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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*{equivariant differential topology} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{topology}{}\paragraph*{{Topology}}\label{topology} [[!include topology - contents]] \hypertarget{differential_geometry}{}\paragraph*{{Differential geometry}}\label{differential_geometry} [[!include synthetic differential geometry - contents]] \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{related_concepts}{Related concepts}\dotfill \pageref*{related_concepts} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} The subject of \emph{equivariant differential topology} is the enhancement of results of [[differential topology]] from plain [[manifolds]]/[[topological spaces]] to those equipped with [[actions]] of some [[group]] ([[G-spaces]]). \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \begin{prop} \label{FixedLociOfSmoothProperActionsAreSubmanifolds}\hypertarget{FixedLociOfSmoothProperActionsAreSubmanifolds}{} \textbf{([[fixed loci]] of [[smooth function|smooth]] [[proper actions]] are [[submanifolds]])} Let $X$ be a [[smooth manifold]], $G$ a [[Lie group]] and $\rho \;\colon\; G \times X \to X$ a \emph{[[proper action|proper]]} [[action]] by [[diffeomorphisms]]. Then the $G$-[[fixed locus]] $X^G \hookrightarrow X$ is a [[smooth manifold|smooth]] [[submanifold]]. If in addition $X$ is equipped with a [[Riemannian metric]] and $G$ acts by [[isometries]] then the [[submanifold]] $X^G$ is a [[totally geodesic submanifold]]. \end{prop} (e.g. \hyperlink{Ziller13}{Ziller 13, theorem 3.5.2}, see also \href{https://math.stackexchange.com/a/1739784/58526}{this MO discussion}) \begin{proof} Let $x \in X^G \subset X$ be any [[fixed point]]. Since this is in particular a closed invariant [[submanifold]], Prop. \ref{ExistenceOfGInvariantTubularNeighbourhoods} applies and shows that an [[open neighbourhood]] of $x$ in $X$ is $G$-equivariantly [[diffeomorphism|diffeomorphic]] to a [[linear representation]] $V \in RO(G)$. The [[fixed locus]] $V^G \subset V$ of that is hence [[diffeomorphism|diffeomorphic]] to an [[open neighbourhood]] of $x$ in $\Sigma$. \end{proof} \begin{remark} \label{}\hypertarget{}{} Without the assumption of [[proper action]] in Prop. \ref{FixedLociOfSmoothProperActionsAreSubmanifolds} the conclusion generally fails. See \href{https://math.stackexchange.com/a/1739768/58526}{this MO comment} for a counter-example. \end{remark} \begin{prop} \label{GActionOnNormalBundleToFixedLocus}\hypertarget{GActionOnNormalBundleToFixedLocus}{} \textbf{($G$-action on [[normal bundle]] to [[fixed locus]])} Let $X$ be a [[smooth manifold]], $G$ a [[Lie group]] and $\rho \;\colon\; G \times X \to X$ a \emph{[[proper action|proper]]} [[action]] by [[diffeomorphisms]]. Then linearization of the $G$-action aroujnd the [[fixed locus]] $X^G \subset X$ equips the [[normal bundle]] $N_X\left( X^G\right)$ with [[smooth function|smooth]] and [[fiber]]-wise [[linear map|linear]] $G$-[[action]]. \end{prop} (e.g. \hyperlink{CrainicStruchiner13}{Crainic-Struchiner 13, Example 1.7}) \begin{prop} \label{ExistenceOfGInvariantRiemannianMetrics}\hypertarget{ExistenceOfGInvariantRiemannianMetrics}{} \textbf{(existence of $G$-invariant [[Riemannian metric]])} Let $X$ be a [[smooth manifold]], $G$ a [[compact Lie group]] and $\rho \;\colon\; G \times X \to X$ a \emph{[[proper action|proper]]} [[action]] by [[diffeomorphisms]]. Then there exists a [[Riemannian metric]] on $X$ with its [[invariant]] with respect to the $G$-[[action]], hence such that all elements of $G$ act by [[isometries]]. \end{prop} (\hyperlink{Bredon72}{Bredon 72, VI Theorem 2.1}, see also \hyperlink{Ziller13}{Ziller 13, Theorem 3.0.2}) \begin{defn} \label{GInvariantTubularNeighbourhood}\hypertarget{GInvariantTubularNeighbourhood}{} \textbf{($G$-invariant [[tubular neighbourhood]])} Let $X$ be a [[smooth manifold]], $G$ a [[Lie group]] and $\rho \;\colon\; G \times X \to X$ a \emph{[[proper action|proper]]} [[action]] by [[diffeomorphisms]]. For $\Sigma \subset X^G \subset X$ a [[closed manifold|closed]] [[smooth manifold|smooth]] [[submanifold]] inside the [[fixed locus]], a \emph{$G$-invariant [[tubular neighbourhood]]} $\mathcal{N}(\Sigma \subset X)$ of $\Sigma$ in $X$ is \begin{enumerate}% \item a [[smooth vector bundle]] $E \to \Sigma$ equipped with a [[fiber]]-wise [[linear map|linear]] $G$-[[action]]; \item an equivariant [[diffeomorphism]] $E \overset{}{\longrightarrow} X$ onto an [[open neighbourhood]] of $\Sigma$ in $X$ which takes the [[zero section]] identically to $\Sigma$. \end{enumerate} \end{defn} \begin{prop} \label{ExistenceOfGInvariantTubularNeighbourhoods}\hypertarget{ExistenceOfGInvariantTubularNeighbourhoods}{} \textbf{(existence of $G$-invariant [[tubular neighbourhoods]])} Let $X$ be a [[smooth manifold]], $G$ a [[Lie group]] and $\rho \;\colon\; G \times X \to X$ a \emph{[[proper action|proper]]} [[action]] by [[diffeomorphisms]]. If $\Sigma \overset{\iota}{\hookrightarrow} X$ is a [[closed manifold|closed]] [[smooth manifold|smooth]] [[submanifold]] inside the $G$-[[fixed locus]] $\backslash$begin\{center\} $\backslash$begin\{xymatrix\} $\backslash$Sigma $\backslash$ar@\{{\tt \symbol{94}}\{(\}-{\tt \symbol{62}}\}rr{\tt \symbol{94}}-\{$\backslash$iota{\tt \symbol{94}}G\} $\backslash$ar@\{{\tt \symbol{94}}\{(\}-{\tt \symbol{62}}\}dr\emph{\{$\backslash$iota\} \&\& X{\tt \symbol{94}}G $\backslash$ar@\{{\tt \symbol{94}}\{(\}-{\tt \symbol{62}}\}dl $\backslash$ \& X $\backslash$end\{xymatrix\} $\backslash$end\{center\}} then \begin{enumerate}% \item $\Sigma$ admits a $G$-invariant [[tubular neighbourhood]] $\Sigma \subset U \subset X$ (Def. \ref{GInvariantTubularNeighbourhood}); \item any two choices of such $G$-invariant tubular neighbourhoods are $G$-equivariantly [[isotopy|isotopic]]; \item there always exists an $G$-invariant tubular neighbourhood parametrized specifically by the [[normal bundle]] $N(\Sigma \subset X)$ of $Sigma$ in $X$, equipped with its induced $G$-[[action]] from Def. \ref{GActionOnNormalBundleToFixedLocus}, and such that the $G$-equivariant [[diffeomorphism]] is given by the [[exponential map]] \begin{displaymath} \exp_\epsilon \;\colon\; N(\Sigma \subset X) \overset{\simeq}{\longrightarrow} \mathcal{N}(\Sigma \subset X) \end{displaymath} \end{enumerate} with respect to a $G$-invariant [[Riemannian metric]] (which exists according to Prop. \ref{ExistenceOfGInvariantRiemannianMetrics}): \end{prop} The existence of the $G$-invariant tubular neighbourhoods is for instance in \hyperlink{Bredon72}{Bredon 72 VI Theorem 2.2}, \hyperlink{Kankaanrinta07}{Kankaanrinta 07, theorem 4.4}. The uniqueness up to equivariant isotopy is in \hyperlink{Kankaanrinta07}{Kankaanrinta 07, theorem 4.4, theorem 4.6}. The fact that one may always use the [[normal bundle]] appears at the end of the proof of \hyperlink{Bredon72}{Bredon 72 VI Theorem 2.2}, and as a special case of a more general statement about invariant tubular neighbourhoods in [[Lie groupoids]] it follows from \hyperlink{PflaumPosthumaTang11}{Pflaum-Posthuma-Tang 11, Theorem 4.1} by applying the construction there to each point in $\Sigma$ for one and the same choice of background metric. See also for instance \hyperlink{PflaumWilkin17}{Pflaum-Wilkin 17, Example 2.5}. \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[equivariant homotopy theory]] \item [[equivariant stable homotopy theory]] \item [[equivariant rational homotopy theory]] \item [[equivariant cohomology]] \end{itemize} \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[Arthur Wasserman]], section 3 of \emph{Equivariant differential topology}, Topology Vol. 8, pp. 127-150, 1969 (\href{https://web.math.rochester.edu/people/faculty/doug/otherpapers/wasserman.pdf}{pdf}) \item [[Glen Bredon]], \emph{Introduction to compact transformation groups}, Academic Press 1972 (\href{http://www.indiana.edu/~jfdavis/seminar/Bredon,Introduction_to_Compact_Transformation_Groups.pdf}{pdf}) \item [[Marja Kankaanrinta]], \emph{Equivariant collaring, tubular neighbourhood and gluing theorems for proper Lie group actions}, Algebr. Geom. Topol. Volume 7, Number 1 (2007), 1-27 (\href{https://projecteuclid.org/euclid.agt/1513796653}{euclid:agt/1513796653}) \item [[Markus Pflaum]], Hessel Posthuma, X. Tang, \emph{Geometry of orbit spaces of proper Lie groupoids}, Journal für die reine und angewandte Mathematik (Crelles Journal) 2014.694 (\href{https://arxiv.org/abs/1101.0180}{arXiv:1101.0180}, \href{https://doi.org/10.1515/crelle-2012-0092}{doi:10.1515/crelle-2012-0092}) \item Wolfgang Ziller, \emph{Group actions}, 2013 (\href{https://www.math.upenn.edu/~wziller/math661/LectureNotesLee.pdf}{pdf}) \item Marius Crainic, Ivan Struchiner, \emph{On the linearization theorem for proper Lie groupoids}, Annales scientifiques de l'École Normale Supérieure, Série 4, Volume 46 (2013) no. 5, p. 723-746 (\href{http://www.numdam.org/item/ASENS_2013_4_46_5_723_0/}{numdam:ASENS\_2013\_4\_46\_5\_723\_0} \href{https://doi.org/10.24033/asens.2200}{doi:10.24033/asens.2200}) \item [[Markus Pflaum]], Graeme Wilkin, \emph{Equivariant control data and neighborhood deformation retractions}, Methods and Applications of Analysis, 2019 (\href{https://arxiv.org/abs/1706.09539}{arXiv:1706.09539}) \end{itemize} \end{document}