13

A Pythagorean triple consists of three positive integers a, b, and c, such that a x a + b x b = c x c. Such a triple is commonly written (a, b, c), and a well-known example is (3, 4, 5). If (a, b, c) is a Pythagorean triple, then so is (ka, kb, kc) for any positive integer k. A primitive Pythagorean triple (PPT) is one in which a, b and c are pairwise coprime. A right triangle whose sides form a Pythagorean triple is called a Pythagorean triangle.

13 forms one of the Pythagorean triangle; 5 x 5 + 12 x 12 = 13 x 13.