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heptagonal numbers

A heptagonal number is a positive integer of the form n(5n - 3)/2 for an integer n >= 1.

The first few heptagonal numbers are: 7, 18, 34, 55, 81, 112, 148, 189, 235, 286, 342, …

632 of the first one million positive integers are heptagonal.

Learn more about heptagonal numbers on wikipedia.

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