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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*{Brauer induction theorem} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{representation_theory}{}\paragraph*{{Representation theory}}\label{representation_theory} [[!include representation theory - 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{SnaithExpansion}{Snaith's explicit Brauer induction}\dotfill \pageref*{SnaithExpansion} \linebreak \noindent\hyperlink{SymondExplicitBrauerInduction}{Symonds' explicit Brauer induction}\dotfill \pageref*{SymondExplicitBrauerInduction} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} The \emph{Brauer induction theorem} (\hyperlink{Brauer46}{Brauer 46}) states that, for [[ground field]] the [[complex numbers]], [[finite dimensional vector space|finite-dimensional]] [[linear representations]] $V$ of a [[finite group]] $G$ all arise as [[virtual representation|virtual combinations]] of [[induced representations]] $ind_{H}^G \big(W\big)$ of just 1-dimensional representations $W$, $dim_{\mathbb{C}}(W) =1$: \begin{equation} [V] \;=\; \underset{ \mathclap{ { H_i \hookrightarrow G } \atop { {W_i \in Rep(H_i)\,,} \atop { dim(W_i) = 1 } } } }{\sum} \, n_i \, \Big[ ind_{H_i}^G \big(W_i\big) \Big] \,, \phantom{AAA} a_i \in \mathbb{Z} \,. \label{GenericExampleOfBrauerInduction}\end{equation} In other words, this says that the [[representation ring]] $R_{\mathbb{C}}(G)$ is generated from [[isomorphism classes]] $\left[ind_{H}^big(W\big)\right]$ of [[induced representations]] $ind_{H}^G \big(W\big)$ of 1-dimensional representations $W$ of [[subgroups]] $H \subset G$. This may be thought of as (implying) a [[splitting principle]] for [[linear representations]] (\hyperlink{Symonds91}{Symonds 91}), for more on this see at \emph{[[characteristic classes of linear representations]]} the section \emph{\href{characteristic+class+of+a+linear+representation#SplittingPrinciple}{splitting principle}}. Brauer induction generalizes the immediate statement that [[finite dimensional vector space|finite-dimensional]] [[permutation representations]] are all [[direct sums]] of [[induced representations]] of the \emph{[[trivial representation|trivial]]} 1-dimensional representation; see at \emph{[[induced representation of the trivial representation]]}. The analogous statement holds true also for [[ground ring]] the [[quaternions]], while for [[ground field]] the [[real numbers]] one has to induce not just from 1-dimensional but also from 2-dimensional representations. Of course, the expansions \eqref{GenericExampleOfBrauerInduction} are not unique. But one may find [[functor|functorial]] choices that satisfy good extra properties, see below \emph{\hyperlink{SnaithExpansion}{Snaith's explicit Brauer induction}} and \emph{\hyperlink{SymondExplicitBrauerInduction}{Symond's explicit Brauer induction}}. \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{SnaithExpansion}{}\subsubsection*{{Snaith's explicit Brauer induction}}\label{SnaithExpansion} (\ldots{}) \hypertarget{SymondExplicitBrauerInduction}{}\subsubsection*{{Symonds' explicit Brauer induction}}\label{SymondExplicitBrauerInduction} We state an explicit and [[natural transformation|natural]] choice of Brauer induction due to \hyperlink{Symonds91}{Symonds 91} (Prop. \ref{SymondsExplicitBrauerInd}) below. Its main property is good compatibility with the [[total Chern classes of linear representations]] via a certain multiplicative transfer map on [[integral cohomology]] (the latter recalled as Lemma \ref{TransferEvens} below). \begin{lemma} \label{TransferEvens}\hypertarget{TransferEvens}{} \textbf{(Evens' multiplicative transfer)} For $G$ a [[finite group]] and $H \subset G$ a [[subgroup]], there is a [[linear map]] \begin{displaymath} \mathcal{N}_H^G \;\colon\; \underset{k \in \mathbb{N}}{\prod} H^{2k}\big( B H, \mathbb{Z}\big) \longrightarrow \underset{k \in \mathbb{N}}{\prod} H^{2k}\big( B G, \mathbb{Z}\big) \end{displaymath} from the [[cohomology ring]] of the [[classifying space]] of $H$ to that of $G$ which is multiplicative in that its respects the product structure, hence the [[cup product]], on both sides \begin{displaymath} \mathcal{N}_H^G\big( \alpha \smile \beta\big) \;=\; \mathcal{N}_H^G\big( \alpha\big) \smile \mathcal{N}_H^G\big( \beta\big) \,. \end{displaymath} \end{lemma} This is due to \hyperlink{Evens63}{Evens 63}. There, the maps themselves are introduced on the bottom of p. 7, while their multiplicativity is stated as Prop. 4 on p. 10. \begin{prop} \label{SymondsExplicitBrauerInd}\hypertarget{SymondsExplicitBrauerInd}{} \textbf{(Symonds' explicit Brauer induction)} For $G \in$ [[FinGrp]] there is a [[linear map]] ([[homomorphism]] of [[abelian groups]]) \begin{displaymath} R_{\mathbb{C}}\big( G \big) \overset {L} {\longrightarrow} \underset{ [H \subset G] }{\prod} \mathbb{Z} \left[ 1dRep_{\mathbb{C}}\big( H \big)_{/\sim} \right] \end{displaymath} from the underlying abelian group of the [[representation ring]] to the [[product group]] of the [[free abelian groups]] that are spanned by the [[isomorphism classes]] of 1-dimensional representations over all [[conjugacy classes]] of [[subgroup]] $H \subset G$, such that \begin{enumerate}% \item $L$ is a [[natural transformation]] of [[functors]] $FinGrp^{op} \to Ab$, hence $L\big( f^\ast V\big) = f^\ast( L(V) )$; \item $L$ is a [[section]] of the [[natural transformation]] \begin{displaymath} \underset{ [H \subset G] }{\prod} R^{1d}_{\mathbb{Z}}\big( H\big) \overset {\sum ind} {\longrightarrow} R_{\mathbb{C}}\big( G \big) \end{displaymath} which applies [[induced representation|induction]] and then sums everything up, in that the [[composition]] $\big( \sum ind \big) \circ L$ is the [[identity morphism|identity]]: \begin{displaymath} \big( \sum ind \big) \circ L(V) \coloneqq \underset{ [H \subset G] }{ \sum } ind_H^G\left[ L(V)_H \right] \;=\; V \end{displaymath} \item $L$ is compatible with the [[total Chern classes of linear representations]] \begin{displaymath} R_{\mathbb{C}}\big( G \big) \overset{c}{\longrightarrow} \underset{ k \in \mathbb{N} }{\prod} H^{2k}\big( B G, \mathbb{Z} \big) \end{displaymath} via their multiplicative transfer $\mathcal{N}_H^G$ (Lemma \ref{TransferEvens}) in that \begin{displaymath} c \big( V \big) \;=\; \underset{ [H \subset G] }{\smile} \mathcal{N}_H^G \Big( c \big( L(V)_H \big) \Big) \,, \end{displaymath} hence in that the following [[commuting diagram|diagram commutes]]: \begin{displaymath} \itexarray{ R_{\mathbb{C}}\big( G\big) &\overset{L}{\longrightarrow}& \underset{ [H \subset G] }{\prod} \mathbb{Z} \left[ 1dRep_{\mathbb{C}}\big( H \big)_{/\sim} \right] \\ {}^{\mathllap{c}}\Big\downarrow && \Big\downarrow {}^{ \underset{ [H \subset G] }{\prod} \left( c \circ \mathcal{N}_H^G \right) } \\ \underset{ k \in \mathbb{N} }{\prod} H^{2k}\big( B G, \mathbb{Z} \big) & \underset{ \underset{ [H \subset G] }{\smile} }{\longleftarrow} & \underset{ [H \subset G] }{\prod} \underset{ k \in \mathbb{Z} } {\Prod} H^{2k}\big( B G, \mathbb{Z}\big) } \end{displaymath} \item a 1-dimensional representation $W \in 1dRep\big(G\big)_{/\sim} \subset R_{\mathbb{C}}\big(G\big)$ is sent to the [[tuple]] $L(W) = (W,0,0, \cdots)$ whose component over $G \subset G$ is $V$ itself, and all whose other components vanish; \item in contrast, if $V \in Rep_{\mathbb{C}}\big( G \big)_{/\sim}$ has no 1-dimensional [[direct sum|direct summand]], then the $G$-compnents of $L(V)$ is zero; \end{enumerate} \end{prop} (\hyperlink{Symonds91}{Symonds 91, Prop. 2.1, Theorem 2.2, and Theorem 2.4}) (\ldots{}) \hypertarget{references}{}\subsection*{{References}}\label{references} Due to \begin{itemize}% \item [[Richard Brauer]], \ldots{} (1946) \end{itemize} See also \begin{itemize}% \item Wikipedia, \emph{\href{https://en.wikipedia.org/wiki/Brauer%27s_theorem_on_induced_characters}{Brauer's theorem on induced characters}} \end{itemize} In the context of [[characteristic classes of linear representations]]: \begin{itemize}% \item [[Peter Symonds]], \emph{A splitting principle for group representations}, Comment. Math. Helv. (1991) 66: 169 (\href{https://eudml.org/doc/140229}{dml:140229}, \href{https://doi.org/10.1007/BF02566643}{doi:10.1007/BF02566643}) \end{itemize} based on \begin{itemize}% \item [[Leonard Evens]], \emph{A Generalization of the Transfer Map in the Cohomology of Groups}, Transactions of the American Mathematical Society Vol. 108, No. 1 (Jul., 1963), pp. 54-65 (\href{https://doi.org/10.1090/S0002-9947-1963-0153725-1}{doi:10.1090/S0002-9947-1963-0153725-1}, \href{https://www.jstor.org/stable/1993825}{jstor:1993825}) \end{itemize} and \begin{itemize}% \item Ove Kroll, \emph{An Algebraic Characterisation of Chern Classes of Finite Group Representations}, Bulletin of the LMS, Volume19, Issue3 May 1987 Pages 245-248 (\href{https://doi.org/10.1112/blms/19.3.245}{doi:10.1112/blms/19.3.245}) \end{itemize} [[!redirects Brauer's theorem]] [[!redirects Brauer's theorem on induced characters]] [[!redirects Brauer induction]] [[!redirects Brauer inductions]] [[!redirects explicit Brauer induction]] [[!redirects explicit Brauer inductions]] \end{document}