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\newtheorem{prop}{Proposition} \newtheorem{cor}{Corollary} \newtheorem*{utheorem}{Theorem} \newtheorem*{ulemma}{Lemma} \newtheorem*{uprop}{Proposition} \newtheorem*{ucor}{Corollary} \theoremstyle{definition} \newtheorem{defn}{Definition} \newtheorem{example}{Example} \newtheorem*{udefn}{Definition} \newtheorem*{uexample}{Example} \theoremstyle{remark} \newtheorem{remark}{Remark} \newtheorem{note}{Note} \newtheorem*{uremark}{Remark} \newtheorem*{unote}{Note} %------------------------------------------------------------------- \begin{document} %------------------------------------------------------------------- \section*{character of a linear representation} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{representation_theory}{}\paragraph*{{Representation theory}}\label{representation_theory} [[!include representation theory - contents]] \hypertarget{linear_algebra}{}\paragraph*{{Linear algebra}}\label{linear_algebra} [[!include homotopy - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{idea}{Idea}\dotfill \pageref*{idea} \linebreak \noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{the_character_homomorphism}{The character homomorphism}\dotfill \pageref*{the_character_homomorphism} \linebreak \noindent\hyperlink{SchurInnerProduct}{Schur inner product}\dotfill \pageref*{SchurInnerProduct} \linebreak \noindent\hyperlink{SpecialCharacterValues}{Special character values}\dotfill \pageref*{SpecialCharacterValues} \linebreak \noindent\hyperlink{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} In [[representation theory]], one defines the \emph{character of a [[linear representation]] $\rho\colon G\to End(V)$ to be the [[group character]] on $G$ given by $g \mapsto Tr \rho(g)$, whenever the [[trace]] in $V$ makes sense (e.g. when $V$ is [[finite-dimensional vector space|finite-dimensional]]). Since such a function is invariant under conjugation, we may equivalently consider it a function on the set of [[conjugacy classes]] of elements in $G$.} Sometimes we also extend a character linearly to the free vector space on the set of conjugacy classes. This version of the character can be identified with the [[bicategorical trace]] of the identity map of the representation, considered as a $k[G]$-$k$-module. There is a different notion of an \emph{infinitesimal character} in [[Harish–Chandra theory]] and also a notion of the \emph{formal character}. There are important formulas concerning characters in representation theory, like [[Weyl character formula]], Kirillov character formula, Demazure character formula and so on. There is also a formula for the [[induced character]] of an [[induced representation]]. \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{the_character_homomorphism}{}\subsubsection*{{The character homomorphism}}\label{the_character_homomorphism} \begin{prop} \label{InCharZeroCharacterMorphismIsInjective}\hypertarget{InCharZeroCharacterMorphismIsInjective}{} \textbf{(in [[characteristic zero]] the [[character homomorphism]] is [[injective function|injective]])} For $G$ a [[finite group]] and $k$ a [[field]] of [[characteristic zero]], the [[character homomorphism]] \begin{displaymath} \itexarray{ Rep_k(G)_{/\sim} &\overset{\chi}{\longrightarrow}& k^{ConjCl(G)} \\ V &\mapsto& \big( [g] \mapsto \chi_V(g) \mathrlap{ \coloneqq \mathrm{tr}_V(g) \big) } } \end{displaymath} is [[injective function|injective]]. \end{prop} (e.g. \hyperlink{tomDieck09}{tom Dieck 09, Theorem (2.1.3)}) \hypertarget{SchurInnerProduct}{}\subsubsection*{{Schur inner product}}\label{SchurInnerProduct} \begin{prop} \label{SchurInnerProductInTermsOfCharacters}\hypertarget{SchurInnerProductInTermsOfCharacters}{} \textbf{(\href{Schur's+lemma#InterpretationInCategoricalAlgebra}{Schur inner product} in terms of [[character of a linear representation|characters]])} Let $G$ be a [[finite group]] of [[order of a group|order]] ${\vert G \vert} \in \mathbb{N}$, and let $V_1, V_2 \in Rep_k(G)$ be two [[linear representation]] over a [[ground field]] $k$ of [[characteristic zero]]. Then the \href{Schur's+lemma#InterpretationInCategoricalAlgebra}{Schur inner product} of the two representation equals the [[convolution product]] of their [[character of a linear representation|characters]], normalized by the [[order of a group|order]] of $G$: \begin{equation} \left\langle V_1, V_2 \right\rangle \;\coloneqq\; dim_k\Big( Hom\big( V_1, V_2 \big) \Big) \;=\; \frac{1}{\left\vert G\right\vert} \underset{g \in G}{\sum} \chi_{V_1}(g^{-1}) \cdot \chi_{V_2}(g) \,. \label{OrthogonalityRelation}\end{equation} \end{prop} (e.g. \hyperlink{tomDieck09}{tom Dieck 09 (2.5)}) \begin{example} \label{NormalizedSumOfCharacters}\hypertarget{NormalizedSumOfCharacters}{} \textbf{(normalized [[sum]] of [[character of a linear representation|characters]] is [[fixed point space]]-[[dimension]])} Let $G$ be a [[finite group]] of [[order of a group|order]] ${\vert G \vert} \in \mathbb{N}$, and let $V \in Rep_k(G)$ be a [[linear representation]] over a [[ground field]] $k$ of [[characteristic zero]]. Then the [[dimension]] of the [[fixed point space]] $V^G$ of $V$ under $G$ equals the normalized [[sum]] of the character values over all group elements: \begin{equation} dim\left( V^G \right) \;=\; \frac{1}{{\vert G\vert}} \underset{g \in G}{\sum} \chi_V(g) \,. \label{SumOverAllCharacterValues}\end{equation} In particular \begin{enumerate}% \item if $V = \rho \neq \mathbf{1}$ is a non-[[trivial representation|trivial]] [[irreducible representation]] then \end{enumerate} \begin{displaymath} \underset{g \in G}{\sum} \chi_{\rho}(g) \;=\; 0 \end{displaymath} \begin{enumerate}% \item if $V = \mathbf{1}$ is the the [[trivial representation|trivial]] [[irreducible representation]] then \end{enumerate} \begin{displaymath} \underset{g \in G}{\sum} \chi_{\mathbf{1}}(g) \;=\; \left\vert G\right\vert \end{displaymath} \end{example} (e.g. \hyperlink{tomDieck09}{tom Dieck 09 (2.1)}) \begin{proof} The first statement is a special case of Prop. \ref{SchurInnerProductInTermsOfCharacters} by observing that the [[fixed point space]] of a [[linear representation]] $V$ is the space of [[homomorphisms]] from the [[trivial representation|trivial]] [[irrep]] \begin{displaymath} dim\big( V^G\big) \;=\; dim\Big( \mathrm{Hom}\big( \mathbf{1}, V \big) \Big) \end{displaymath} and that the [[character of a linear representation|character]] of $\mathbf{1}$ is [[constant function|constant]] on $1 \in k$. The second statement follows from this by [[Schur's lemma]], which says that $\langle \mathbf{1}, \rho\rangle = 0$ if the [[irrep]] $\rho \neq \mathbf{1}$. The last statement may similarly be seen as the complementary special case of \eqref{SumOverAllCharacterValues} obtained from $\langle \mathbf{1},\mathbf{1}\rangle =1$, but of course it also follows directly from the fact that the [[character of a linear representation|character]] of the [[trivial representation|trivial]] [[irrep]] is [[constant function|constant]] on 1. \end{proof} \hypertarget{SpecialCharacterValues}{}\subsubsection*{{Special character values}}\label{SpecialCharacterValues} \begin{prop} \label{CharactersAreCyclotomicIntegers}\hypertarget{CharactersAreCyclotomicIntegers}{} \textbf{([[characters are cyclotomic integers]])} Let $G$ be a [[finite group]], and let $V$ be a [[finite-dimensional vector space|finite-dimensional]] [[linear representation]] over a [[ground field]] $k$ Then the values of the [[character of a representation|character]] $\chi_V \colon G \to k$ of $V$ are [[cyclotomic integers]] over $k$, for some [[root of unity]]. \end{prop} (see e.g. \hyperlink{characters+are+cyclotomic+integers#Naik}{Naik}) In particular it follows that: \begin{prop} \label{}\hypertarget{}{} If the [[ground field]] $k$ in Prop. \ref{CharactersAreCyclotomicIntegers} has [[characteristic zero]], then a [[character of a linear representation]] which takes values in the [[rational numbers]] $\mathbb{Q} \hookrightarrow k$ in fact already takes values in the [[integers]] $\mathbb{Z} \hookrightarrow\mathbb{Q} \hookrightarrow k$. \end{prop} (see e.g. \hyperlink{characters+are+cyclotomic+integers#Yang}{Yang}) \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[equivariant K-theory]] \item [[fractional D-brane]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[Tammo tom Dieck]], chapter 2 of \emph{Representation theory}, 2009 (\href{http://www.uni-math.gwdg.de/tammo/rep.pdf}{pdf}) \end{itemize} For more see the references at \emph{[[representation theory]]}. Character rings of [[compact Lie groups]] are discussed in \begin{itemize}% \item [[Graeme Segal]], \emph{The representation ring of a compact Lie group}, Publications Math\'e{}matiques de l'Institut des Hautes \'E{}tudes Scientifiques January 1968, Volume 34, Issue 1, pp 113-128 (\href{http://archive.numdam.org/numdam-bin/fitem?id=PMIHES_1968__34__113_0}{NUMDAM}) \item Masaru Tackeuchi, \emph{A remark on the character ring of a compact Lie group}, J. Math. Soc. Japan Volume 23, Number 4 (1971), 555-705 (\href{http://projecteuclid.org/euclid.jmsj/1259849785}{Euclid}) \item Troels Roussauc Johansen, \emph{Character Theory for Finite Groups and Compact Lie Groups} \href{http://www.math.upb.de/~johansen/character-theory.pdf}{pdf} \end{itemize} Discussion in the more general context of [[equivariant cohomology|equivariant]] [[complex oriented cohomology theory]] ([[transchromatic character]]) is in \begin{itemize}% \item [[Michael Hopkins]], [[Nicholas Kuhn]], [[Douglas Ravenel]], \emph{Generalized group characters and complex oriented cohomology theories}, J. Amer. Math. Soc. 13 (2000), 553-594 (\href{http://www.ams.org/journals/jams/2000-13-03/S0894-0347-00-00332-5/}{publisher}, \href{http://www.math.rochester.edu/people/faculty/doug/mypapers/hkr.pdf}{pdf}) \end{itemize} for review see \begin{itemize}% \item Arpon Raksit, \emph{Characters in global equivariant homotopy theory}, 2015 \href{http://web.stanford.edu/~arpon/math/files/senior-thesis.pdf}{pdf} \end{itemize} Examples of [[characters of linear representations]] of [[finite groups]] are discussed and listed at \begin{itemize}% \item [[James Montaldi]], \emph{\href{http://www.maths.manchester.ac.uk/~jm/wiki/Representations/Representations}{representations}}, 2008 \end{itemize} [[!redirects characters of linear representations]] [[!redirects character of a representation]] [[!redirects characters of representations]] [[!redirects character homomorphism]] [[!redirects character homomorphisms]] [[!redirects character morphism]] [[!redirects character morphisms]] [[!redirects character table]] [[!redirects character tables]] \end{document}