Yes, this is a Number Theory problem (Mathematicians call the Integers, Z, for Zahlen, a German word.) However, remember for example, when factoring x^2 - 1 = y, y in Z, the left hand side becomes (x+1)(x-1) = y. That is, x is still in Z. However, factoring x^2 + 1 requires the Gaussian Integers Z, i.e., x^2 + 1 is not factorable in Z, but in Z, ( x + sqrt(-1) ) ( x - sqrt(-1) ) = y. If you expand and collect the left hand side, you will see that it equals x + 1. nitro