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Permutations which are the union of an increasing and a decreasing subsequence. (English) Zbl 0885.05011

Electron. J. Comb. 5, Research paper R6, 13 p. (1998); printed version J. Comb. 5, 87-99 (1998).
Summary: It is shown that there are \({2n\choose n}-\sum_{m=0}^{n-1}2^{n-m-1}{2m\choose m}\) permutations which are the union of an increasing sequence and a decreasing sequence.

MSC:

05A15 Exact enumeration problems, generating functions
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