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Bell numbers

A Bell number counts the ways to partition a set of n elements into nonempty subsets, for an integer n >= 0.

The first few bell numbers are: 2, 5, 15, 52, 203, 877, 4140, 21147, 115975, 678570, …

11 of the first one million positive integers are bell.

Learn more about bell numbers on wikipedia.

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