is this true even if (a,b) ≠ 1? if (a,b) = 1, yes, i know that it's isomorphic to Z/(a^2+b^2)Z, although i would like to see a cleaner version of the isomorphism than what i came up with.
but what if a+bi = d(k+mi)? for example, what is Z[i]/<4+6i>? really needing some help, here...
Ok, I missed that coprimality condition, but the answer's still the same, though it must be fixed.
You really want to read http://home.wlu.edu/~dresdeng/papers/factorrings.pdf , in particular