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 $2$.
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 $2$.