If X_1,...,X_n is a partition of a 3n-set X into 3-blocks then a(n) is equal to the number of permutations f of X such that f(X_i) <> X_i, (i=1,...,n).

A116219

If X_1,...,X_n is a partition of a 3n-set X into 3-blocks then a(n) is equal to the number of permutations f of X such that f(X_i) <> X_i, (i=1,...,n).