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centered hexagonal numbers

A centered hexagonal number counts a central dot surrounded by hexagonal layers, and has the form 3n(n - 1) + 1 for an integer n >= 1.

The first few centered hexagonal numbers are: 7, 19, 37, 61, 91, 127, 169, 217, 271, 331, 397, …

577 of the first one million positive integers are centered hexagonal.

Learn more about centered hexagonal numbers on wikipedia.

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