburg, Shlomo Flam (the Lutzker Maggid), Asher Zebi of Ostrowo, Zev Wolf of Zhitomyr, and Shneur Zalman of Liadi.

Name
The most common transliterations are Dov-Ber, Dov Baer and DovBer; rarely used forms are Dob Ber or Dobh Ber, which often depend on the region in Eastern Europe where Jews resided and hence the influence of the local Yiddish dialects. The name דוב-בער Dov-Ber is traceable back to the דב, and בער. It is thus an example of a bilingual tautological name.

Dov Ber of Mezeritch was known as the Maggid—"Preacher" or literally "Sayer," one who preaches and admonishes to go in God's ways—of Mezritsh (the German form Meseritz is sometimes used instead of Mezeritch). Towards the end of his life he was also known as the Maggid of Rivne, the town where he was buried.

Biography
Dov Ber was born in Lokachi, Volhynia in 1710, according to the Jewish Encyclopedia, though his year of birth is unknown and some sources place it around 1700. Little is known about him before he became a disciple of the Baal Shem Tov. A Hasidic legend states that, when he was five years old, his family home burst into flames. On hearing his mother weeping, he asked: "Mother, do we have to be so unhappy because we have lost a house?" She replied that she was mourning the family tree, which was destroyed, and is traced to King David by way of Rabbi Yohanan, the sandal-maker and master in the Talmud. The boy replied: "And what does that matter! I shall get you a new family tree which begins with me!"

When he was young, he reportedly lived in great poverty with his wife. One legend relates that when a child was born, they had no money to pay the midwife. His wife complained and the Maggid went outside to "curse" Israel. He went outside and said: "O children of Israel, may abundant blessings come upon you!" When his wife complained a second time, he went outside again and cried: "Let all happiness come to the children of Israel—but they shall give their money to thorn bushes and stones!" The baby was too weak to cry, and the Maggid sighed rather than "cursing". Immediately the answer came, and a voice said: "You have lost your share in the coming world." The Maggid replied: "Well, then, the reward has been done away with. Now I can begin to serve in good earnest."

Death
He is buried in Hannopil, beside Zusha of Hanipol.

Visit to Baal Shem Tov
Dov Ber was originally a student of the Pnei Yehoshua later became an admirer of Isaac Luria's system of Kabbalah, which was becoming popular at that time, and was aware of Moshe Chaim Luzzatto, whose writings, then only in manuscript, were well known among the Polish mystics of the period. Dov Ber followed the Lurian school, living the life of an ascetic, fasting a great deal, praying intensely, and living in poverty. He is reported to have become a cripple as a result of poor nourishment.

One account has it that on account of his poor health he was persuaded to seek out the Baal Shem Tov for a cure. He arrived at the Baal Shem Tov's house, expecting to hear expositions of profound mysteries, but instead was told stories of the latter's everyday life. Hearing only similar stories at each subsequent visit, Dov Ber decided to return home. Just as he was about to leave, he was summoned again to the Baal Shem Tov's house. The Baal Shem Tov opened an "Eitz Chaim" of Rabbi Chaim Vital (Rabbi Isaac Luria's chief disciple), and asked him to explain a certain passage. Dov Ber did so to the best of his ability, but the Baal Shem Tov declared that he had not understood the real meaning of the passage. Dov Ber reviewed it once more and insisted that his interpretation was correct. The Baal Shem Tov then proceeded to explain the passage himself. Legend has it that, as he spoke, the darkness suddenly gave way to light, and angels appeared and listened to the Baal Shem Tov's words. "Your explanations," he said to Dov Ber, "were correct, but your deductions were thoughts without any soul in them." This experience persuaded Dov Ber to stay with the Baal Shem Tov.

Dov Ber is reported to have learned from the Baal Shem Tov to value everyday things and events, and to emphasize the proper attitude with which to study Torah. The mystical philosophy of the Baal Shem Tov rejected the emphasis on mortification of the body in Musar and Kabbalistic traditions, seeing the greater spiritual advantage in transforming the material into a vehicle for holiness, rather than breaking it. This could be achieved by the perception of the omnipresent Divine immanence in all things, from understanding the inner mystical Torah teachings of Hasidic thought. Under the guidance of the Baal Shem Tov, Dov Ber abandoned his ascetic lifestyle, and recovered his health, though his left foot remained lame. The Baal Shem Tov said that "before Dov Ber came to me, he was already a pure golden menorah (candelebrum). All I needed to do was ignite it." Regarding his holiness, the Baal Shem Tov also reputedly said that if Dov Ber had not been lame, and had been able to ritually immerse in the mikvah, then he could have been able to bring the Mashiach.

Hasidic leadership
"Suddenly, Reb Tzvi got up and said, “My father appeared and told me that the Shechina now dwells in Mezritch.” Then Reb Tzvi removed the white cloak of leadership and put it onto the shoulders of Reb Dov Ber of Mezritch. And so, Reb Dov Ber, who later became known as the Mezritcher Maggid, assumed leadership of the Chassidic movement. And so it was"

Immediately after the death of the Baal Shem Tov in 1760, his son Rabbi Tsvi became the next Rebbe. After only a year he gave up this position. Among the disciples of the Baal Shem Tov, two stood out as contenders to succeed him, Dov Ber and Yacov Yoseph of Polonne. Yacov Yoseph would later become the author of the first Hasidic book published ("Toldos Yaacov Yosef" in 1780), one of the most direct records of the teachings of the Baal Shem Tov. By collective consent, the Maggid assumed the leadership of Hasidism. In effect he became the architect of the Hasidic movement and is responsible for its successful dissemination.

The Maggid was housebound because of his poor physical condition. Jewish philosopher Solomon Maimon records an encounter with the Maggid in his memoirs, in which he passes a strong negative judgement on the Hasidic movement. He relates that the Maggid passed the entire week in his room, permitting only a few confidants to enter. He appeared in public only on Shabbat, dressed in white satin. On those occasions he prayed with people, and kept open house for anyone who wanted to dine with him. After the meal he would reportedly begin to chant, and placing his hand upon his forehead, would ask those present to quote any verse from the Bible. These served as texts for the Maggid's subsequent sermon. Solomon Maimon wrote: "He was such a master in his craft that he combined these disjointed verses into an harmonious whole."

He attracted a remarkable group of scholarly and saintly disciples, including most of his fellow students of the Baal Shem Tov. The Baal Shem Tov had travelled across Jewish areas, reaching out to and inspiring the common folk, whose sincerity he cherished. He sought to revive the broken spirit of the simple Jews. At the same time, he would also seek out the great scholars of Talmud and Kabbalah, to win them over to Hasidism, to whom he taught the inner meaning of his teachings. Many Hasidic tales relate the stories of the Baal Shem Tov's travels, accompanied by his close disciples, and led by his non-Jewish wagon driver. Dov Ber, in contrast, set up his court in Mezhirichi, where his lameness restricted him, and devoted his main focus to articulating the mystical-philosophical system within the Baal Shem Tov's teachings to his close circle of disciples, who would lead the future movement. The simple folk were also able to visit during the Sabbath public attendancies of Dov Ber, and receive spiritual encouragement and comfort. The Maggid's court became the spiritual seat and place of pilgrimage of the second generation of the Hasidic movement, and moved its centre north from the Baal Shem Tov's residence in Medzhybizh. This move benefited the growth of the movement, as it was closer to new territories in Galicia, Poland and Belarus to reach. It was also nearer to the centre of Rabbinic opposition in Lithuania, who perceived of the new movement as a spiritual threat. The disciples of Dov Ber related that:

With the move of Rabbi Dov Ber, the Shechina (Divine Presence) "Packed up Her belongings and moved from Medzhybizh to Mezeritch, and all we can do is follow"

The elite group of disciples, the "Chevraya Kaddisha" ("Holy Society"), included Rabbis Aharon of Karlin, Menachem Mendel of Vitebsk, Levi Yitzchok of Berditchev, Elimelech of Lizhensk, Zusha of Hanipol, Shmelka (later Chief Rabbi of Nikolsburg), Pinchas Horowitz (later Chief Rabbi of Frankfurt-am-Main and author of profound Talmudic commentaries), and Shneur Zalman of Liadi (author of the Tanya, and by instructions of his master, author of an updated version of the Shulchan Aruch code of Jewish Law for the new movement). These disciples, being themselves great Talmudic authorities and well-versed in Kabbalah and Hasidic philosophy, were successful in turning Hasidus into a vast movement.

Opposition of the rabbis
Hasidism spread rapidly as a result of Dov Ber's powerful personality, gaining footholds in Volhynia, Lithuania, and Ukraine. The dissolution of the "Four-Lands" synod in 1764 proved favorable to its spread. The local rabbis were annoyed by the growth of the movement, but could not easily do anything about it. The Gaon of Vilna was the only rabbi whose reputation extended beyond the borders of Lithuania. When Hasidism appeared in Vilna, the Vilna Gaon enacted the first major excommunication against Hasidism, which was issued on April 11, 1772. The Vilna Gaon believed the movement was antagonistic to Talmudic rabbinism and was suspicious that it was a remnant of the recent Sabbatean movement. See Hasidim and Mitnagdim.

The Maggid's pupils Rabbi Menachem Mendel of Vitebsk and Rabbi Shneur Zalman of Liadi tried to visit the Vilna Gaon to bring about reconciliation, but the Vilna Gaon declined to meet them. Lubavitch legend has it that had the Gaon met with these two Rabbis, the Mashiach (Messiah) would have come.

The ban issued at Vilna drew the eyes of the world toward Hasidism. Rabbi Dov Ber ignored the opposition, but it is blamed in part for his death in Mezhirichi on December 15, 1772.

Published writings
The Maggid left no writings of his own. Many of his teachings were recorded by his disciples and appeared in anthologies  "MaggiD DebaraV le-Ya'akoV" (מגיד דבריו ליעקב the last letters of which title spell "Dov"), known also under the title of Likkutei Amarim ("Collected Sayings"), published at Korets in 1780 (second edition with additions Korets, 1784), and frequently reprinted; Likkutim Yekarim ("Precious Collections"), published at Lemberg in 1792; Or Torah (the largest collection) published in Korets, 1804; Or Ha'emet published in Husiatin, 1899; Kitvei Kodesh (small collection) published in Lemberg. 1862; Shemu'ah Tovah (small collection) published in Warsaw. 1938. A number of manuscripts with additional teachings are in the National Library of the Hebrew University. They consist of excerpts from his sermons, transcribed and compiled by his students. The first to be published (Likkutei Amarim) was collated by his relative, Rabbi Shlomo Flam of Lutzk, who, as he himself notes, was unhappy with the manuscript but did not have time to edit it properly.

There is a great deal of overlapping between all these texts, but each contains teachings that do not appear in the others. Work and editing has been done on them: Maggid Devarav Layaakov was edited by Rabbi Avraham Yitzchak Kohn (Jerusalem, 1961). Later, a critical edition was edited by Prof. Rivkah Shatz-Uffenheimer (Hebrew University, Jerusalem, 1976). Kehot Publishing of Chabad put out another edition edited by Rabbi Jacob Immanuel Schochet (Brooklyn, NY, 2008). These editions all contain introductions, annotations and indices. Or Torah has appeared in an annotated edition with introduction, commentaries, cross-references and indices, authored by Schochet (Brooklyn, NY, 2006). Likkutim Yekarim is in annotated edition by Rabbi Avraham Yitzchak Kohn (Jerusalem, 1974).

View of God
For the Maggid, God manifests Himself in creation, which is only one aspect of His activity, and which is therefore in reality a self-limitation. Just as God in His goodness limited Himself, and thus descended to the level of the world and man, so it is the duty of the latter to strive to unite with God. The removal of the outer shell of mundane things, or "the ascension of the [divine] spark," being a recognition of the presence of God in all earthly things, it is the duty of man, should he experience pleasure, to receive it as a divine manifestation, for God is the source of all pleasure.

On the ecstasy of prayer
Rabbi Dov Ber's view of prayer was that it is the purpose of the life on earth to advance until the perfect union with God is attained. Thus the vegetable kingdom serves as food for the animal kingdom, in order that the lower manifestation of divinity, existing in the former, may be developed into a higher one. Man being the highest manifestation has a duty to attain the highest pinnacle in order to be united with God. The way to achieve this, he argued, is through prayer, in which man forgets himself and his surroundings, and concentrates all his thought and feeling upon union with God.

Like the Neo-Platonists, he said that when a man becomes so absorbed in the contemplation of an object that his whole power of thought is concentrated upon one point, his self becomes unified with that point. So prayer in such a state of real ecstasy, effecting a union between God and man, is extremely important, and may even be able to overcome the laws of nature.

Role of the tzadik
Rabbi Dov Ber taught that only the tzadik is able to remove all his thoughts from earthly things and concentrate completely on God. Because of his union with God, he is the connecting link between God and creation, and thus the channel of blessing and mercy. The love that men have for the tzadik provides a path to God. The duty of the ordinary mortal is therefore to love the tzadik and be a student of his. In this connection Hasidim cite the classical Jewish teaching that Scripture considers one who serves Torah scholars to be cleaving to the Almighty Himself.

title: Dov Ber of Mezeritch


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UUCP

UUCP (Unix-to-Unix Copy) is a suite of computer programs and protocols allowing remote execution of commands and transfer of files, email and netnews between computers.

A command named uucp is one of the programs in the suite; it provides a user interface for requesting file copy operations. The UUCP suite also includes uux (user interface for remote command execution), uucico (the communication program that performs the file transfers), uustat (reports statistics on recent activity), uuxqt (execute commands sent from remote machines), and uuname (reports the UUCP name of the local system). Some versions of the suite include uuencode/uudecode (convert 8-bit binary files to 7-bit text format and vice versa).

Although UUCP was originally developed on Unix in the 1970s and 1980s, and is most closely associated with Unix-like systems, UUCP implementations exist for several non-Unix-like operating systems, including DOS, OS/2, OpenVMS (for VAX hardware only), AmigaOS, classic Mac OS, and even CP/M.

History
UUCP was originally written at AT&T Bell Laboratories by Mike Lesk. By 1978 it was in use on 82 UNIX machines inside the Bell system, primarily for software distribution. It was released in 1979 as part of Version 7 Unix.

The first UUCP emails from the U.S. arrived in the United Kingdom in 1979 and email between the UK, the Netherlands and Denmark started in 1980, becoming a regular service via EUnet in 1982.

The original UUCP was rewritten by AT&T researchers Peter Honeyman, David A. Nowitz, and Brian E. Redman around 1983. The rewrite is referred to as HDB or HoneyDanBer uucp, which was later enhanced, bug fixed, and repackaged as BNU UUCP ("Basic Network Utilities").

Each of these versions was distributed as proprietary software, which inspired Ian Lance Taylor to write a new free software version from scratch in 1991. Taylor UUCP was released under the GNU General Public License. Taylor UUCP addressed security holes which allowed some of the original network worms to remotely execute unexpected shell commands. Taylor UUCP also incorporated features of all previous versions of UUCP, allowing it to communicate with any other version and even use similar config file formats from other versions.

UUCP was also implemented for non-UNIX operating systems, most-notably DOS systems. Packages such as UUSLAVE/GNUUCP (John Gilmore, Garry Paxinos, Tim Pozar), UUPC/extended (Drew Derbyshire of Kendra Electronic Wonderworks) and FSUUCP (Christopher Ambler of IODesign), brought early Internet connectivity to personal computers, expanding the network beyond the interconnected university systems. FSUUCP formed the basis for many bulletin board system (BBS) packages such as Galacticomm's Major BBS and Mustang Software's Wildcat! BBS to connect to the UUCP network and exchange email and Usenet traffic. As an example, UFGATE (John Galvin, Garry Paxinos, Tim Pozar) was a package that provided a gateway between networks running Fidonet and UUCP protocols.

FSUUCP was the only other implementation of Taylor's enhanced 'i' protocol, a significant improvement over the standard 'g' protocol used by most UUCP implementations.

Technology
Before the widespread availability of Internet access, computers were only connected by smaller local area networks within a company or organization. They were also often equipped with modems so they could be used remotely from character-mode terminals via dial-up telephone lines. UUCP used the computers' modems to dial out to other computers, establishing temporary, point-to-point links between them. Each system in a UUCP network has a list of neighbor systems, with phone numbers, login names and passwords, etc. When work (file transfer or command execution requests) is queued for a neighbor system, the uucico program typically calls that system to process the work. The uucico program can also poll its neighbors periodically to check for work queued on their side; this permits neighbors without dial-out capability to participate.

Over time, dial-up links were replaced by Internet connections, and UUCP added a number of new link layer protocols. These newer connections also reduced the need for UUCP at all, as newer application protocols developed to take advantage of the new networks. Today, UUCP is rarely used over dial-up links, but is occasionally used over TCP/IP. The number of systems involved, as of early 2006, ran between 1500 and 2000 sites across 60 enterprises. UUCP's longevity can be attributed to its low cost, extensive logging, native failover to dialup, and persistent queue management.

Sessions
UUCP is normally started by having a user log into the target system and then running the UUCP program. In most cases, this is automated by logging into a known user account used for transfers, whose account's shell has been set to uucico. Thus, for automated transfers, another machine simply has to open a modem connection to the called machine and log into the known account.

When uucico runs, it will expect to receive commands from another UUCP program on the caller's machine and begin a session. The session has three distinct stages:

 * 1) Initial handshake
 * 2) File request(s)
 * 3) Final handshake

Initial handshake
On starting, uucico will respond by sending an identification string,, where \20 is the control-P character, and \0 is a trailing null. The caller's UUCP responds with, where options is a string containing zero or more Unix-like option switches. These can include packet and window sizes, the maximum supported file size, debugging options, and others.

Depending on the setup of the two systems, the call may end here. For instance, when the caller responds with their system name, the called system may optionally hang up if it does not recognize the caller, sending the  response string and then disconnecting.

File requests
If the two systems successfully handshake, the caller will now begin to send a series of file requests. There are four types:

 * S causes a file to be Sent from the caller to the called system (upload). The from and to names are provided, allowing the filename to be changed on the receiver. When the S command is received on the called system, it responds with SY if it succeeded and it is ready to accept the file, or SNx if it failed, where x is a failure reason. If an SY is received by the caller, it begins uploading the file using the protocol selected during the initial handshake (see below). When the transfer is complete, the called system responds with CY if it successfully received the file, or CN5 if it failed.
 * R is a Request for the called system to send a file to the caller (download). It is otherwise similar to S, using RY and RN to indicate the command was accepted and it will begin to send data or had a problem, and expecting a CY and CN5 from the caller at the end of the transfer.
 * X uploads commands to be eXecuted on the called system. This can be used to make that system call another and deliver files to it. The called system responds with XY if it succeeded, or XN if it failed.
 * H, for Hangup, indicates the caller is done. The called system responds with HY if it succeeded, or HN if it failed.

Final handshake
After sending an H command, the calling system sends a final packet \20OOOOOO\0 (control-P, six ohs, null-terminator) and the called system responds with \20OOOOOO\0 (control-P, seven ohs, null-terminator). Some systems will simply hang up on the successful reception of the H command and not bother with the final handshake.

g-protocol
Within the suite of protocols in UUCP, the underlying g-protocol is responsible for transferring information in an error-free form. The protocol originated as a general-purpose system for packet delivery, and thus offers a number of features that are not used by the UUCP package as a whole. These include a secondary channel that can send command data interspersed with a file transfer, and the ability to renegotiate the packet and window sizes during transmission. These extra features may not be available in some implementations of the UUCP stack.

The packet format consisted of a 6-byte header and then between zero and 4096 bytes in the payload. The packet starts with a single \020 (control-P). This is followed by a single byte, known as "K", containing a value of 1 to 8 indicating a packet size from 32 to 4096 bytes, or a 9 indicating a control packet. Many systems only supported K=2, meaning 64 bytes. The next two bytes were a 16-bit checksum of the payload, not including the header. The next byte is the data type and finally, the last byte is the XOR of the header, allowing it to be checked separately from the payload.

The control byte consists of three bit-fields in the format TTXXXYYY. TT is the packet type, 0 for control packets (which also requires K=9 to be valid), 1 for alternate data (not used in UUCP), 2 for data, and 3 indicates a short packet that re-defines the meaning of K. In a data packet, XXX is the packet number for this packet from 0 to 7, and YYY is the last that was received correctly. This provides up to 8 packets in a window. In a control packet, XXX indicates the command and YYY is used for various parameters. For instance, transfers are started by sending a short control packet with TT=0 (control), XXX=7 and YYY the number of packets in a window, then sending another packet with XXX=6 and YYY as the packet length (encoded as it would be in K) and then a third packet that is identical to the first but XXX=5.

g-protocol uses a simple sliding window system to deal with potentially long latencies between endpoints. The protocol allows packets to size from 64 to 4096 8-bit bytes, and windows that include 1 to 7 packets. In theory, a system using 4k packets and 7 packet windows (4096x7) would offer performance matching or beating the best file-transfer protocols like ZMODEM. In practice, many implementations only supported a single setting of 64x3. As a result, the g-protocol has an undeserved reputation for poor performance. Confusion over the packet and window sizes led to the G-protocol, differing only in that it always used 4096x3. Taylor UUCP did not support G, but did support any valid requested window or packet size, so remote systems starting G would work fine with Taylor's g, while two Taylor systems could negotiate even faster connections.

Telebit modems used protocol spoofing to improve the performance of g-protocol transfers by noticing end-of-packet markers being sent to the remote system and immediately sending an ACK back to the local host, pretending that the remote system had already received the packet and decoded it correctly. This triggered the software stack to send the next packet, so rapidly that the transfer became almost continuous. The data between the two modems was error-corrected using a proprietary protocol based on MNP that ran over Telebit's half-duplex connections much better than g-protocol would normally, because in the common 64x3 case the remote system would be sending a constant stream of ACKs that would overflow the low-speed return channel. Combined with the modem's naturally higher data rates, they greatly improved overall throughput and generally performed about seven times the speed of a 2400 bit/s modem. They were widely used on UUCP hosts as they could quickly pay for themselves in reduced long-distance charges.

Other protocols
UUCP implementations also include other transfer protocols for use over certain links.

f-protocol is designed to run over 7-bit error-corrected links. This was originally intended for use on X.25 links, which were popular for a time in the 1980s. It does not packetize data, instead, the entire file is sent as a single long string followed by a whole-file checksum. The similar x-protocol appears to have seen little or no use. d-protocol was similar to x, but intended for use on Datakit networks that connected many of Bell Labs offices.

t-protocol originated in the BSD versions of UUCP and like some similar ones, is designed to run over 8-bit error-free TCP/IP links. It has no error correction at all, and the protocol consists simply of breaking up command and file data into 512 or 1024-byte packets to easily fit within typical TCP frames.

e-protocol ("e" for Ethernet) was developed by Clem Cole at MASSCOMP and was widely released by Brian Redman in the later HoneyDanBer versions. It was developed and released before the t-protocol, but the t-protocol was more commonly used because the BSD version of UUCP was the dominant implementation. The e-protocol differs from the t-protocol only in that commands are not packetized and are instead sent as normal strings, while files are padded to the nearest 20 bytes.

Mail routing
The uucp and uuxqt capabilities could be used to send email between machines, with suitable mail user interfaces and delivery agent programs. A simple UUCP mail address was formed from the adjacent machine name, an exclamation mark (often pronounced bang), followed by the user name on the adjacent machine. For example, the address barbox!user would refer to user user on adjacent machine barbox.

Mail could furthermore be routed through the network, traversing any number of intermediate nodes before arriving at its destination. Initially, this had to be done by specifying the complete path, with a list of intermediate host names separated by bangs. For example, if machine barbox is not connected to the local machine, but it is known that barbox is connected to machine foovax which does communicate with the local machine, the appropriate address to send mail to would be foovax!barbox!user.

User barbox!user would generally publish their UUCP email address in a form such as …!bigsite!foovax!barbox!user. This directs people to route their mail to machine bigsite (presumably a well-known and well-connected machine accessible to everybody) and from there through the machine foovax to the account of user user on barbox. Publishing a full path would be pointless, because it would be different, depending on where the sender was. (e.g. Ann at one site may have to send via path gway!tcol!canty!uoh!bigsite!foovax!barbox!user, whereas from somewhere else, Bill has to send via the path pdp10!router22!bigsite!foovax!barbox!user). Many users would suggest multiple routes from various large well-known sites, providing even better and perhaps faster connection service from the mail sender.

Bang path
An email address of this form was known as a bang path. Bang paths of eight to ten machines (or hops) were not uncommon in 1981, and late-night dial-up UUCP links could cause week-long transmission times. Bang paths were often selected by both transmission time and reliability, as messages would often get lost. Some hosts went so far as to try to "rewrite" the path, sending mail via "faster" routes—this practice tended to be frowned upon.

The "pseudo-domain" ending .uucp was sometimes used to designate a hostname as being reachable by UUCP networking, although this was never formally registered in the domain name system (DNS) as a top-level domain. The uucp community administered itself and did not mesh well with the administration methods and regulations governing the DNS; .uucp works where it needs to; some hosts punt mail out of SMTP queue into uucp queues on gateway machines if a .uucp address is recognized on an incoming SMTP connection.

Usenet traffic was originally transmitted over the UUCP protocol using bang paths. These are still in use within Usenet message format Path header lines. They now have only an informational purpose, and are not used for routing, although they can be used to ensure that loops do not occur.

In general, like other older e-mail address formats, bang paths have now been superseded by the "@ notation", even by sites still using UUCP. A UUCP-only site can register a DNS domain name, and have the DNS server that handles that domain provide MX records that cause Internet mail to that site to be delivered to a UUCP host on the Internet that can then deliver the mail to the UUCP site.

UUCPNET and mapping
UUCPNET was the name for the totality of the network of computers connected through UUCP. This network was very informal, maintained in a spirit of mutual cooperation between systems owned by thousands of private companies, universities, and so on. Often, particularly in the private sector, UUCP links were established without official approval from the companies' upper management. The UUCP network was constantly changing as new systems and dial-up links were added, others were removed, etc.

The UUCP Mapping Project was a volunteer, largely successful effort to build a map of the connections between machines that were open mail relays and establish a managed namespace. Each system administrator would submit, by e-mail, a list of the systems to which theirs would connect, along with a ranking for each such connection. These submitted map entries were processed by an automatic program that combined them into a single set of files describing all connections in the network. These files were then published monthly in a newsgroup dedicated to this purpose. The UUCP map files could then be used by software such as "pathalias" to compute the best route path from one machine to another for mail, and to supply this route automatically. The UUCP maps also listed contact information for the sites, and so gave sites seeking to join UUCPNET an easy way to find prospective neighbors.

Connections with the Internet
Many UUCP hosts, particularly those at universities, were also connected to the Internet in its early years, and e-mail gateways between Internet SMTP-based mail and UUCP mail were developed. A user at a system with UUCP connections could thereby exchange mail with Internet users, and the Internet links could be used to bypass large portions of the slow UUCP network. A "UUCP zone" was defined within the Internet domain namespace to facilitate these interfaces.

With this infrastructure in place, UUCP's strength was that it permitted a site to gain Internet e-mail and Usenet connectivity with only a dial-up modem link to another cooperating computer. This was at a time when true Internet access required a leased data line providing a connection to an Internet Point of Presence, both of which were expensive and difficult to arrange. By contrast, a link to the UUCP network could usually be established with a few phone calls to the administrators of prospective neighbor systems. Neighbor systems were often close enough to avoid all but the most basic charges for telephone calls.

Remote commands
uux is remote command execution over UUCP. The uux command is used to execute a command on a remote system, or to execute a command on the local system using files from remote systems. The command is run by the uucico daemon, which handles remote execution requests as simply another kind of file to batch-send to the remote system whenever a next-hop node is available. The remote system will then execute the requested command and return the result, when the original system is available. Both of these transfers may be indirect, via multi-hop paths, with arbitrary windows of availability. Even when executing a command on an always-available neighbor, uux is not instant.

Decline
UUCP usage began to die out with the rise of Internet service providers offering inexpensive SLIP and PPP services. The UUCP Mapping Project was formally shut down in late 2000.

The UUCP protocol has now mostly been replaced by the Internet TCP/IP based protocols SMTP for mail and NNTP for Usenet news.

In July 2012, Dutch Internet provider XS4ALL closed down its UUCP service, claiming it was "probably one of the last providers in the world that still offered it"; it had only 13 users at that time (prior to its shut-down it had refused requests from new users for several years).

Current uses and legacy
One surviving feature of UUCP is the chat file format, largely inherited by the Expect software package.

UUCP was in use over special-purpose high cost links (e.g. marine satellite links) long after its disappearance elsewhere, and still remains in legacy use. In addition to legacy use, in 2021 new and innovative UUCP uses are growing, especially for telecommunications in the HF band, for example, for communities in the Amazon rainforest for email exchange and other uses. A patch to Ian's UUCP was contributed to UUCP Debian Linux package to adapt for the HERMES (High-Frequency Emergency and Rural Multimedia Exchange System) project, which provides UUCP HF connectivity.

In the mid 2000s, UUCP over TCP/IP (often encrypted, using the SSH protocol ) was proposed for use when a computer does not have any fixed IP addresses but is still willing to run a standard mail transfer agent (MTA) like Sendmail or Postfix.

Bang-like paths are still in use within the Usenet network, though not for routing; they are used to record, in the header of a message, the nodes through which that message has passed, rather than to direct where it will go next. "Bang path" is also used as an expression for any explicitly specified routing path between network hosts. That usage is not necessarily limited to UUCP, IP routing, email messaging, or Usenet.

The concept of delay-tolerant networking protocols was revisited in the early 2000s. Similar techniques as those used by UUCP can apply to other networks that experience delay or significant disruption.

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

title: UUCP


text:
Talk:Normal (geometry)

What does it all mean!?
"A surface normal holds the three dimensional direction a surface faces in a 3 component vector representation."

Can't this be stated a little more clearly? I don't understand what is meant.

S.

 * Probably. It could also benefit from a simple graphic.  I know what it means—I'll try to get around to it. —Frecklefoot 18:52, 28 Aug 2003 (UTC)

 * Okay, I edited the definition and added a graphic. I hope it is clearer now.  If not, mention it here and I'll try to clrify it even more. :-) —Frecklefoot 20:34, 28 Aug 2003 (UTC)

 * I took off the stub notice, it looks pretty well fleshed out now. (if someone more mathematical disagrees, they can put it back. --ObscureAuthor 20:37, 28 Aug 2003 (UTC)

Outward-pointing normal
I reverted recent changes to this article as they were not carefully written.

 * The picture with an inward pointing normal is not good looking, and a bit confusing.

 * The concepts of inward and outward pointing normals do not make sense unless one defines what is the "inside" and what is the "outside" of a surface, so I guess it does not make sense to all surfaces.

 * The wording "Normals do not penetrate the surface." does not make sense.

As such, the new changes had a point, but were poorly written to the point of being incorrect. Comments? Oleg Alexandrov (talk) 22:34, 20 May 2006 (UTC)

 * Theory says a smooth surface in 3D has a tangent plane at each point. Displacements between points in that plane are perpendicular to exactly two opposite directions. A normal vector is a non-zero vector pointing in one of the two directions; often, but not always, a normal vector has unit length. If a vector is an equivalence class of parallel, equal-length, and consistently directed displacements, which is how a mathematician might view the matter, then it makes no sense to speak of vectors penetrating anything. If a surface is not closed or not orientable (like a Möbius strip), then we cannot speak of inside or outside. We may draw pictures of vectors as physical arrows in space, but we should not confuse the picture with reality. --KSmrqT 01:16, 22 May 2006 (UTC)
 * But how does this relate to an ellipsoidal surface? For instance, look at Figure 1.4 on the top of pg.8, here (PDF):  Would that nearly horizontal ring connecting points A and B be the curvature of the "normal section"?  If it is, what would the "ring" that connects the two points, cutting through the center of the ellipsoid (i.e., a "great circle"), [[Image:Storcirkel.png|50px]], be the curvature of?:  The "central section" or "subtended section"?  Orthodromic section? ~Kaimbridge ~13:14, 22 May 2006 (UTC)

Surface Normal Outward Normal Left and Right hand rules
Hi KSmrq Re: Article Surface Normal

The word "outward" was edited out of the caption of the image with the advice to stay away from that adjective. However S. P. Timoshenko, recognized as the father of Engineering Elasticity, in his book Theory of Elasticity uses the symbol "N" to represent "outward normal to the surface of a body" The images in the book showing normals are exactly identical to the image in the article.

If an outward normal is to be recognized, shouldn't an inward normal be also recognized? The inward normal vector represents a pressure

If one of the two normals is determined by the Right-hand rule, isn't the other normal, in the opposite direction, uniquely determined by the Left-hand rule?

If you do not mind would you kindly respond Subhash 01:04, 17 June 2006 (UTC)

Retrieved from "http://en.wikipedia.org/wiki/User:Subhash15/Trial2"

 * I am happy to respond.
 * The title of the book refers to the "surface of a body", which a mathematician might paraphrase as the boundary of a solid in 3D. Such a solid has a well-defined inside and outside, so the terms "outward" and "inward" can have meaning for the surface. However, mathematicians deal with many surfaces that are not boundaries of solids, including some for which it is demonstrably impossible to distinguish or define "outward" and "inward". A mundane example is a triangle in space; which side is which? But the triangle still has two sides, which is not always so. For example, the famous Möbius strip, a cylindrical strip with a half twist, has only one side. An engineer would never encounter such a surface as the boundary of a solid, but mathematicians encounter them often. Elsewhere in engineering the mathematician's view is needed, so please don't be mislead by one special case.
 * I am responding here because I do not monitor the talk page of the surface normal article, but in future you should conduct such discussions where all interested parties can see, learn, and participate, on the article talk page. --KSmrqT 04:28, 17 June 2006 (UTC)
 * (the section up to here copied from User talk:KSmrq by Oleg Alexandrov (talk) at 07:58, 17 June 2006 (UTC))
 * I totally agree with KSmrq here. Oleg Alexandrov (talk) 07:58, 17 June 2006 (UTC)

Transformation
I just added a link to pseudovector to the article, since a surface normal, being a cross product, is a pseudovector. But I'm trying to figure out if it's more than that, I think it'll have to do with covariance and contravariance. If I have the vectors (1,0,-1) and (0,1,0) on a plane, the normal to that plane is obviously in the (1,0,1) direction. But if I stretch that plane by a factor of 2 in z, the tangent vectors become (1,0,-2) and (0,1,0). But the normal isn't in the (1,0,2) direction, it's in the (1,0,1/2) direction. What's up with that? Is it a co- or contravariant pseudovector or something like that? —Ben FrantzDale 20:50, 8 May 2007 (UTC)

 * To find a normal to a surface, I don't need to take the cross product of tangent vectors to the surface. That is, a surface normal needn't be a pseudovector.  Also, be careful about saying "the" normal when there isn't one (only "a" normal).

 * Dunno quite what your confusion is in your computations, but maybe considering this will help: the z-component of the tangents influences the x- and y-components of the normal (if you use the cross product). So when you stretch the z-components of the tangent by a factor of 2, you need to stretch the x- and y-components of the normal by a factor of 2.  In other words, you don't want (1,0,2); you want (2,0,1).  Lunch 21:57, 8 May 2007 (UTC)

 * Regarding scaling, I'm saying I think the normal may be a covariant vector (or is it contravariant?). It isn't just an arbitrary rule that you need to transform it strangely, it must be well-defined. As for it being a pseudovector, it must be in as much as it's the result of a cross product. But the more I think about it, it seems like a surface normal really shouldn't flip direction sign under any inversion—i.e., it should stay outward-pointing from the surface it's on even if you invert the surface in x. —Ben FrantzDale 23:13, 8 May 2007 (UTC)

 * With regards to whether a normal is covariant or contravariant, it seems to me to depend on your point of view; that is, you could do it either way. For instance, in three dimensions, you can write a cross product as a contraction of the alternating tensor with the two vectors of interest, but how you assign covariant and contravariant indices seems to be immaterial.  On the other hand, what do you mean by "transform it"?

 * With regards to sign changes, as I understand it, geometers aren't just concerned with linear transformations when thinking about problems of orientability. For a concrete example, consider the unit circle in the complex plane under the transformation $$z\rightarrow 1/z$$.  This swaps "inside" for "outside" but leaves the surface (the cirlce) alone.

 * Also consider how this generalizes to more dimensions. The choice of normal determines a connection (or vice versa); the choice of an ambient space affects this.  But maybe you're not intersted in geometry, but more in algebra.  Then there's exterior products and outer products.

 * I dunno if I'm the best person to help here, and I don't think I'm really understanding your confusion. It'd probably be best to talk to someone at your school and bang this out in person.  Lunch 22:14, 10 May 2007 (UTC)

 * Here's what I was thinking about:
 * Basically, if you stretch an object, by (2,1,1) for example, you would like the normal vector to stay perpendicular to vectors on the surface. That is, if you are transforming the surface vectors by M, you want the normal, n to become $$\mathbf{n}'$$ such that
 * $$ \mathbf{n}'\cdot M\mathbf{v} = 0$$ for all v on the surface.
 * As that article points out, we start with
 * $$\mathbf{n}\cdot\mathbf{x} = \mathbf{n}^T\mathbf{x} = 0$$
 * so
 * $$\mathbf{n}^T M^{-1} M \mathbf{x} = 0$$.
 * Thus
 * $$\mathbf{n}' = \mathbf{n}^T M^{-1} = (M^{-1})^T \mathbf{n}$$.
 * That is, the normal vector transforms by the inverse of the transpose of the transformation. I think this may be a covariant transformation, but I'm not sure. —Ben FrantzDale 23:27, 10 May 2007 (UTC)

 * That's a contravariant transformation. Ugh, covariant transformation is a mess; differential form is better.  Better still is to talk to one of your professors.  Lunch 00:55, 11 May 2007 (UTC)

 * Cool. Thanks. That sounds right. I'll try to incite some cleanup/merging among covariant transformation, differential form, and covariance and contravariance. —Ben FrantzDale 10:11, 11 May 2007 (UTC)

3D-centrism
Surface normals are defined (and used) for general surfaces in n-dimensional spaces. This article doesn't even mention the possibility of a normal in n-dimensional space. While the 3D examples here are intuitive and may be useful for most people arriving at this page, I think a more general exposition and a section on higher dimensions are necessary. Anyone? --Zvika 06:30, 29 August 2007 (UTC)
 * Well, you are talking about "hypersurface normal", or otherwise about "normal subspace to a surface". If you want a section about this, it should be further down the article, where it won't confuse people who don't know much beyond 3D euclidean geometry. Oleg Alexandrov (talk) 06:43, 29 August 2007 (UTC)

I am talking about a normal to a hypersurface (I think many would still call it a surface, even if they are talking about n-dimensional space). This was just a suggestion since I got to this page looking for a formula for an n-dimensional normal of a (hyper)surface. I am willing to accept that most of the stuff about this should be rather towards the end of the article, but I think a paragraph about this extension should appear in the lead too. --Zvika 06:50, 29 August 2007 (UTC)

I was bold and added a section on this, as well as removing the word "three-dimensional" from the lead. Feel free to modify. --Zvika 13:38, 29 August 2007 (UTC)

On the other hand, there is no text and no separate article dedicated to the 2-dimensional case, so made a brief mention of it.kostmo (talk) 02:58, 19 October 2008 (UTC)

Hi, do you know how to get the normal to a hypersurface given curvilinear coordinates? Something similar to $${\partial \mathbf{x} \over \partial s}\times {\partial \mathbf{x} \over \partial t}$$ for surfaces in 3-D. Thanks, --71.106.173.110 (talk) 07:17, 17 July 2009 (UTC)
 * I must do a little observation: the word "surface" is universally reserved for two-dimensional manifolds embedded in three-dimensional space, so I am going to change the section name "n-dimensional surface" to the correct and universally accepted "hypersurface". If someone has something to object, He is welcome. Daniele.tampieri (talk) 13:25, 3 September 2009 (UTC)

the mobius strip
It seems to have an 'outer' normal in both directions. But the parameterization is differentiable and the normal vector is a function (which mean it'll only give 1 normal at a point). If we chose a normal in 1 direction at a point A then run across the surface back to that point the normal points in the other direction (since the normal vector is continuous we do that right?), admiting two normals "in the same direction at the same point"... WHY? —Preceding unsigned comment added by Cod4 better than halo3 (talk • contribs) 07:56, 29 August 2008 (UTC)

 * As you point out, a Mobius strip isn't orientable. As I understand it, a Mobius strip can be thought of not embedded in R3 in which case a "surface normal" isn't meaningful. So in answer to your question of "WHY?": Because a Mobius strip has one side. If you wanted to represent one on a computer embedded in R3, you'd make it out of polygons each of which had a given surface normal, then you'd have a discontinuity somewhere around the loop. —Ben FrantzDale (talk) 21:03, 29 August 2008 (UTC)

Only convex polygons?
At the moment, the page says "For a convex polygon (such as a triangle), a surface normal can be calculated as the vector cross product of two (non-parallel) edges of the polygon."

 * I'm pretty sure that will also work for concave polygons ... —Preceding unsigned comment added by 157.193.173.205 (talk) 11:30, 7 December 2009 (UTC)
 * It will work up to sign. If you want an oriented surface normal (in which the sign matters), then obviously crossing consecutive edges of a concave pair of edges would give you the wrong sign. —Ben FrantzDale (talk) 13:25, 7 December 2009 (UTC)

Unnecessary complication
Not entirely sure, but why does one need to solve:
 * $$\left( A{{A}^{*}}+b{{b}^{*}} \right)n=b$$

I mean, I know it's originated from a constrained LS, but it seems to be an insane move to solve this system in order to obtain $$n$$.

The condition number of $$A{{A}^{*}}+b{{b}^{*}}$$ might be horrid, since there are no promises about $$A$$, and this is just the start of it.

Wouldn't it be much simpler to generate a vector $$b$$ which isn't in the span of $$A$$ and then just solve:
 * $$n^*\left[ A,b \right]=\left[ {{0}_{N-1}},1 \right]$$

Here there is no need to multiply matrices, generate unneeded matrices ($$bb^*$$) if one doesn't know how to bypass this, and most importantly the condition number should be low enough for randomly selected $$b$$. 79.182.0.184 (talk) 12:52, 29 March 2010 (UTC)

Edited, fixed typo. 79.182.0.184 (talk) 21:36, 29 March 2010 (UTC)

Partial Derivatives of Implicitly Defined Functions
"If a surface S is given implicitly as the set of points (x,y,z) satisfying F(x,y,z) = 0, then, a normal at a point (x,y,z) on the surface is given by the gradient

since the gradient at any point is perpendicular to the level set, and F(x,y,z) = 0 (the surface) is a level set of F."

I have not been able to find "how to find the gradient or partial derivatives of an implicitly defined function" anywhere in wikipdia. --   Here is a short example: "it is true that the gradient of Ax^2 + By^2 + Cz^2 + D = 0 is (2Ax, 2By, 2Cz)." - source: my instructor. -- this should be added to the articles for Gradient, Partial Derivatives, and maybe other articles as well. BriEnBest (talk) 00:10, 16 November 2010 (UTC)

Terminology is still confusing
When describing how planes can be calculated from normals, the text says "For a plane given by the equation r = a + αb + βc, where a is a vector to get onto the plane". I have NO IDEA what 'vector to get onto the plane' is supposed to mean, and I am not unfamiliar with the material. swestrup (talk) 20:31, 29 March 2011 (UTC)

 * You are right; it's not clear. What it is trying to do is parametrize a plane by defining all points, r, on that plane as a function of α and β. That is, the point r(0,0) is on the plane at position a. r(0,1) is on the plane at a+c, etc. That is, a is a vector from the origin to some point on the plane and c and d define directions on the surface. Does that make sense? —Ben FrantzDale (talk) 23:25, 29 March 2011 (UTC)

Generalization to manifolds
While it is IMO important to provide a generalization, is the generalization of the vector space orthogonal to the tangent space of the manifold valid usage, and not pushing the generalization too far? For example, do we speak of the normal surface to a curve in 3-space? I would simply call this the space orthogonal to the tangent space; the term "normal" is already not great because of its double-use to mean "normalized". The generalization I would expect would be a normal vector or line to an (n−1)-manifold in a Euclidean n-space. But then, I'm not a mathematician; input gladly accepted. — Quondum tc 18:48, 19 December 2011 (UTC)

 * The general notion of normal (vector) space is an important mathematical notion which deserves to be fully treated in Wikipedia, which is presently not. For example, in Differential geometry of curves, an orthogonal basis of the normal plane is defined which consists in the curvature vector and the binormal vector. Another example: as the normal space is the orthogonal space of the tangent space, the normal space plays a role in the theory of duality of manifolds and varieties. However I am unable to be more precise, being not a specialist of this theory and being unable to follow the Wikipedia pages on duality theory, which are written in a style which is much too abstract for me. D.Lazard (talk) 20:08, 19 December 2011 (UTC)

 * I am merely concerned about encroaching on the perfectly good term orthogonal, which does cover spaces orthogonal to linear vector subspaces. The only added concept is the idea of a tangent space. I guess part of my reticence stems from the other use of the term "normal" to mean "of norm 1", which occurs in the same contexts as "orthogonal". However,  the criterion is how the term gets used in the literature, not my preferences. I was only wanting to be sure that the term "normal" does get used rather than "orthogonal" in the more abstract context. — Quondum tc 21:42, 19 December 2011 (UTC)

Why is it called a Normal?
I'm an artist, who dabbles in the intersection of math and art. Normal maps are frequently called bump maps in game dev. Pretty obvious reason: because it encodes bumps, right?

The more correct and general term is "normal map" because that's what it's a map of, the normal of the surface. But WHY is it called a normal? What is the history?

I think it's important for wikipedia to include historical context for mathematical ideas. Mathematical terms are not pre-existing platonic Forms, they have historical and cultural context that helps ground a student's engagement with the abstract.

Does anyone know why Normal has become the standard terminology for this? What is the history for it? Who first coined the term, and why did they choose Normal?

This is especially important considering how Normal is used in many other aspects of math for different things. Having an understanding of the history of the idea helps distinguish those different uses of the term. Thedonquixotic (talk) 19:55, 24 March 2023 (UTC)

"Unit normal" listed at Redirects for discussion
The redirect [//en.wikipedia.org/w/index.php?title=Unit_normal&redirect=no Unit normal] has been listed at redirects for discussion to determine whether its use and function meets the redirect guidelines. Readers of this page are welcome to comment on this redirect at  until a consensus is reached. 1234qwer1234qwer4 12:37, 12 November 2023 (UTC)

Looking for a reference to one sentence
Do you have a reference to the claim: "Since a surface does not have a tangent plane at a singular point, it has no well-defined normal at that point."נצה מ.ה. (talk) 14:14, 27 December 2023 (UTC)

title: Talk:Normal (geometry)


text:
Arno Schmidt

Arno Schmidt (18 January 1914 – 3 June 1979) was a German author and translator. He is little known outside of German-speaking areas, in part because his works present a formidable challenge to translators. Although he is not one of the popular favourites within Germany, critics and writers often consider him to be one of the most important German-language writers of the 20th century.

Biography
Born in Hamburg, the son of a police constable, Schmidt moved in 1928, after the death of his father (1883–1928), with his mother (1894–1973), to her hometown of Lauban (in Lusatia, then Lower Silesia, now Poland) and attended secondary school in Görlitz as well as a trade school there. After finishing school, he was unemployed for some months and then, in 1934, began a commercial apprenticeship at a textile company in Greiffenberg. After finishing his apprenticeship he was hired by the same company as a stock accountant. Around this time, at his company, he met his future wife, Alice Murawski. The couple married on 21 August 1937; they had no children.

At the outset of World War II, in 1939, Schmidt was drafted into the Wehrmacht, where his mathematical skills led him to be assigned to the artillery corps. He first served in Alsace and after 1941 in fairly quiet Norway. In 1945, Schmidt volunteered for active front duty in Northern Germany, in order to be granted a brief home visit. As the war was obviously lost, he used this visit to organise his wife's and his own escape to the west of Germany, in order to evade capture by the Red Army, which was known for its much harsher treatment of prisoners of war and German civilians. Schmidt gave himself up to British forces in Lower Saxony. As refugees, Schmidt and his wife lost almost all of their possessions, including their cherished book collection.

After an interlude as a British POW and later as an interpreter at a police school, Schmidt began his career as a freelance writer in 1946. Since Schmidt's pre-war home in Lauban was now under Polish administration, Schmidt and his wife were among the millions of refugees moved by the authorities to numerous places in what was to become West Germany. During this time of uncertainty and extreme poverty, the Schmidts were sustained by CARE Packages his sister sent them from the US (his sister Lucie had emigrated to the US in 1939, together with her husband Rudy Kiesler, a Jewish German communist). Temporary accommodations led the Schmidts to Cordingen (near Bomlitz), Gau-Bickelheim, and Kastel (the latter two in the newly formed state of Rhineland-Palatinate). In Kastel, he was accused in court of blasphemy and moral subversion, then still considered a crime in some of the Catholic regions of Germany. As a result, Schmidt and his wife moved to the Protestant city of Darmstadt in Hesse, where the suit against him was dismissed. In 1958, the Schmidts moved to the small village of Bargfeld, where they were to stay for the rest of their lives, Schmidt dying in 1979, his wife Alice in 1983.

Writing style and personal philosophy
Schmidt was a strict individualist, almost a solipsist. Disaffected by his experience of Nazi Germany, he had an extremely pessimistic world view. In 1951's Schwarze Spiegel (Dark Mirrors), he describes his utopia as an empty world after an anthropogenic apocalypse. Although he was not a deist in the conventional sense, he maintained that the world was created by a monster called Leviathan, whose predatory nature was passed on to humans. Still, he thought this monster could not be too powerful to be attacked, if it behoved humanity.

His writing style is characterised by a unique and witty style of adapting colloquial language, which won him quite a few fervent admirers. Moreover, he developed an orthography by which he thought to reveal the true meaning of words and their connections amongst each other. One of the most cited examples is the use of “Roh=Mann=Tick" instead of "Romantik" (revealing romanticism as the craze of unsubtle men). The atoms of words holding the nuclei of original meaning he called Etyme (etyms).

Bottom's Dream
His theory of etyms is developed in his magnum opus, Bottom's Dream (Zettels Traum), in which an elderly writer comments on Edgar Allan Poe's works in a stream of consciousness, while discussing a Poe translation with a couple of translators and flirting with their teenage daughter. Schmidt also accomplished a translation of Edgar Allan Poe's works himself (1966–1973, together with Hans Wollschläger). Some critics even dismissed Bottom's Dream as non-art, or sheer nonsense, and Schmidt himself as a "psychopath". But Schmidt's reputation as esoteric, and that of his work as non-art, has faded and he is now seen as an important, if highly eccentric, German writer of the 20th century.

Other, minor works
In the 1960s, he authored a series of plays for German radio stations presenting forgotten or little known and—in his opinion—vastly underrated authors, e.g. Johann Gottfried Schnabel, Karl Philipp Moritz, Leopold Schefer, Karl Ferdinand Gutzkow, and others. These "plays" are basically talks about literature with two or three participants plus voices for quotations (Schmidt lent his voice for his translations of Finnegans Wake quoted in Der Triton mit dem Sonnenschirm [1961]). Eleven of these so-called "Radio-Essays" were republished on 12 audio CDs in 2003.

The final years
1971 Schmidt was nominated for the Nobel Prize in Literature by Lars Gyllensten, a member of the Swedish Academy. But as none of Schmidt's works sold more than a few thousand copies (he openly admitted that he only wrote for the small handful of people who could appreciate his work), he lived in extreme poverty. During the last few years of his life, he was financially supported by the philologist and writer Jan Philipp Reemtsma, the heir of the German cigarette manufacturer Philipp F. Reemtsma. Schmidt's final completed novel was Abend mit Goldrand (1975) which was praised by some critics for its verbal inventiveness, although many had a difficult time digesting the erotic themes of the book. He died in a hospital in Celle on 3 June 1979 after suffering a stroke.

Posthumous legacy
Dalkey Archive Press has reissued five volumes of Schmidt's work translated by John E. Woods. The series includes Collected Novellas, Collected Stories, Nobodaddy's Children, Two Novels, and most recently, Bottom's Dream. The reissues were scheduled to coincide with "Rediscovering Arno Schmidt events in the US, UK, and continental Europe." The Arno Schmidt Foundation (Arno Schmidt Stiftung) in Bargfeld, sponsored by Jan Philipp Reemtsma, is publishing his complete works.

Schmidt is mentioned in the Roberto Bolaño novel 2666.

Arno Schmidt Prize
The Arno Schmidt Prize, awarded by the Arno Schmidt Foundation and awarded from 1981 to 1988, was endowed with DM 50,000 (about €25,000). The prize has only been awarded four times:
 * 1982: Hans Wollschläger
 * 1984: Wolfgang Koeppen
 * 1986: Peter Rühmkorf
 * 1988: Karlheinz Deschner

After that the award was converted into the two-year Arno Schmidt scholarship, endowed with 36,000 euros.

Arno Schmidt scholarship winners

 * 1992/1993: Ulrich Holbein
 * 1994/1995: Andreas Eschbach
 * 1996/1997: Werner Kofler
 * 2000/2001: Kurt Drabert
 * 2000: Georg Martin Oswald
 * 2002/2003: Friederike Kretzen
 * 2005/2006: Reinhard Jirgl
 * 2007/2008: Werner Fritsch
 * 2015/2016: Andreas Maier
 * 2018/2019: Georg Klein

Novels

 * Brand's Haide (1951). Brand's Heath
 * Schwarze Spiegel (1951). Dark Mirrors
 * Aus dem Leben eines Fauns (1953). Scenes from the Life of a Faun
 * Das steinerne Herz (1954). The Stony Heart
 * KAFF auch Mare Crisium (1960). B/Moondocks
 * Nobodaddy's Kinder (1963). Nobodaddy's Children; collects Aus dem Leben eines Fauns, Brand's Haide, Schwarze Spiegel
 * Zettels Traum (1970). Bottom's Dream
 * Die Schule der Atheisten (1972). The School for Atheists
 * Abend mit Goldrand (1975). Evening Edged in Gold
 * Julia, oder die Gemälde (unfinished, 1983).

Novellas and short stories

 * Leviathan (1949). Includes: Enthymesis, Gadir, Leviathan
 * Die Umsiedler – 2 Prosastudien (1953). Includes: Die Umsiedler (The Displaced) and Alexander
 * Seelandschaft mit Pocahontas (1955). Lake Scenery with Pocahontas
 * Kosmas oder Vom Berge des Nordens (1955)
 * Tina oder über die Unsterblichkeit (1956)
 * Goethe und einer seiner Bewunderer (1957)
 * Die Gelehrtenrepublik (1957). The Egghead Republic (trans. Michael Horovitz, 1979) and Republica Intelligentsia (trans. John E. Woods, 1994)
 * Rosen und Porree (1959). Collects Pocahontas, Die Umsiedler, Alexander and Kosmas
 * Kühe in Halbtrauer (1964). Later published as Ländliche Erzählungen (Country Matters)
 * Trommler beim Zaren (1966). Short story anthology.

Dialogues

 * Massenbach, Cooper, Brockes, Fouqué, Pape, Schnabel, Europa, Wieland, Meyern, Meisterdiebe, Klopstock, Moritz
 * Joyce, May, Stifter, Krakatau, Herder, Vorspiel, Oppermann, Wezel, Kreisschlösser, Müller, Tieck, Schefer, Dickens
 * Das Zweite Programm, Joyce, May, Frenssen, Stifter, Gutzkow, Lafontaine, Joyce, Collins, Bulwer-Lytton, Spindler

Literary theory

 * Fouqué und einige seiner Zeitgenossen, biography of Friedrich de la Motte Fouqué, 1958 (2nd, extended ed. 1960)
 * Dya Na Sore (1958)
 * Belphegor (1961)
 * Sitara und der Weg dorthin, biography of Karl May, 1963
 * Die Ritter vom Geist (1965)
 * Der Triton mit den Sonnenschirm (1969). The Triton with the Parasol

English translations

 * The Egghead Republic – 1979 (Die Gelehrtenrepublik, trans. Michael Horovitz)
 * Evening Edged in Gold – 1980 (Abend mit Goldrand, trans. John E. Woods)
 * Scenes from the Life of a Faun – 1983 (Aus dem Leben eines Fauns, trans. John E. Woods; revised in 1995)
 * Collected Early Fiction, 1949–1964, in four volumes (all trans. John E. Woods):
 * Volume 1: Collected Novellas – 1994; collects Enthymesis (1949), Leviathan (1949), Gadir (1949), Alexander (1953), The Displaced (1953), Lake Scenery with Pocahontas (1955), Cosmas (1955), Tina (1956), Goethe (1957) and Republica Intelligentsia (1957)
 * Volume 2: Nobodaddy's Children – 1995; collects Scenes from the Life of a Faun (1953), Brand's Heath (1951) and Dark Mirrors (1951)
 * Volume 3: Collected Stories – 1996; collects Aus der Inselstraße (Tales from Island Street) (16 stories, 1955–1962), Stürenburg-Geschichten (Stürenburg Stories) (9 stories, 1955–1959), Country Matters (9 stories and the novella Caliban über Setebos (Caliban upon Stetebos), 1960–1964)
 * Volume 4: Two Novels – 1997; collects The Stony Heart (1954) and B/Moondocks (1960)
 * Radio Dialogs I – 1999 (trans. John E. Woods)
 * The School for Atheists – 2001 (Die Schule der Atheisten, trans. John E. Woods)
 * Radio Dialogs II – 2003 (trans. John E. Woods)
 * Bottom's Dream – 2016 (Zettels Traum, trans. John E. Woods)

title: Arno Schmidt


text:
MIX (abstract machine)

MIX is a hypothetical computer used in Donald Knuth's monograph, The Art of Computer Programming (TAOCP). MIX's model number is 1009, which was derived by combining the model numbers and names of several contemporaneous, commercial machines deemed significant by the author. Also, "MIX" read as a Roman numeral is 1009.

The 1960s-era MIX has since been superseded by a new (also hypothetical) computer architecture, MMIX, to be incorporated in forthcoming editions of TAOCP.

Software implementations for both the MIX and MMIX architectures have been developed by Knuth and made freely available (named "MIXware" and "MMIXware", respectively). Several derivatives of Knuth's MIX/MMIX emulators also exist. GNU MDK is one such software package; it is free and runs on a wide variety of platforms.

Their purpose for education is quite similar to John L. Hennessy's and David A. Patterson's DLX architecture, from Computer Organization and Design - The Hardware Software Interface.

Architecture
MIX is a hybrid binary–decimal computer. When programmed in binary, each byte has 6 bits (values range from 0 to 63). In decimal, each byte has 2 decimal digits (values range from 0 to 99). Bytes are grouped into words of five bytes plus a sign. Most programs written for MIX will work in either binary or decimal, so long as they do not try to store a value greater than 63 in a single byte.

A word has the range −1,073,741,823 to 1,073,741,823 (inclusive) in binary mode, and −9,999,999,999 to 9,999,999,999 (inclusive) in decimal mode. The sign-and-magnitude representation of integers in the MIX architecture distinguishes between “−0” and “+0.” This contrasts with modern computers, whose two's-complement representation of integer quantities includes a single representation for zero, but whose range for a given number of bits includes one more negative integer than the number of representable positive integers.

Registers
There are 9 registers in MIX:
 * rA: Accumulator (full word, five bytes and a sign).
 * rX: Extension (full word, five bytes and a sign).
 * rI1, rI2, rI3, rI4, rI5, rI6: Index registers (two bytes and a sign).
 * rJ: Jump address (two bytes, always positive).

A byte is assumed to be at least 6 bits. Most instructions can specify which of the "fields" (bytes) of a register are to be altered, using a suffix of the form (first:last). The zeroth field is the one-bit sign.

MIX also records whether the previous operation overflowed, and has a one-trit comparison indicator (less than, equal to, or greater than).

Memory and input/output
The MIX machine has 4000 words of memory (each with 5 bytes and a sign), addressed from 0 to 3999. A variety of input and output devices are also included:
 * Tape units (devices 0...7).
 * Disk or drum units (devices 8...15).
 * Card reader (device 16).
 * Card punch (device 17).
 * Line printer (device 18).
 * Typewriter terminal (device 19).
 * Paper tape (device 20).

Instructions
Each machine instruction in memory occupies one word, and consists of 4 parts: the address (2 bytes and the sign of the word) in memory to read or write; an index specification (1 byte, describing which rI index register to use) to add to the address; a modification (1 byte) that specifies which parts of the register or memory location will be read or altered; and the operation code (1 byte). All operation codes have an associated mnemonic.

MIX programs frequently use self-modifying code, in particular to return from a subroutine, as MIX lacks an automatic subroutine return stack. Self-modifying code is facilitated by the modification byte, allowing the program to store data to, for example, the address part of the target instruction, leaving the rest of the instruction unmodified.

MIX programs are typically constructed using the MIXAL assembly language; for an example, see the list hello world programs page.

Implementations
MIX has been implemented in software by:
 * Knuth's MIXWare and the derived GNU MDK;
 * 9front's mix(1); and
 * Hardware::Simulator::MIX on CPAN.

An implementation of MIX was created for the iCE40HX8K FPGA board in 2021.

title: MIX (abstract machine)


text:
Diels–Alder reaction

In organic chemistry, the Diels–Alder reaction is a chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene derivative. It is the prototypical example of a pericyclic reaction with a concerted mechanism. More specifically, it is classified as a thermally-allowed [4+2] cycloaddition with Woodward–Hoffmann symbol [π4s + π2s]. It was first described by Otto Diels and Kurt Alder in 1928. For the discovery of this reaction, they were awarded the Nobel Prize in Chemistry in 1950. Through the simultaneous construction of two new carbon–carbon bonds, the Diels–Alder reaction provides a reliable way to form six-membered rings with good control over the regio- and stereochemical outcomes. Consequently, it has served as a powerful and widely applied tool for the introduction of chemical complexity in the synthesis of natural products and new materials. The underlying concept has also been applied to π-systems involving heteroatoms, such as carbonyls and imines, which furnish the corresponding heterocycles; this variant is known as the hetero-Diels–Alder reaction. The reaction has also been generalized to other ring sizes, although none of these generalizations have matched the formation of six-membered rings in terms of scope or versatility. Because of the negative values of ΔH° and ΔS° for a typical Diels–Alder reaction, the microscopic reverse of a Diels–Alder reaction becomes favorable at high temperatures, although this is of synthetic importance for only a limited range of Diels-Alder adducts, generally with some special structural features; this reverse reaction is known as the retro-Diels–Alder reaction.

Mechanism
The reaction is an example of a concerted pericyclic reaction. It is believed to occur via a single, cyclic transition state, with no intermediates generated during the course of the reaction. As such, the Diels–Alder reaction is governed by orbital symmetry considerations: it is classified as a [π4s + π2s] cycloaddition, indicating that it proceeds through the suprafacial/suprafacial interaction of a 4π electron system (the diene structure) with a 2π electron system (the dienophile structure), an interaction that leads to a transition state without an additional orbital symmetry-imposed energetic barrier and allows the Diels–Alder reaction to take place with relative ease.

A consideration of the reactants' frontier molecular orbitals (FMO) makes plain why this is so. (The same conclusion can be drawn from an orbital correlation diagram or a Dewar-Zimmerman analysis.) For the more common "normal" electron demand Diels–Alder reaction, the more important of the two HOMO/LUMO interactions is that between the electron-rich diene's ψ2 as the highest occupied molecular orbital (HOMO) with the electron-deficient dienophile's π* as the lowest unoccupied molecular orbital (LUMO). However, the HOMO–LUMO energy gap is close enough that the roles can be reversed by switching electronic effects of the substituents on the two components. In an inverse (reverse) electron-demand Diels–Alder reaction, electron-withdrawing substituents on the diene lower the energy of its empty ψ3 orbital and electron-donating substituents on the dienophile raise the energy of its filled π orbital sufficiently that the interaction between these two orbitals becomes the most energetically significant stabilizing orbital interaction. Regardless of which situation pertains, the HOMO and LUMO of the components are in phase and a bonding interaction results as can be seen in the diagram below. Since the reactants are in their ground state, the reaction is initiated thermally and does not require activation by light.

The "prevailing opinion"  is that most Diels–Alder reactions proceed through a concerted mechanism; the issue, however, has been thoroughly contested. Despite the fact that the vast majority of Diels–Alder reactions exhibit stereospecific, syn addition of the two components, a diradical intermediate has been postulated (and supported with computational evidence) on the grounds that the observed stereospecificity does not rule out a two-step addition involving an intermediate that collapses to product faster than it can rotate to allow for inversion of stereochemistry.

There is a notable rate enhancement when certain Diels–Alder reactions are carried out in polar organic solvents such as dimethylformamide and ethylene glycol, and even in water. The reaction of cyclopentadiene and butenone for example is 700 times faster in water relative to 2,2,4-trimethylpentane as solvent. Several explanations for this effect have been proposed, such as an increase in effective concentration due to hydrophobic packing or hydrogen-bond stabilization of the transition state.

The geometry of the diene and dienophile components each propagate into stereochemical details of the product. For intermolecular reactions especially, the preferred positional and stereochemical relationship of substituents of the two components compared to each other are controlled by electronic effects. However, for intramolecular Diels–Alder cycloaddition reactions, the conformational stability of the structure of the transition state can be an overwhelming influence.

Regioselectivity
Frontier molecular orbital theory has also been used to explain the regioselectivity patterns observed in Diels–Alder reactions of substituted systems. Calculation of the energy and orbital coefficients of the components' frontier orbitals provides a picture that is in good accord with the more straightforward analysis of the substituents' resonance effects, as illustrated below.

In general, the regioselectivity found for both normal and inverse electron-demand Diels–Alder reaction follows the ortho-para rule, so named, because the cyclohexene product bears substituents in positions that are analogous to the ortho and para positions of disubstituted arenes. For example, in a normal-demand scenario, a diene bearing an electron-donating group (EDG) at C1 has its largest HOMO coefficient at C4, while the dienophile with an electron withdrawing group (EWG) at C1 has the largest LUMO coefficient at C2. Pairing these two coefficients gives the "ortho" product as seen in case 1 in the figure below. A diene substituted at C2 as in case 2 below has the largest HOMO coefficient at C1, giving rise to the "para" product. Similar analyses for the corresponding inverse-demand scenarios gives rise to the analogous products as seen in cases 3 and 4. Examining the canonical mesomeric forms above, it is easy to verify that these results are in accord with expectations based on consideration of electron density and polarization.

In general, with respect to the energetically most well-matched HOMO-LUMO pair, maximizing the interaction energy by forming bonds between centers with the largest frontier orbital coefficients allows the prediction of the main regioisomer that will result from a given diene-dienophile combination. In a more sophisticated treatment, three types of substituents (Z withdrawing: HOMO and LUMO lowering (CF3, NO2, CN, C(O)CH3), X donating: HOMO and LUMO raising (Me, OMe, NMe2), C conjugating: HOMO raising and LUMO lowering (Ph, vinyl)) are considered, resulting in a total of 18 possible combinations. The maximization of orbital interaction correctly predicts the product in all cases for which experimental data is available. For instance, in uncommon combinations involving X groups on both diene and dienophile, a 1,3-substitution pattern may be favored, an outcome not accounted for by a simplistic resonance structure argument. However, cases where the resonance argument and the matching of largest orbital coefficients disagree are rare.

Stereospecificity and stereoselectivity
Diels–Alder reactions, as concerted cycloadditions, are stereospecific. Stereochemical information of the diene and the dienophile are retained in the product, as a syn addition with respect to each component. For example, substituents in a cis (trans, resp.) relationship on the double bond of the dienophile give rise to substituents that are cis (trans, resp.) on those same carbons with respect to the cyclohexene ring. Likewise, cis,cis- and trans,trans-disubstituted dienes give cis substituents at these carbons of the product whereas cis,trans-disubstituted dienes give trans substituents:

Diels–Alder reactions in which adjacent stereocenters are generated at the two ends of the newly formed single bonds imply two different possible stereochemical outcomes. This is a stereoselective situation based on the relative orientation of the two separate components when they react with each other. In the context of the Diels–Alder reaction, the transition state in which the most significant substituent (an electron-withdrawing and/or conjugating group) on the dienophile is oriented towards the diene π system and slips under it as the reaction takes place is known as the endo transition state. In the alternative exo transition state, it is oriented away from it. (There is a more general usage of the terms endo and exo in stereochemical nomenclature.)

In cases where the dienophile has a single electron-withdrawing / conjugating substituent, or two electron-withdrawing / conjugating substituents cis to each other, the outcome can often be predicted. In these "normal demand" Diels–Alder scenarios, the endo transition state is typically preferred, despite often being more sterically congested. This preference is known as the Alder endo rule. As originally stated by Alder, the transition state that is preferred is the one with a "maximum accumulation of double bonds." Endo selectivity is typically higher for rigid dienophiles such as maleic anhydride and benzoquinone; for others, such as acrylates and crotonates, selectivity is not very pronounced. The most widely accepted explanation for the origin of this effect is a favorable interaction between the π systems of the dienophile and the diene, an interaction described as a secondary orbital effect, though dipolar and van der Waals attractions may play a part as well, and solvent can sometimes make a substantial difference in selectivity. The secondary orbital overlap explanation was first proposed by Woodward and Hoffmann. In this explanation, the orbitals associated with the group in conjugation with the dienophile double-bond overlap with the interior orbitals of the diene, a situation that is possible only for the endo transition state. Although the original explanation only invoked the orbital on the atom α to the dienophile double bond, Salem and Houk have subsequently proposed that orbitals on the α and β carbons both participate when molecular geometry allows. Often, as with highly substituted dienes, very bulky dienophiles, or reversible reactions (as in the case of furan as diene), steric effects can override the normal endo selectivity in favor of the exo isomer.

The diene
The diene component of the Diels–Alder reaction can be either open-chain or cyclic, and it can host many different types of substituents. It must, however, be able to exist in the s-cis conformation, since this is the only conformer that can participate in the reaction. Though butadienes are typically more stable in the s-trans conformation, for most cases energy difference is small (~2–5 kcal/mol).

A bulky substituent at the C2 or C3 position can increase reaction rate by destabilizing the s-trans conformation and forcing the diene into the reactive s-cis conformation. 2-tert-butyl-buta-1,3-diene, for example, is 27 times more reactive than simple butadiene. Conversely, a diene having bulky substituents at both C2 and C3 is less reactive because the steric interactions between the substituents destabilize the s-cis conformation.

Dienes with bulky terminal substituents (C1 and C4) decrease the rate of reaction, presumably by impeding the approach of the diene and dienophile.

An especially reactive diene is 1-methoxy-3-trimethylsiloxy-buta-1,3-diene, otherwise known as Danishefsky's diene. It has particular synthetic utility as means of furnishing α,β–unsaturated cyclohexenone systems by elimination of the 1-methoxy substituent after deprotection of the enol silyl ether. Other synthetically useful derivatives of Danishefsky's diene include 1,3-alkoxy-1-trimethylsiloxy-1,3-butadienes (Brassard dienes) and 1-dialkylamino-3-trimethylsiloxy-1,3-butadienes (Rawal dienes). The increased reactivity of these and similar dienes is a result of synergistic contributions from donor groups at C1 and C3, raising the HOMO significantly above that of a comparable monosubstituted diene.

Unstable (and thus highly reactive) dienes can be synthetically useful, e.g. o-quinodimethanes can be generated in situ. In contrast, stable dienes, such as naphthalene, require forcing conditions and/or highly reactive dienophiles, such as N-phenylmaleimide. Anthracene, being less aromatic (and therefore more reactive for Diels–Alder syntheses) in its central ring can form a 9,10 adduct with maleic anhydride at 80 °C and even with acetylene, a weak dienophile, at 250 °C.

The dienophile
In a normal demand Diels–Alder reaction, the dienophile has an electron-withdrawing group in conjugation with the alkene; in an inverse-demand scenario, the dienophile is conjugated with an electron-donating group. Dienophiles can be chosen to contain a "masked functionality". The dienophile undergoes Diels–Alder reaction with a diene introducing such a functionality onto the product molecule. A series of reactions then follow to transform the functionality into a desirable group. The end product cannot be made in a single DA step because equivalent dienophile is either unreactive or inaccessible. An example of such approach is the use of α-chloroacrylonitrile (CH2=CClCN). When reacted with a diene, this dienophile will introduce α-chloronitrile functionality onto the product molecule. This is a "masked functionality" which can be then hydrolyzed to form a ketone. α-Chloroacrylonitrile dienophile is an equivalent of ketene dienophile (CH2=C=O), which would produce same product in one DA step. The problem is that ketene itself cannot be used in Diels–Alder reactions because it reacts with dienes in unwanted manner (by [2+2] cycloaddition), and therefore "masked functionality" approach has to be used. Other such functionalities are phosphonium substituents (yielding exocyclic double bonds after Wittig reaction), various sulfoxide and sulfonyl functionalities (both are acetylene equivalents), and nitro groups (ketene equivalents).

Hetero-Diels–Alder
Diels–Alder reactions involving at least one heteroatom are also known and are collectively called hetero-Diels–Alder reactions. Carbonyl groups, for example, can successfully react with dienes to yield dihydropyran rings, a reaction known as the oxo-Diels–Alder reaction, and imines can be used, either as the dienophile or at various sites in the diene, to form various N-heterocyclic compounds through the aza-Diels–Alder reaction. Nitroso compounds (R-N=O) can react with dienes to form oxazines. Chlorosulfonyl isocyanate can be utilized as a dienophile to prepare Vince lactam.

Lewis acid activation
Lewis acids, such as zinc chloride, boron trifluoride, tin tetrachloride, or aluminium chloride, can catalyze Diels–Alder reactions by binding to the dienophile. Traditionally, the enhanced Diels-Alder reactivity is ascribed to the ability of the Lewis acid to lower the LUMO of the activated dienophile, which results in a smaller normal electron demand HOMO-LUMO orbital energy gap and hence more stabilizing orbital interactions.

Recent studies, however, have shown that this rationale behind Lewis acid-catalyzed Diels–Alder reactions is incorrect. It is found that Lewis acids accelerate the Diels–Alder reaction by reducing the destabilizing steric Pauli repulsion between the interacting diene and dienophile and not by lowering the energy of the dienophile's LUMO and consequently, enhancing the normal electron demand orbital interaction. The Lewis acid binds via a donor-acceptor interaction to the dienophile and via that mechanism polarizes occupied orbital density away from the reactive C=C double bond of the dienophile towards the Lewis acid. This reduced occupied orbital density on C=C double bond of the dienophile will, in turn, engage in a less repulsive closed-shell-closed-shell orbital interaction with the incoming diene, reducing the destabilizing steric Pauli repulsion and hence lowers the Diels–Alder reaction barrier. In addition, the Lewis acid catalyst also increases the asynchronicity of the Diels–Alder reaction, making the occupied π-orbital located on the C=C double bond of the dienophile asymmetric. As a result, this enhanced asynchronicity leads to an extra reduction of the destabilizing steric Pauli repulsion as well as a diminishing pressure on the reactants to deform, in other words, it reduced the destabilizing activation strain (also known as distortion energy). This working catalytic mechanism is known as Pauli-lowering catalysis, which is operative in a variety of organic reactions.

The original rationale behind Lewis acid-catalyzed Diels–Alder reactions is incorrect,  because besides lowering the energy of the dienophile's LUMO, the Lewis acid also lowers the energy of the HOMO of the dienophile and hence increases the inverse electron demand LUMO-HOMO orbital energy gap. Thus, indeed Lewis acid catalysts strengthen the normal electron demand orbital interaction by lowering the LUMO of the dienophile, but, they simultaneously weaken the inverse electron demand orbital interaction by also lowering the energy of the dienophile's HOMO. These two counteracting phenomena effectively cancel each other, resulting in nearly unchanged orbital interactions when compared to the corresponding uncatalyzed Diels–Alder reactions and making this not the active mechanism behind Lewis acid-catalyzed Diels–Alder reactions.

Asymmetric Diels–Alder
Many methods have been developed for influencing the stereoselectivity of the Diels–Alder reaction, such as the use of chiral auxiliaries, catalysis by chiral Lewis acids, and small organic molecule catalysts. Evans' oxazolidinones, oxazaborolidines,  bis-oxazoline–copper chelates, imidazoline catalysis, and many other methodologies exist for effecting diastereo- and enantioselective Diels–Alder reactions.

Hexadehydro Diels–Alder
In the hexadehydro Diels–Alder reaction, alkynes and diynes are used instead of alkenes and dienes, forming an unstable benzyne intermediate which can then be trapped to form an aromatic product. This reaction allows the formation of heavily functionalized aromatic rings in a single step.

Applications and natural occurrence
The retro-Diels–Alder reaction is used in the industrial production of cyclopentadiene. Cyclopentadiene is a precursor to various norbornenes, which are common monomers. The Diels–Alder reaction is also employed in the production of vitamin B6.

History

The work by Diels and Alder is described in a series of 28 articles published in the Justus Liebigs Annalen der Chemie and Berichte der deutschen chemischen Gesellschaft from 1928 to 1937. The first 19 articles were authored by Diels and Alder, while the later articles were authored by Diels and various other coauthors.

Applications in total synthesis
The Diels–Alder reaction was one step in an early preparation of the steroids cortisone and cholesterol. The reaction involved the addition of butadiene to a quinone.

Diels–Alder reactions were used in the original synthesis of prostaglandins F2α and E2. The Diels–Alder reaction establishes the relative stereochemistry of three contiguous stereocenters on the prostaglandin cyclopentane core. Activation by Lewis acidic cupric tetrafluoroborate was required.

A Diels–Alder reaction was used in the synthesis of disodium prephenate, a biosynthetic precursor of the amino acids phenylalanine and tyrosine.

A synthesis of reserpine uses a Diels–Alder reaction to set the cis-decalin framework of the D and E rings.

In another synthesis of reserpine, the cis-fused D and E rings was formed by a Diels–Alder reaction. Intramolecular Diels–Alder of the pyranone below with subsequent extrusion of carbon dioxide via a retro [4+2] afforded the bicyclic lactam. Epoxidation from the less hindered α-face, followed by epoxide opening at the less hindered C18 afforded the desired stereochemistry at these positions, while the cis-fusion was achieved with hydrogenation, again proceeding primarily from the less hindered face.

A pyranone was similarly used as the dienophile in the total synthesis of taxol. The intermolecular reaction of the hydroxy-pyrone and α,β–unsaturated ester shown below suffered from poor yield and regioselectivity; however, when directed by phenylboronic acid the desired adduct could be obtained in 61% yield after cleavage of the boronate with neopentyl glycol. The stereospecificity of the Diels–Alder reaction in this instance allowed for the definition of four stereocenters that were carried on to the final product.

A Diels–Alder reaction is a key step in the synthesis of (-)-furaquinocin C.

Tabersonine was prepared by a Diels–Alder reaction to establish cis relative stereochemistry of the alkaloid core. Conversion of the cis-aldehyde to its corresponding alkene by Wittig olefination and subsequent ring-closing metathesis with a Schrock catalyst gave the second ring of the alkaloid core. The diene in this instance is notable as an example of a 1-amino-3-siloxybutadiene, otherwise known as a Rawal diene.

(+)-Sterpurene can be prepared by asymmetric D-A reaction that featured a remarkable intramolecular Diels–Alder reaction of an allene. The [2,3]-sigmatropic rearrangement of the thiophenyl group to give the sulfoxide as below proceeded enantiospecifically due to the predefined stereochemistry of the propargylic alcohol. In this way, the single allene isomer formed could direct the Diels–Alder reaction to occur on only one face of the generated 'diene'.

The tetracyclic core of the antibiotic (-)-tetracycline was prepared with a Diels–Alder reaction. Thermally initiated, conrotatory opening of the benzocyclobutene generated the o-quinodimethane, which reacted intermolecularly to give the tetracycline skeleton. The dienophile's free hydroxyl group is integral to the success of the reaction, as hydroxyl-protected variants did not react under several different reaction conditions.

Takemura et al. synthesized cantharidin in 1980 by Diels–Alder reaction, utilizing high pressure.

Synthetic applications of the Diels–Alder reaction have been reviewed extensively.

title: Diels–Alder reaction