- Erdős number
The

**Erdős number**(IPA2|ɛrdøːʃ), honoring the late Hungarian mathematicianPaul Erdős , is a way of describing the "collaborative distance" between a person and Erdős, as measured by authorship of mathematical papers.It was created by friends as a humorous tribute to the enormous output of Erdős, one of the most prolific modern writers of mathematical papers, and has become well-known in scientific circles as a

tongue-in-cheek measurement of mathematical prominence.**Definition**In order to be assigned an Erdős number, an author must co-write a mathematical paper with an author with a finite Erdős number. Paul Erdős is the one person having an Erdős number of zero. If the lowest Erdős number of a coauthor is "k", then the author's Erdős number is "k" + 1.

Erdős wrote around 1,500 mathematical articles in his lifetime, mostly co-written. He had 511 direct collaborators [

*[*] ; these are the people with Erdős number 1. The people who have collaborated with them (but not with Erdős himself) have an Erdős number of 2 (8,162 people as of 2007), those who have collaborated with people who have an Erdős number of 2 (but not with Erdős or anyone with an Erdős number of 1) have an Erdős number of 3, and so forth. A person with no such coauthorship chain connecting to Erdős has no Erdős number (or an undefined one).*http://www.oakland.edu/enp/Erdos0 Erdős Number Project*]There is room for ambiguity over what constitutes a link between two authors; the Erdős Number Project Web site says "Our criterion for inclusion of an edge between vertices u and v is some research collaboration between them resulting in a published work. Any number of additional co-authors is permitted," but they do not include non-research publications such as elementary textbooks, joint editorships, obituaries, and the like. The “Erdős number of the second kind” restricts assignment of Erdős numbers to papers with only two collaborators. [

*Grossman "et al." “Erdös numbers of the second kind,” in " Facts about Erdös Numbers and the Collaboration Graph". Erdös Number Project, [*]*http://www.oakland.edu/enp/trivia.html#en2k*] , retrievedJanuary 9 ,2008 .The Erdős number was most likely first defined in print by Casper Goffman, an analyst whose own Erdős number is 1. [

*[*] Goffman published his observations about Erdős's prolific collaboration in a 1969 article entitled "And what is your Erdős number?" [*http://www.math.purdue.edu/about/purview/fall96/paul-erdos.html Erdős' obituary by Michael Golomb's*]*cite journal|author=Goffman, Casper|title=And what is your Erdős number?|journal=American Mathematical Monthly|volume=76|year=1969|doi=10.2307/2317868|pages=791*]The AMS collaboration distance calculator allows an online calculation of an individual's Erdős number.

**Impact**Erdős numbers have been a part of the

folklore of mathematicians throughout the world for many years. Amongst all working mathematicians at the turn of the millennium who have a finite Erdős number, the numbers range up to 15, the median is 5, the average Erdős number is 4.65; [*[*] and almost everyone with a finite Erdős number has a number less than 8. Due to the very high frequency of interdisciplinary collaboration in science today, very large numbers of non-mathematicians in many other fields of science also have finite Erdős numbers. For example, political scientist*http://www.oakland.edu/enp/ Erdős Number Project*]Steven Brams has an Erdős number 2. In biomedical research, it is common for statisticians to be among the authors of publications,and many statisticians can be linked viaJohn Tukey , who has Erdős number 2, to Erdős. Similarly, the prominent geneticistEric Lander and the mathematicianDaniel Kleitman have collaborated on papers, [*[http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=pubmed&cmd=search&term=lander+kleitman A dictionary-based approach for gene annotation. [J Comput Biol. 1999 Fall-Winter - PubMed Result ]*] [*[*] and since Kleitman has an Erdős number of 1, [*http://www-math.mit.edu/~djk/list.html Prof. Daniel Kleitman's Publications Since 1980 more or less*]*cite journal | last = Erdős | first = Paul | authorlink = Paul Erdős | coauthors =*] a large fraction of the genetics and genomics community can be linked via Lander and his numerous collaborators. According to Alex Lopez-Ortiz, all theDaniel Kleitman | title = On Collections of Subsets Containing No 4-Member Boolean Algebra | journal = Proceedings of the American Mathematical Society | volume = 28 | issue = 1 | pages = 87–90 |date=April 1971 | doi = 10.2307/2037762

Fields and Nevanlinna prize winners during the three cycles in 1986 to 1994 have Erdős number at most 9. Similarly, manylinguists have finite Erdős numbers, many due to chains of collaboration withNoam Chomsky , whose number is 4. [*[*]*http://semantics-online.org/2004/01/my-erds-number-is-8 My Erdös Number is 8 < Semantics etc*]Tompa [

*cite journal|author=Tompa, Martin|title=Figures of merit|journal=ACM SIGACT News|volume=20|issue=1|pages=62–71|year=1989|doi=10.1145/65780.65782 cite journal|author=Tompa, Martin|title=Figures of merit: the sequel|journal=ACM SIGACT News|volume=21|issue=4|pages=78–81|year=1990|doi=10.1145/101371.101376*] proposed adirected graph version of the Erdős number problem, by orienting edges of the collaboration graph from the alphabetically earlier author to the alphabetically later author and defining the "monotone Erdős number" of an author to be the length of a "longest" path from Erdős to the author in this directed graph. He finds a path of this type of length 12.Also, Michael Barr suggests "rational Erdős numbers", generalizing the idea that a person who has written p joint papers with Erdős should be assigned Erdős number 1/p. From the collaboration multigraph of the second kind (although he also has a way to deal with the case of the first kind) — with one edge between two mathematicians for "each" joint paper they have produced — form an electrical network with a one-ohm resistor on each edge. The total resistance between two nodes tells how "close" these two nodes are.

Earlier mathematicians published fewer papers than modern ones, and more rarely published jointly-written papers. The earliest person known to have a finite positive Erdős number is either

Richard Dedekind (born 1831, Erdős number 7) orGeorg Frobenius (born 1849, Erdős number 3), depending on the standard of publication eligibility [*[*] . It seems that older historic figures such as*http://www.oakland.edu/enp/erdpaths.html Erdős Number Project - Paths to Erdős*]Leonhard Euler do not have finite Erdős numbers.**Effect of Erdős' death on the Erdős number**Given that Erdős died in 1996 and no works of his remain to be published, it is no longer possible for a person to be newly assigned an Erdős number of 1. Likewise, once everyone with an Erdős number of 1 has died and have no work remaining to be published, it will be impossible for a new person to obtain an Erdős number of 2, and so on.

**Outside mathematics****Bacon number**The

Bacon number (as in the gameSix Degrees of Kevin Bacon ) is an application of the same idea to the movie industry, connecting actors that appeared in a film together to the actorKevin Bacon .A small number of people are connected to both Erdős and Bacon and thus have an

Erdős-Bacon number . One example is the actressDanica McKellar , best known for playing Winnie Cooper on the TV series, "The Wonder Years ". Her Erdős number is 4 and her Bacon number is 2. The lowest knownErdős-Bacon number is three forDaniel Kleitman , a mathematics professor at MIT; his Erdős number is 1 and his Bacon number is 2.**Kibo number**The surrealist

Usenet personality Kibo has had a similar number applied to people in contact with him. A Kibo number of one is defined as someone who has at minimum receivede-mail from Kibo. Mail from someone with a Kibo number will make the recipient's number the sender's Kibo number plus one. [*cite book*]

url=http://books.google.com/books?id=71jirhtZCCUC

title=Advanced Learner's Dictionary of Computers

publisher=Anmol Publications

isbn=8126104740**tringfield number**The Stringfield number is an application of the same idea to the field of

Ufology , connecting those who co-investigated or co-researchedUFO cases with the lateLeonard H. Stringfield . [*[*]*http://www.anomalyresponse.org/stringfieldnumber.php Stringfield Number Project*]**eBay auctions**On

April 20 ,2004 , [*http://williamtozier.com/slurry Bill Tozier*] , a researcher with an Erdős number of 4, offered the chance for collaboration to attain an Erdős number of 5 in an auction oneBay . The final bid was $1,031, though apparently the winning bidder had no intention to pay. [*[*] The winner (who already had an Erdős number of 3) considered it a "mockery", and said "papers have to be worked and earned, not sold, auctioned or bought".*http://www.zephoria.org/thoughts/archives/2004/04/21/decrease_your_erdos_number.html Decrease Your Erdős Number*]Another

eBay auction offered an Erdős number of 2 for a prospective paper to be submitted for publication to "Chance" (a magazine of theAmerican Statistical Association ) about skill in theWorld Series of Poker and theWorld Poker Tour . It closed on22 July 2004 with a winning bid of $127.40. This is noteworthy because with the exception of a few co-written articles to be published posthumously, 2 is the lowest number that can now be achieved.**Cultural anecdotes**It is jokingly said that the famous American baseball player

Hank Aaron has an Erdős number of 1 because he autographed a baseball with Erdős whenEmory University awarded them both honorary degrees on the same day.As of June 2007, the

University of Memphis mathematical sciences faculty has the most faculty members with Erdős number 1 of any mathematics department. The Erdős-1 mathematicians areBéla Bollobás ,Ralph Faudree ,Jeno Lehel ,Cecil C. Rousseau , andRichard Schelp . Three of these faculty members are among the top ten most frequent co-authors with Erdős. [*http://www.oakland.edu/enp/Erdos0*] [*http://www.msci.memphis.edu/faculty.html*]**ee also**

*List of people by Erdős number

*Shusaku number

*Hirsch number

*Six degrees of separation

*Small-world network

*Small world phenomenon **References****Further reading***cite journal | author = Goffman, Casper | title = And What Is Your Erdös Number? | journal = American Mathematical Monthly | volume = 76 | issue = 7 | year = 1969 | pages = 791 | doi = 10.2307/2317868

*cite journal | title = Famous Trails to Paul Erdős | author = De Castro, Rodrigo; Grossman, Jerrold W. | journal = The Mathematical Intelligencer | volume = 21 | issue = 3 | year = 1999 | pages = 51–63 | url = http://www.oakland.edu/enp/trails.ps | id = MathSciNet | id = 1709679 Original Spanish version in "Rev. Acad. Colombiana Cienc. Exact. Fís. Natur."**23**(89) 563–582, 1999, MathSciNet | id = 1744115.**External links*** Jerry Grossman, [

*http://www.oakland.edu/enp The Erdös Number Project*] . Contains statistics and a complete list of all mathematicians with an Erdős number less than or equal to 2.

* [*http://www.oakland.edu/enp/collab.pdf "On a Portion of the Well-Known Collaboration Graph"*] , Jerrold W. Grossman and Patrick D. F. Ion.

* [*http://vlado.fmf.uni-lj.si/pub/networks/doc/erdos/erdos.pdf "Some Analyses of Erdős Collaboration Graph"*] , Vladimir Batagelj and Andrej Mrvar.

* American Mathematical Society, [*http://www.ams.org/mathscinet/collaborationDistance.html MR Collaboration Distance*] . A search engine for Erdős numbers and collaboration distance between other authors. Special access required.

* [*http://www.sciencenews.org/articles/20040612/bob8.asp "Theorems for Sale"*] (From Science News, Vol. 165, No. 24, June 12, 2004)

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