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Possible Duplicate:
Number of equivalence relations splitting set into sets with exactly 3 elements

Given a set $S$ ($|S|$ even), I'm looking for the number of partitions of $S$, so that all elements of such partition have a cardinality of $2$.

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    I voted to close this as a duplicate (see above) because the only difference to that question is the cardinality of the subsets, which is easily generalizable in the answers given there.2011-08-26

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