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centered pentagonal numbers

A centered pentagonal number counts a central dot surrounded by pentagonal layers, and has the form 5n(n - 1)/2 + 1 for an integer n >= 1.

The first few centered pentagonal numbers are: 6, 16, 31, 51, 76, 106, 141, 181, 226, 276, 331, …

632 of the first one million positive integers are centered pentagonal.

Learn more about centered pentagonal numbers on wikipedia.

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