Question

A positive integer m can be expressed as the sum of three squares if it is of the form p + q + r where p, q, r ≥ 0, and p, q, r are all perfect squares. For instance, 2 can be written as
0+1+1 but 7 cannot be expressed as the sum of three squares. The first numbers that
cannot be expressed as the sum of three squares are 7, 15, 23, 28, 31, 39, 47, 55, 60, 63,
71, … (Refer Legendre's three-square theorem from Wikipedia)

Answer 1

What is this sum man?