Subcase 2.2 x ≥ 1. Since 2k − 1 is odd, (2k − 1)x + (2k)y is odd. This implies that z2 is odd and finally we get z is odd. Then z2 ≡ 1 (mod 4). Since z2 −(2k)y ≡ 1 (mod 4) it follows that (2k −1)x ≡ 1 (mod 4). But(2k −1)x ≡ 3x(mod 4).
Thus 3x ≡ 1 (mod 4) and it is not hard to see that x is even.
Now, let us consider