Problem 233
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≤ 1011 such that f(N) = 420 ?

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http://projecteuler.net/problem=233