Monday Math: Why 1 Is Not a Prime

I'm currently rereading Jim Butcher's Turn Coat. There is a scene, early on, in which Dresden has a run-in with a naagloshi, a skinwalker out of Navajo legend. The creature is so frightful that seeing it through a wizard's Sight reduces him to gibbering terror; in his struggle to regain control over himself, Dresden begins calculating prime numbers. (And pretty impressively; by the time he reaches a place of temporary sanctuary, he's well into the 2000s.) Unfortunately - to a mathematician's eye - he begins with the number 1.

There may have been a time when mathematicians regarded 1 as a prime number. That time is long past. This may seem confusing, since 1 does meet the condition we're taught in school - that of not being divisible by any number except 1 and itself. However, like much that is taught in school, that criterion has long since been discarded. I'll talk about why, and about various related things, under the cut.

( A Prime Cut! )