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centered square numbers

A centered square number counts a central dot surrounded by square layers, and has the form 2n(n - 1) + 1 for an integer n >= 1.

The first few centered square numbers are: 5, 13, 25, 41, 61, 85, 113, 145, 181, 221, 265, …

707 of the first one million positive integers are centered square.

Learn more about centered square numbers on wikipedia.

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