From charlesreid1

Problem Statement

Lattice Points on a Circle

Let f(N) be the number of points with integer coordinates that are on a circle passing through (0,0), (N,0), (0,N), and (N,N).

It can be shown that f(10000) = 36.

What is the sum of all positive integers N ≤ 10^11 such that f(N) = 420?

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