A generating tree for 321,hexagon-avoiding permutations

Julian West

Malaspina University-College, British Columbia, Canada

October 27,
refreshments at 3:45pm


The 321,hexagon-avoiding permutations are introduced by Sara Billey and Gregory Warrington in a recent preprint. In the language of permutations with forbidden subsequences, they are the permutations avoiding all of the patterns 46718235, 46781235, 56718234, 56781234, and 321.

Standard `generating tree' techniques lead to a recursive construction of the 321,hexagon-avoiding permutations. The nodes on level n of the generating tree correspond to the 321,hexagon-avoiding permutations on n symbols. Each node is further labelled with four parameters which summarize the important features of the permutation; the subtree below a node can be reconstructed from these four parameters alone.

Speaker's Contact Info: westj(at-sign)mala.bc.ca

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Combinatorics Seminar, Mathematics Department, MIT, sara(at-sign)math.mit.edu

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