If X_1,...,X_n is a partition of a 2n-set X into 2-blocks (or pairs) then a(n) is equal to the number of permutations f of X such that f(X_i) != X_i for all i=1,...n.

A116218

If X_1,...,X_n is a partition of a 2n-set X into 2-blocks (or pairs) then a(n) is equal to the number of permutations f of X such that f(X_i) != X_i for all i=1,...n.

Terms

    a(0) =1a(1) =0a(2) =20a(3) =592a(4) =35088a(5) =3252608a(6) =437765440a(7) =80766186240

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