Callan, David and Mansour, Toufik and Shattuck, Mark - Wilf classification of triples of 4-letter patterns I

dmtcs:3218 - Discrete Mathematics & Theoretical Computer Science, March 24, 2017, Vol 19 no. 1
Wilf classification of triples of 4-letter patterns I

Authors: Callan, David and Mansour, Toufik and Shattuck, Mark

This is the first of two papers in which we determine all 242 Wilf classes of triples of 4-letter patterns by showing that there are 32 non-singleton Wilf classes. There are 317 symmetry classes of triples of 4-letter patterns and after computer calculation of initial terms, the problem reduces to showing that counting sequences that appear to be the same (agree in the first 16 terms) are in fact identical. This amounts to counting avoiders for 107 representative triples. The insertion encoding algorithm (INSENC) applies to many of them and some others have been previously counted. Thus there remain 36 triples. In this paper, we find the generating function for the first 18 of these triples and in a second paper, we treat the other 18. The generating function turns out to be algebraic in each case. Our methods are both combinatorial and analytic, including decompositions by left-right maxima and by initial letters. Sometimes this leads to an algebraic equation for the generating function, sometimes to a functional equation or a multi-index recurrence that succumbs to the kernel method. A bijection is used in one of the cases (Case 50).


Source : oai:HAL:hal-01321512v3
Volume: Vol 19 no. 1
Section: Combinatorics
Published on: March 24, 2017
Submitted on: March 24, 2017
Keywords: Wilf-equivalence,pattern avoidance,kernel method,insertion encoding algorithm,[MATH] Mathematics [math]


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