Some good answers and discussion at https://math.stackexchange.com/questions/120/why-is-1-not-a-prime-number/ https://math.stackexchange.com/questions/120/why-i

Some good answers and discussion at https://math.stackexchange.com/questions/120/why-is-1-not-a-prime-number/ https://math.stackexchange.com/questions/120/why-is-1-not-a-... In particular, the papers "What is the smallest prime?" https://arxiv.org/abs/1209.2007 https://arxiv.org/abs/1209.2007 and "The History of the Primality of One: A Selection of Sources" https://cs.uwaterloo.ca/journals/JIS/VOL15/Caldwell2/cald6.html https://cs.uwaterloo.ca/journals/JIS/VOL15/Caldwell2/cald6.h... have good history on the matter (e.g. 1, 2, and 3 were all answers at various points — note that at some point in history 1 wasn't even considered a number).