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centered triangular numbers

A centered triangular number is formed by arranging successive layers of points around a central point in a triangular pattern.

The first few centered triangular numbers are: 4, 10, 19, 31, 46, 64, 85, 109, 136, 166, 199, …

816 of the first one million positive integers are centered triangular.

Learn more about centered triangular numbers on wikipedia.

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