Proof:
Let, n be a sum-difference number. Then there exist integers w, x, y, and z such that, n = xy = wz with x – y = w + z. Let, k be a positive integer. Then nk2 = (kx) (ky) = (kw) (kz). Since, x – y = w + z, we know k(x – y) = k(w + z) which gives kx – ky = kw + kz. Thus, nk2 is also a sum-difference number