Lattice points on a circle
Published on 20 February 2009 at 09:00 pm [Server Time]
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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