Question 42
Prove that the product of any two elements of the set
$$
\{m \mid m = a^2 - 5b^2,\ a, b \in \mathbb{N}\}
$$
belongs to that set as well.
$$
\{m \mid m = a^2 - 5b^2,\ a, b \in \mathbb{N}\}
$$
belongs to that set as well.
MathNet 2011 Question 42
Math Olympiad Vault · MathNet 2011