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Motzkin numbers

A Motzkin number counts the ways to draw nonintersecting chords between n points on a circle, allowing points to remain unused, for an integer n >= 0.

The first few motzkin numbers are: 2, 4, 9, 21, 51, 127, 323, 835, 2188, 5798, 15511, …

16 of the first one million positive integers are motzkin.

Learn more about motzkin numbers on wikipedia.

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