Question 10
Find all positive integers $k$ such that there exist positive integers $a$, $b$ such that
$$
a^2 + 4 = (k^2 - 4)b^2.
$$
$$
a^2 + 4 = (k^2 - 4)b^2.
$$
MathNet 2023-part-06 — 191 problems (EN–VI) Question 10
Math Olympiad Vault · MathNet 2023-part-06 — 191 problems (EN–VI)