Stirling numbers of the second kind $S(n, k)$ count the number of ways to partition a set of $n$ elements into $k$ nonempty subsets. What if there were duplicate elements in the set? That is, the set is a multiset?
Stirling numbers of the second kind on Multiset
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combinatorics
discrete-mathematics
stirling-numbers
multisets
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2I think [math] is a pretty useless tag in this site; I've deleted it and added [counting] – 2011-01-10
5 Answers
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There is no known formulation for a general multiset. However, a paper at JIS tackles the case where the element 1 occurs multiple times.
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Here are two links to get you started:
Eulerian numbers of the second kind may be helpful (for counting ascents, descents, etc., though i think)
Additionally some more useful information may be found in Stanley's book
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You divide the Stirling number by the possible permutations of identical elements as this is just changing order within the stars and bars example.