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octagonal numbers

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

The first few octagonal numbers are: 8, 21, 40, 65, 96, 133, 176, 225, 280, 341, 408, …

577 of the first one million positive integers are octagonal.

Learn more about octagonal numbers on wikipedia.

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