\documentclass[11pt,reqno]{amsart} \usepackage[a4paper,margin=30mm]{geometry} \pagestyle{plain} \usepackage{amsmath,amssymb,mathtools} \usepackage{microtype} \usepackage{booktabs} \usepackage[colorlinks=true,linkcolor=blue,citecolor=blue,urlcolor=blue]{hyperref} % ---------------------------------------------------------------- % Theorem Environments % ---------------------------------------------------------------- \newtheorem{theorem}{Theorem}[section] \newtheorem{proposition}[theorem]{Proposition} \newtheorem{lemma}[theorem]{Lemma} \newtheorem{corollary}[theorem]{Corollary} \newtheorem*{conjecture}{Huppert's Conjecture} \theoremstyle{remark} \newtheorem{remark}[theorem]{Remark} \numberwithin{equation}{section} % ---------------------------------------------------------------- % Operators and Macros % ---------------------------------------------------------------- \DeclareMathOperator{\Irr}{Irr} \DeclareMathOperator{\cd}{cd} \DeclareMathOperator{\SL}{SL} \DeclareMathOperator{\PSL}{PSL} \DeclareMathOperator{\PGL}{PGL} \DeclareMathOperator{\GL}{GL} \DeclareMathOperator{\Aut}{Aut} \DeclareMathOperator{\Ind}{Ind} \DeclareMathOperator{\diag}{diag} \newcommand{\FF}{\mathbb{F}} \newcommand{\ZZ}{\mathbb{Z}} % ---------------------------------------------------------------- % Title and Author Metadata % ---------------------------------------------------------------- \title[A Counterexample to Huppert's Conjecture]{A Counterexample to Huppert's Conjecture} \author{Shihao Wang} \address{Shihao Wang, Qiuzhen College, Tsinghua University, Beijing, China} \email{wangshihao25@mails.tsinghua.edu.cn} \author{Zhiwei Zheng} \address{Zhiwei Zheng, Yau Mathematical Sciences Center, Tsinghua University, Beijing, China} \email{zhengzhiwei@mail.tsinghua.edu.cn} \subjclass[2020]{Primary 20C15; Secondary 20D06} \date{} \begin{document} \begin{abstract} Let $S = \PSL_4(5)$ and let $G$ be the unique subgroup of $\PGL_4(5)$ containing $S$ with $[G:S] = 2$. We prove that $G$ and $S$ have the same set of irreducible complex character degrees. Meanwhile, $G$ is centerless and is not isomorphic to $S \times A$ for any finite abelian group $A$. This provides a counterexample to Huppert's conjecture. \end{abstract} \maketitle % ================================================================ \section{Introduction}\label{sec:intro} % ================================================================ For a finite group $H$, let $\Irr(H)$ denote the set of its irreducible complex characters and \[ \cd(H) = \{\chi(1) : \chi \in \Irr(H)\} \] the set of its irreducible character degrees, considered without multiplicities. A natural problem in representation theory is to understand how much of the structure of $H$ is encoded by the set $\cd(H)$. While $\cd(H)$ cannot distinguish between $H$ and a direct product $H \times A$ with a finite abelian group $A$, Huppert \cite[p.~829]{HuppertI} made the following conjecture: \begin{conjecture} Let $S$ be a nonabelian finite simple group and $H$ a finite group. If $\cd(H) = \cd(S)$, then $H \cong S \times A$ for some finite abelian group $A$. \end{conjecture} Huppert's conjecture has been verified for all sporadic simple groups \cite[pp.~289--290]{ADTW2013}, the alternating groups of degree at least $5$ \cite[Theorem~1.1]{BTZ2017}, and numerous families of simple groups of Lie type of low rank (see, for instance, \cite{HuppertI,NTW2012} and the references therein). For the four-dimensional projective special linear groups $\PSL_4(q)$, Nguyen, Tong-Viet, and Wakefield \cite[Theorem~1.1]{NTW2012} established the conjecture for all prime powers $q \ge 13$. For $q\equiv1\pmod4$ in this range, they show that $\cd(H)\ne\cd(\PSL_4(q))$ for every $\PSL_4(q)