Let p be either a prime number, or -1. Prove that the following norm defined on Z[sqrt(p)] (a + bsqrt(p), a,b are in Z )

N(a+bsqrt(p))= /a^2 -pb^2/. ( the bars mean absolute value)

If N(a+bsqrt(p)) is a prime number, then a+bsqrt(p) is irreducible.