eli5-Why is the number 1 not considered a prime number even though there isn’t any number that divides it?
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The common definition of prime is a little bit incomplete, and when you add the concept of unit everything starts making sense
A unit is a number that is invertible, that is, u is a number such that it exists a number a that satisfies a*u = 1. Then, you define a prime as a number that is NOT a unit, and is only divisible by itself times a unit or a unit
In the natural numbers there is only one unit: 1, and because of it being a unit, it is not considered a prime
If you thought about primes with both positive and negative integers, though, you’d have two units: 1 and -1, and numbers like -2, -5 and -7 would also be primes
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