9161

This number is a prime.

The primitive right triangle (5289, 7480, 9161) has a remarkable property: c = 9161 is prime, and not only does a^2 + b^2 equal a square (c^2, obviously) but, in addition, a + b is a square (5289 + 7480 = 113^2). There are only three smaller right triangles like this, with hypotenuses 41, 389, and 4549. [Keith]