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Catalan numbers

A Catalan number has the form C(2n, n)/(n + 1) for an integer n >= 0, where C denotes a binomial coefficient. It counts balanced arrangements of n pairs of parentheses.

The first few catalan numbers are: 2, 5, 14, 42, 132, 429, 1430, 4862, 16796, 58786, 208012, …

13 of the first one million positive integers are catalan.

Learn more about catalan numbers on wikipedia.

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