Wikipedia isn't a good source, but it uses 3 as an element of Z[√-5] as an example so it's quite relevant: https://en.m.wikipedia.org/wiki/Irreducible_element h

Wikipedia isn't a good source, but it uses 3 as an element of Z[√-5] as an example so it's quite relevant: https://en.m.wikipedia.org/wiki/Irreducible_element https://en.m.wikipedia.org/wiki/Irreducible_element For a more serious reference any abstract algebra book covering rings and UFDs should do, for example it is in Dummit & Foote.