Proof. Suppose that there are non-negative integers x, y and z such that
5
x + 7y = z
2
. By Lemma 2.2 and 2.3, we have x ≥ 1 and y ≥ 1. Note that z
is even. Then z
2 ≡ 0 (mod 4). Moreover, 5x ≡ 1 (mod 4). This implies that
7
y ≡ 3 (mod 4). Thus, y is odd. It follows that 7y ≡ 2 (mod 5) or 7y ≡ 3 (mod
5). We obtain that z
2 ≡ 2 (mod 5) or z
2 ≡ 3 (mod 5). This is impossible.