Crystals and trees: quasi-Kashiwara operators, monoids of binary trees, and Robinson--Schensted-type correspondences

TL;DR

This paper develops a new crystal-type combinatorial structure for the sylvester and Baxter monoids, linking them to Robinson--Schensted correspondences and analyzing their algebraic properties.

Contribution

It introduces a novel 'crystal-type' structure for sylvester and Baxter monoids, extending crystal graph concepts beyond classical cases.

Findings

Both monoids arise from the new structure similarly to the plactic monoid.

The structure interacts with Robinson--Schensted--Knuth correspondences for these monoids.

Results on factorizations and identities within these monoids are established.

Abstract

Kashiwara's crystal graphs have a natural monoid structure that arises by identifying words labelling vertices that appear in the same position of isomorphic components. The celebrated plactic monoid (the monoid of Young tableaux), arises in this way from the crystal graph for the $q$-analogue of the general linear Lie algebra $\mathfrak{gl}_{n}$, and the so-called Kashiwara operators interact beautifully with the combinatorics of Young tableaux and with the Robinson--Schensted--Knuth correspondence. The authors previously constructed an analogous `quasi-crystal' structure for the related hypoplactic monoid (the monoid of quasi-ribbon tableaux), which has similarly neat combinatorial properties. This paper constructs an analogous `crystal-type' structure for the sylvester and Baxter monoids (the monoids of binary search trees and pairs of twin binary search trees, respectively). Both monoids are shown to arise from this structure just as the plactic monoid does from the usual crystal graph. The interaction of the structure with the sylvester and Baxter versions of the Robinson-Schensted-Knuth correspondence is studied. The structure is then applied to prove results on the number of factorizations of elements of these monoids, and to prove that both monoids satisfy non-trivial identities.