# What does the Combinatorica function: NumberOfPermutationsByType return?

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.

## 1 Answer

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]}]]