Interval structures in the Bruhat and weak orders

We study the appearance of notable interval structures—lattices, modular lattices, distributive lattices, and boolean lattices—in both the Bruhat and weak orders of Coxeter groups. We collect and expand upon known results for principal order ideals, including pattern characterizations and enumerations for the symmetric group. This segues naturally into a similar analysis for arbitrary intervals, although the results are less characterizing for the Bruhat order at this generality. In counterpoint, however, we obtain a full characterization for intervals starting at rank one in the symmetric group, for each of the four structure types, in each of the two posets. Each category can be enumerated, with intriguing connections to Fibonacci and Catalan numbers. We conclude with suggestions for further directions and questions, including an interesting analysis of the intervals formed between a permutation and each generator in its support.

Keywords

Coxeter group, Bruhat order, weak order, interval, lattice, boolean, Catalan, Fibonacci

2010 Mathematics Subject Classification

Primary 20F55. Secondary 05A05, 05A15, 06A07.

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Research partially supported by Simons Foundation Collaboration Grant for Mathematicians 277603, and by a University Research Council Competitive Research Leave from DePaul University.

Received 18 May 2020

Accepted 31 January 2021

Published 31 January 2022