3. Main Results
Theorem 3.1. The Diophantine equation 5x + 43y = z2 has no nonnegative integer solution where y and z are non-negative integers.
Proof. Suppose that there are non-negative integers x, y and z such that
5x +43y = z2. By Lemma 2.2, we have y ≥ 1. Note that z is even. Then z2 ≡ 0
(mod 4). Since 5x ≡ 1 (mod 4), it follows that 43y ≡ 3 (mod 4). Then 3y ≡ 3
(mod 4). This implies that y is odd. Note that 43y ≡ 2 (mod 5) or 43y ≡ 3
(mod 5). By Lemma 2.3, we have x ≥ 1. Since 5x ≡ 0 (mod 5), it follows that