The cycle index of a permutation group G is the average of the cycle index monomials of all the permutations g in G.
More formally, let G be a permutation group of order m and degree n. Every permutation g in G has a unique decomposition into disjoint cycles, say c1 c2 c3 ... Let the length of a cycle c be denoted by |c|.
Now let jk(g) be the number of cycles of g of length k, where
We associate to g the monomial
in the variables a1, a2, ... an.
Then the cycle index Z(G) of G is given by
Read more about Cycle Index: Example, Types of Actions, Applications
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