I am using Mathematica 9. If I input:
NumberOfPermutationsByType[{2, 2, 1, 1}]
Mathematica returns 1/8.
I was expecting 45, the number of permutations of {1,2,3,4,5,6} with cycle lengths 2,2,1,1.
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asked Nov 7 '15 at 21:53
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The type of a permutation of length $n$ is $\{\lambda_1, \lambda_2, \ldots, \lambda_n\}$ where $\lambda_i$ is the number of cycles of length $i$. Therefore, the number of permutations of $\{1, 2, 3, 4, 5, 6\}$ that have two 1-cycles and two 2-cycles is
NumberOfPermutationsByType[{2, 2, 0, 0, 0, 0}]
(* 45 *)
The best reference for Combinatorica is Steven Skiena's book, in particular section 3.1.2 (page 99) shows the actual implementation of this function:
NumberOfPermutationsByType[l_List] :=
(Length[l]!)/Apply[Times, Table[l[[i]]!i^(l[[i]]), {i, Length[l]}]]
