This problem is looking for the number of vertices on the hexagonal grid inside the circle of radius r. The center of the hexagon is not counted as a point for this problem, and this is true for every hexagon inside the circle.
Project Euler: Problem 2, Sum of even Fibonacci
Back to basics 8 - Matrix Diagonals
Rotate and display numbered tile
Chebyshev polynomials of the 1st Kind
Chebyshev polynomials of the 2nd Kind
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