Why isn't one a prime number?
by 
ethicalbro
ANSWERS: 4
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1 used to be considered a prime. Now it is considered to be a "unit". The reason that it was demoted is the Fundamental Theorem of Arithmetic. This states that every number greater than one is the product of one or more primes AND if you put the primes in order, this "prime fingerprint" is the only possible product. For instance 60 = 2*2*3*5 no other product of primes (in order) makes sixty and there's no other number with this product. This means numbers can be "broken down" into easier prime pieces. But for the theorem to work, 1 must not be prime. For 60 = 1*2*2*3*5 and 1*1*1*1*1*2*2*3*5 and 1*1*1*2*2*3*5 and so on. There's no unique product any more.
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Purely by convention. More theorems are made cleaner by calling 1 not a prime than by calling it a prime. If one is a prime, a lot of theorems would have to say "any prime other than 1" rather than "any prime".
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It is a prime number. It just isn't considered in a lot of calculations and formlae. How could it not be?
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