Editing Prime number (section)

=== Primality of one ===
Most early Greeks did not even consider 1 to be a number,<ref name="crxk-34">{{cite journal | last1 = Caldwell | first1 = Chris K. | last2 = Reddick | first2 = Angela | last3 = Xiong | first3 = Yeng | last4 = Keller | first4 = Wilfrid | issue = 9 | journal = [[Journal of Integer Sequences]] | mr = 3005523 | page = Article 12.9.8 | title = The history of the primality of one: a selection of sources | url = https://cs.uwaterloo.ca/journals/JIS/VOL15/Caldwell2/cald6.html | volume = 15 | year = 2012 }} For a selection of quotes from and about the ancient Greek positions on the status of 1 and 2, see in particular pp. 3–4. For the Islamic mathematicians, see p. 6.</ref><ref>{{cite book|title=Speusippus of Athens: A Critical Study With a Collection of the Related Texts and Commentary|volume=39|series=Philosophia Antiqua : A Series of Monographs on Ancient Philosophy|first=Leonardo|last=Tarán|publisher=Brill|year=1981|isbn=978-90-04-06505-5|pages=35–38|url=https://books.google.com/books?id=cUPXqSb7V1wC&pg=PA35}}</ref> so they could not consider its primality. A few scholars in the Greek and later Roman tradition, including [[Nicomachus]], [[Iamblichus]], [[Boethius]], and [[Cassiodorus]], also considered the prime numbers to be a subdivision of the odd numbers, so they did not consider {{tmath|2}} to be prime either. However, Euclid and a majority of the other Greek mathematicians considered {{tmath|2}} as prime. The [[Mathematics in medieval Islam|medieval Islamic mathematicians]] largely followed the Greeks in viewing 1 as not being a number.<ref name="crxk-34"/> By the Middle Ages and Renaissance, mathematicians began treating 1 as a number, and by the 17th century some of them included it as the first prime number.<ref>{{harvnb|Caldwell|Reddick|Xiong|Keller|2012}}, pp. 7–13. See in particular the entries for Stevin, Brancker, Wallis, and Prestet.</ref> In the mid-18th century, [[Christian Goldbach]] listed 1 as prime in his correspondence with [[Leonhard Euler]];<ref>{{harvnb|Caldwell|Reddick|Xiong|Keller|2012}}, pp. 6–7.</ref> however, Euler himself did not consider 1 to be prime.<ref>{{harvnb|Caldwell|Reddick|Xiong|Keller|2012}}, p. 15.</ref> Many 19th century mathematicians still considered 1 to be prime,<ref name="cx"/> and [[Derrick Norman Lehmer]] included 1 in his ''list of primes less than ten million'' published in 1914.{{sfn|Conway|Guy|1996|pp=130}} Lists of primes that included 1 continued to be published as recently {{nowrap|as 1956.<ref>{{cite book | last=Riesel | first=Hans | author-link= Hans Riesel | title=Prime Numbers and Computer Methods for Factorization | publisher=Birkhäuser | location=Basel, Switzerland | isbn=978-0-8176-3743-9 | year=1994|page=36|edition=2nd|mr=1292250|url=https://books.google.com/books?id=ITvaBwAAQBAJ&pg=PA36 | doi=10.1007/978-1-4612-0251-6 }}</ref><ref name="cg-bon-129-130">{{cite book | last1=Conway | first1=John Horton | author1-link=John Horton Conway | last2=Guy | first2=Richard K. | author2-link=Richard K. Guy | title=The Book of Numbers |title-link=The Book of Numbers (math book) | publisher=Copernicus | location=New York | isbn=978-0-387-97993-9 | year=1996 | pages = [https://archive.org/details/bookofnumbers0000conw/page/129 129–130] | mr=1411676 | doi=10.1007/978-1-4612-4072-3 }}</ref>}} However, by the early 20th century mathematicians began to agree that 1 should not be listed as prime, but rather in its own special category as a "[[Unit (ring theory)|unit]]".<ref name="cx"/><!--pp=6-8-->

If 1 were to be considered a prime, many statements involving primes would need to be awkwardly reworded. For example, the fundamental theorem of arithmetic would need to be rephrased in terms of factorizations into primes greater than 1, because every number would have multiple factorizations with any number of copies of&nbsp;1.<ref name="cx">{{cite journal | last1 = Caldwell | first1 = Chris K. | last2 = Xiong | first2 = Yeng | issue = 9 | journal = [[Journal of Integer Sequences]] | mr = 3005530 | page = Article 12.9.7 | title = What is the smallest prime? | url = https://cs.uwaterloo.ca/journals/JIS/VOL15/Caldwell1/cald5.pdf | volume = 15 | year = 2012}}</ref> Similarly, the [[sieve of Eratosthenes]] would not work correctly if it handled 1 as a prime, because it would eliminate all multiples of 1 (that is, all other numbers) and output only the single number&nbsp;1.<ref name="cg-bon-129-130"/> Some other more technical properties of prime numbers also do not hold for the number 1: for instance, the formulas for [[Euler's totient function]] or for the [[Sum-of-divisors function|sum of divisors function]] are different for prime numbers than they are for&nbsp;1.<ref>For the totient, see {{harvnb|Sierpiński|1988}}, [https://books.google.com/books?id=ktCZ2MvgN3MC&pg=PA245 p. 245]. For the sum of divisors, see {{cite book|title=How Euler Did It|series=MAA Spectrum|first=C. Edward|last=Sandifer|publisher=Mathematical Association of America|year=2007|isbn=978-0-88385-563-8|page=59|url=https://books.google.com/books?id=sohHs7ExOsYC&pg=PA59}}</ref>