Counting Permutations with a prescribed number of `forbidden' patterns
(with John Noonan)[Adv. Appl. Math. 17(1996), 381-407]
Stack-Permutations are those that avoid the pattern acb. Wilf asked,
and Julian West and several others considered, the problem of enumerating
permutations that avoid other, longer, patterns. Here we go yet one
step beyond, by tackling the problem of enumerating permutations
that contain the `forbidden' patterns exactly r times.
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Mireille Bousquet-Melou and Ira Gessel made some
interesting comments
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