For questions about the study of finite or countable discrete structures, specifically how to count or enumerate elements in a set (perhaps of all possibilities) or any subset. It includes questions on permutations, combinations, bijective proofs, and generating functions.

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seating at two round tables

We have $n$ people sitting in two round tables like the picture. We randomly change the place of the people we decide two of them and change their places. We can only change the place of the ...