I have seen 1 called a trivial factor, but I have never seen it excluded entirely from a factor list: perhaps it’s a cultural thing like how 0 is/isn’t a natural number depending on where you are from.
On further research it seems like my earlier critique about requiring exactly two prime factors is a little off in any case, as it would exclude e.g. 4 (which only has one prime factor). It seems like semi primes must be a product of exactly two prime numbers so I think any definition based on number of factors is doomed to over- or under- define these semi primes as they could have either three or four factors.
The definitions often exclude 1. In the case where you include it you could then say a semi prime has exactly three factors.
I have seen 1 called a trivial factor, but I have never seen it excluded entirely from a factor list: perhaps it’s a cultural thing like how 0 is/isn’t a natural number depending on where you are from.
On further research it seems like my earlier critique about requiring exactly two prime factors is a little off in any case, as it would exclude e.g. 4 (which only has one prime factor). It seems like semi primes must be a product of exactly two prime numbers so I think any definition based on number of factors is doomed to over- or under- define these semi primes as they could have either three or four factors.