From Wikipedia:

Until the 19th century, most mathematicians considered the number 1 a prime, with the definition being just that a prime is divisible only by 1 and itself but not requiring a specific number of distinct divisors. There is still a large body of mathematical work that is valid despite labelling 1 a prime, such as the work of Stern and Zeisel. Derrick Norman Lehmer's list of primes up to 10,006,721, reprinted as late as 1956,So there you have it. They decided in the last 43 years that 1 could no longer be prime so that one of their theorems could be correct. Why does that remind me of someone fudging statistics?^{[2]}started with 1 as its first prime.^{[3]}Henri Lebesgue is said to be the last professional mathematician to call 1 prime.^{[citation needed]}The change in label occurred so that the fundamental theorem of arithmetic, as stated, is valid,i.e., “each number has a unique factorization into primes.”^{[4]}^{[5]}

I know, I know... soapbox, right? First the divide-by-zero post, then this... How about this, as soon as we can explain all our math in simple fractals without saying things like "except 0 or 1" or "with n>1", then I'll change my position on it.

Thus Theorem 1 of Hardy & Wright (1979) takes the form, “Every positive integer, except 1, is a product of primes”bah humbug ;)