How many perfect matchings are contained in a given bipartite graph? An exercise in Godsil's 1993 Algebraic Combinatorics solicits proof that this question's answer is an integral involving a certain rook polynomial. Though not widely known, this result appears implicitly in Riordan's 1958 An Introduction to Combinatorial Analysis. It was stated more explicitly and proved independently by S. A. ~Joni and G. -C. ~Rota JCTA 29 (1980), 59--73 and C. D. ~Godsil Combinatorica 1 (1981), 257--262. Another generation later, perhaps it's time both to simplify the proof and to broaden the formula's reach.
Emerson et al. (Fri,) studied this question.
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