# Families of nested graphs with compatible symmetric-group actions

Authors
• 1 University of Oregon, Department of Mathematics, Fenton Hall, Eugene, OR, 97401, USA , Eugene (United States)
• 2 Indiana University - Bloomington, Department of Mathematics, Rawles Hall, Bloomington, IN, 47405, USA , Bloomington (United States)
Type
Published Article
Journal
Selecta Mathematica
Publisher
Springer International Publishing
Publication Date
Nov 08, 2019
Volume
25
Issue
5
Identifiers
DOI: 10.1007/s00029-019-0520-9
Source
Springer Nature
Keywords
For fixed positive integers n and k, the Kneser graph KGn,k\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$KG_{n,k}$$\end{document} has vertices labeled by k-element subsets of {1,2,⋯,n}\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\{1,2,\dots ,n\}$$\end{document} and edges between disjoint sets. Keeping k fixed and allowing n to grow, one obtains a family of nested graphs, each of which is acted on by a symmetric group in a way which is compatible with these inclusions and the inclusions of each symmetric group into the next. In this paper, we provide a framework for studying families of this kind using the FI\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\,\mathrm{FI}\,}}$$\end{document}-module theory of Church et al. (Duke Math J 164(9):1833–1910, 2015), and show that this theory has a variety of asymptotic consequences for such families of graphs. These consequences span a range of topics including enumeration, concerning counting occurrences of subgraphs, topology, concerning Hom-complexes and configuration spaces of the graphs, and algebra, concerning the changing behaviors in the graph spectra.