Wikipedia: Catalan number
In combinatorial mathematics, the Catalan numbers form a sequence of natural numbers that occur in various counting problems, often involving recursively defined objects. They are named for the Belgian mathematician Eugène Charles Catalan (1814–1894).
The nth Catalan number is given directly in terms of binomial coefficients by
The first Catalan numbers (sequence A000108 in OEIS) for n = 0, 1, 2, 3, … are