2,841 research outputs found
Avoiding patterns in irreducible permutations
International audienceWe explore the classical pattern avoidance question in the case of irreducible permutations, i.e., those in which there is no index such that . The problem is addressed completely in the case of avoiding one or two patterns of length three, and several well known sequences are encountered in the process, such as Catalan, Motzkin, Fibonacci, Tribonacci, Padovan and Binary numbers. Also, we present constructive bijections between the set of Motzkin paths of length and the sets of irreducible permutations of length (respectively fixed point free irreducible involutions of length ) avoiding a pattern for . This induces two new bijections between the set of Dyck paths and some restricted sets of permutations
Distributions of several infinite families of mesh patterns
Br\"and\'en and Claesson introduced mesh patterns to provide explicit
expansions for certain permutation statistics as linear combinations of
(classical) permutation patterns. The first systematic study of avoidance of
mesh patterns was conducted by Hilmarsson et al., while the first systematic
study of the distribution of mesh patterns was conducted by the first two
authors.
In this paper, we provide far-reaching generalizations for 8 known
distribution results and 5 known avoidance results related to mesh patterns by
giving distribution or avoidance formulas for certain infinite families of mesh
patterns in terms of distribution or avoidance formulas for smaller patterns.
Moreover, as a corollary to a general result, we find the distribution of one
more mesh pattern of length 2.Comment: 27 page
Pattern-avoiding alternating words
A word is alternating if either
(when the word is up-down) or (when the word is
down-up). In this paper, we initiate the study of (pattern-avoiding)
alternating words. We enumerate up-down (equivalently, down-up) words via
finding a bijection with order ideals of a certain poset. Further, we show that
the number of 123-avoiding up-down words of even length is given by the
Narayana numbers, which is also the case, shown by us bijectively, with
132-avoiding up-down words of even length. We also give formulas for
enumerating all other cases of avoidance of a permutation pattern of length 3
on alternating words
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