cannonball problem

== English == === 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 === cannonball problem (mathematics) The challenge of proving that the only solution of the Diophantine equation : ∑ n = 1 N n 2 = M 2 {\displaystyle \sum _{n=1}^{N}n^{2}=M^{2}\;} with N > 1 is when N = 24 and M = 70.