Combinatorial specifications for juxtapositions of permutation classes

Brignall, Robert and Sliacan, Jakub (2019). Combinatorial specifications for juxtapositions of permutation classes. Electronic Journal of Combinatorics, 26(4), article no. P4.4.

URL: https://www.combinatorics.org/ojs/index.php/eljc/a...

Abstract

We show that, given a suitable combinatorial specification for a permutation class C, one can obtain a specification for the juxtaposition (on either side) of C with Av(21) or Av(12), and that if the enumeration for C is given by a rational or algebraic generating function, so is the enumeration for the juxtaposition. Furthermore this process can be iterated, thereby providing an effective method to enumerate any "skinny" k×1 grid class in which at most one cell is non-monotone, with a guarantee on the nature of the enumeration given the nature of the enumeration of the non-monotone cell.

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