Let's Expand Rota's Twelvefold Way For Counting Partitions!

Abstract

Rota's Twelvefold Way gave formulas for the numbers of partitions which could be formed in twelve scenarios. This proposed AMM article expands Rota's 4 x 3 table. The resulting 6 x 5 table considers a broader collection of splitting-distributing-grouping-arranging scenarios, each of which can be visualized with the distribution of m items into certain kinds of bins. The additional counts or scenarios include: the Bell numbers B(m), the partition numbers p(m), arrangements of m books on b shelves, standings of m teams in a league, arrangements of m books into b scattered stacks, and pairings of 2m items. Teaching remarks are included. The two additional rows (due to K. Bogart) consider ordering the items within the bins. One additional column distributes the items into an unspecified number of bins, each receiving at least one item. The other (due to T. Brylawski) distributes the items into bins such that the number of bins containing a given number of items is specified. The quotient and summation relationships amongst the thirty counts are stated. A closely related table formed by the same six rows and seven certain columns is used to complete and to organize a 6 x 7 family of counting sequences in the On-Line Encyclopedia of Integer Sequences.