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Dynamics and limits in algebraic combinatorics

Time: Fri 2020-12-11 15.00

Location: Via zoom, (English)

Subject area: Mathematics

Doctoral student: Samu Potka , Matematik (Avd.)

Opponent: Professor Victor Reiner, University of Minnesota

Supervisor: Professor Svante Linusson, Matematik (Avd.)

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This thesis consists of the following six articles.

  • Properties of the Edelman-Greene bijection. Edelman and Greene constructed a correspondence between reduced words of the reverse permutation and standard Young tableaux. We prove that for any reduced word the shape of the region of the insertion tableau containing the smallest possible entries evolves exactly as the upper-left component of the permutation's (Rothe) diagram. Properties of the Edelman-Greene bijection restricted to 132-avoiding and 2143-avoiding permutations are presented. We also consider the Edelman-Greene bijection applied to non-reduced words.
  • On random shifted standard Young tableaux and 132-avoiding sorting networks. We study shifted standard Young tableaux (SYT). The limiting surface of uniformly random shifted SYT of staircase shape is determined, with the integers in the SYT as heights. This implies via properties of the Edelman-Greene bijection results about random 132-avoiding sorting networks, including limit shapes for trajectories and intermediate permutations. Moreover, the expected number of adjacencies in SYT is considered. It is shown that on average each row and each column of a shifted SYT of staircase shape contains precisely one adjacency.
  • The cyclic sieving phenomenon on circular Dyck paths. We give a q-enumeration of circular Dyck paths, which is a superset of the classical Dyck paths enumerated by the Catalan numbers. These objects have recently been studied by Alexandersson and Panova. Furthermore, we show that this q-analogue exhibits the cyclic sieving phenomenon under a natural action of the cyclic group. The enumeration and cyclic sieving is generalized to Möbius paths. We also discuss properties of a generalization of cyclic sieving, which we call subset cyclic sieving, and introduce the notion of Lyndon-like cyclic sieving that concerns special recursive properties of combinatorial objects exhibiting the cyclic sieving phenomenon.
  • The exact phase diagram for a semipermeable TASEP with nonlocal boundary jumps. We consider a finite one-dimensional totally asymmetric simple exclusion process (TASEP) with four types of particles, 1, 0, -1, and *, in contact with reservoirs. Particles of species 0 can neither enter nor exit the lattice, and those of species * are constrained to lie at the first and last site. Particles of species 1 enter from the left reservoir into either the first or second site, move rightwards, and leave from either the last or penultimate site. Conversely, particles of species -1 enter from the right reservoir into either the last or penultimate site, move leftwards, and leave from either the first or second site. This dynamics is motivated by a natural random walk on the Weyl group of type D. We compute the exact nonequilibrium steady state distribution using a matrix ansatz building on earlier work of Arita. We then give explicit formulas for the nonequilibrium partition function as well as densities and currents of all species in the steady state, and derive the phase diagram.
  • Limiting directions for random walks in classical affine Weyl groups. Let W be a finite Weyl group and W_a the corresponding affine Weyl group. A random element of W_a can be obtained as a reduced random walk on the alcoves of W_a. By a theorem of Lam (Ann. Prob. 2015), such a walk almost surely approaches one of |W| many directions. We compute these directions when W is B_n, C_n and D_n and the random walk is weighted by Kac and dual Kac labels. This settles Lam's questions for types B and C in the affirmative and for type D in the negative. The main tool is a combinatorial two row model for a totally asymmetric simple exclusion process called the D*-TASEP, with four parameters. By specializing the parameters in different ways, we obtain TASEPs for each of the Weyl groups mentioned above. Computing certain correlations in these TASEPs gives the desired limiting directions.
  • Refined Catalan and Narayana cyclic sieving. We prove several new instances of the cyclic sieving phenomenon (CSP) on Catalan objects of type A and type B. Moreover, we refine many of the known instances of the CSP on Catalan objects. For example, we consider triangulations refined by the number of "ears", non-crossing matchings with a fixed number of short edges, and non-crossing configurations with a fixed number of loops and edges.