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pentagonal numbers

A pentagonal number is a positive integer of the form n(3n - 1)/2 for an integer n >= 1.

The first few pentagonal numbers are: 5, 12, 22, 35, 51, 70, 92, 117, 145, 176, 210, …

816 of the first one million positive integers are pentagonal.

Learn more about pentagonal numbers on wikipedia.

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