cannonball problem

Etymology
It can be visualized as the problem of taking a square arrangement of cannonballs on the ground and building a square pyramid out of them.

Proper noun

 * 1)  The challenge of proving that the only solution of the Diophantine equation :$$\sum_{n=1}^{N} n^2 = M^2\;$$ with N > 1 is when N = 24 and M = 70.