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John Francis Riordan (April 22, 1903 – August 27, 1988) was an American mathematician and a pioneering figure in the field of combinatorics. Born in Derby, Connecticut, Riordan was a graduate of Yale University and pursued an academic path that defied the narrow confines of his era. In his early life, he wrote poetry, essays, and even a book of short stories, On the Make , published in 1929. This duality of mathematical rigor and artistic sensibility made him a unique figure among his peers.
Used for permuting elements with a specific number of cycles, or partitioning a set into a specific number of non-empty subsets.