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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*{Pontrjagin-Thom collapse map} \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{manifolds_and_cobordisms}{}\paragraph*{{Manifolds and cobordisms}}\label{manifolds_and_cobordisms} [[!include manifolds and cobordisms - contents]] \hypertarget{contents}{}\section*{{Contents}}\label{contents} \noindent\hyperlink{Idea}{Idea}\dotfill \pageref*{Idea} \linebreak \noindent\hyperlink{definition}{Definition}\dotfill \pageref*{definition} \linebreak \noindent\hyperlink{ComponentDefinitionInTopologicalSpaces}{Component definition in topological spaces}\dotfill \pageref*{ComponentDefinitionInTopologicalSpaces} \linebreak \noindent\hyperlink{AbstractDefinitionInTermsOfDuality}{Abstract definition in terms of duality}\dotfill \pageref*{AbstractDefinitionInTermsOfDuality} \linebreak \noindent\hyperlink{properties}{Properties}\dotfill \pageref*{properties} \linebreak \noindent\hyperlink{general}{General}\dotfill \pageref*{general} \linebreak \noindent\hyperlink{relation_between_cohomotopy_and_cobordism}{Relation between cohomotopy and cobordism}\dotfill \pageref*{relation_between_cohomotopy_and_cobordism} \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} Given an [[embedding of topological spaces|embedding]] of [[smooth manifolds]] $i \colon X \hookrightarrow Y$ of [[codimension]] $n$, the \emph{Thom collapse map} (\hyperlink{Thom54}{Thom 54}) is the [[continuous function]] from $X$ to the [[n-sphere]] which assigns \textbf{asymptotic normal distance} from the [[submanifold]], measured \begin{enumerate}% \item in [[direction vector|direction]] [[orthogonality|perpendicular]] to the submanifold, with respect to a [[normal framing]]; \item asymptotically, regarding all points outside a [[tubular neighbourhood]] as being [[one-point compactification|at infinity]]. \end{enumerate} \begin{quote}% graphics grabbed from \href{Cohomotopy+charge#SatiSchreiber19}{SS 19} \end{quote} For maximal codimension $n$, hence for 0-dimensional [[submanifolds]], hence for [[configuration space of points|configurations of points]], this is alternatively known as the ``electric field map'' (\href{cohomotopy+charge#Salvatore01}{Salvatore 01} following \href{cohomotopy+charge#Segal73}{Segal 73, Section 1}, see also \href{cohomotopy+charge#Knudsen18}{Knudsen 18, p. 49}) or the ``scanning map'' (\href{cohomotopy+charge#Kallel98}{Kallel 98}). The [[homotopy class]] of the Thom collpase map may be regarded as the \emph{[[Cohomotopy charge]]} of the submanifolds, as measured in $n$-[[Cohomotopy]]-[[generalized cohomology|cohomology theory]]. The PT collapse is a useful approximation to the would-be [[left inverse]] of the [[embedding of topological spaces]] As such, it is is used to define pushforward of [[cohomology]]-classes along $i$ (``[[Umkehr maps]]''). It also appears as the key step in [[Thom's theorem]]. \hypertarget{definition}{}\subsection*{{Definition}}\label{definition} \hypertarget{ComponentDefinitionInTopologicalSpaces}{}\subsubsection*{{Component definition in topological spaces}}\label{ComponentDefinitionInTopologicalSpaces} All [[topological spaces]] in the following are taken to be [[compact space|compact]]. Consider $X$ and $Y$ two [[manifolds]] and \begin{displaymath} i \colon X \hookrightarrow Y \end{displaymath} an [[embedding]]. Write \begin{itemize}% \item $N_i X \coloneqq i^* T Y/ T X$ for the [[normal bundle]]; \item $Th(N_i X)$ for the [[Thom space]] of the normal bundle; \item $f \colon N_i X \longrightarrow Y$ for any choice of [[tubular neighbourhood]] of $i$. \end{itemize} \begin{defn} \label{CollapseMap}\hypertarget{CollapseMap}{} The \textbf{collapse map} (or the \emph{Pontrjagin-Thom construction}) associated to $i$ and the choice of tubular neighbourhood $f$ is \begin{displaymath} c_i \colon Y \to Y/(Y - f(N_i X)) \stackrel{\simeq}{\to} Th(N_i X) \,, \end{displaymath} where the first morphism is the [[projection]] onto the [[quotient]] and the second is the canonical [[homeomorphism]] to the [[Thom space]] of the [[normal bundle]]. \end{defn} \begin{defn} \label{RefinedCollapseMap}\hypertarget{RefinedCollapseMap}{} Since in the construction of remark \ref{CollapseMap} every point of $N_i X$ is associated to a particular point of $X$, the collapse map lifts to a map \begin{displaymath} Y \longrightarrow X_+ \wedge Th(N_i X) \end{displaymath} from $Y$ to the [[smash product]] of the [[Thom space]] (canonically regarded as a [[pointed topological space]]) and the topological space $X$ with a base point adjoined. \end{defn} (e.g. \hyperlink{Rudyak98}{Rudyak 98, p. 317}) \begin{example} \label{ForEmbeddingsIntoAnNSphere}\hypertarget{ForEmbeddingsIntoAnNSphere}{} Of particular interest is the case where $Y$ in the above is a [[Cartesian space]] $\mathbb{R}^{dim X + k}$ or rather its [[one-point compactification]], the [[n-sphere|sphere]] $S^{dim X + k}$. By the [[Whitney embedding theorem]], for every $n \in \mathbb{N}$ there exists an $k \in \mathbb{N}$ such that every [[manifold]] $X$ of [[dimension]] $n$ has an [[embedding]] $X \hookrightarrow \mathbb{R}^{n+k} \to S^{n+k}$. In this case the collapse map of def. \ref{CollapseMap} has the form \begin{displaymath} S^{n+k} \longrightarrow Th(N_i X) \,. \end{displaymath} Composing this further with the canonical map $N_i X \longrightarrow E O(k) \underset{O(k)}{\times} \mathbb{R}^{k}$ to the universal vector bundle of rank $k$ yields a map \begin{displaymath} S^{n+k} \longrightarrow M O(k) \end{displaymath} from to the $k$th space in the [[Thom spectrum]] $M O$. This hence defines an element in the [[homotopy group of a spectrum|homotopy group]] $\pi_{k}(M O)$ of the [[Thom spectrum]]. [[Thom's theorem]] says that all elements in the homotopy groups of $M O$ arise this way, and that they retain precisely the information of the [[cobordism]] [[equivalence class]] of manifolds $X$. In this case the refined Thom collapse map of def. \ref{RefinedCollapseMap} is of the form \begin{displaymath} S^{n+k} \longrightarrow X_+ \wedge Th(N_i X) \,. \end{displaymath} \end{example} \begin{remark} \label{}\hypertarget{}{} The refined map in example \ref{ForEmbeddingsIntoAnNSphere} lifts to a morphism of [[spectra]] \begin{displaymath} \mathbb{S} \longrightarrow \Sigma_+^\infty X \wedge \Sigma^{-n-k} Th(N_i X) \end{displaymath} where $\mathbb{S}$ denotes the [[sphere spectrum]] and $Th(N_i X)$ now the [[Thom spectrum]] of the normal bundle. This morphism is the [[unit of an adjunction]] which exhibits the [[suspension spectrum]] $\Sigma_+^\infty X$ as a [[dualizable object]] in the [[stable homotopy category]], with [[dual object]] $\Sigma^{-n-k} Th(N_i X)$. See at \emph{[[Atiyah duality]]} and at \emph{[[n-duality]]}. \end{remark} Equivalently, one may proceed as follows. For a framed manifold i.e. a manifold $M^n$ with a chosen trivialization of the normal bundle $N_i (M^n)$ in some $\mathbf{R}^{n+r}$ one has $T N_i(M^n)\cong \Sigma^r(M^n_+)$ where $M^n_+$ is the union of $M^n$ with a disjoint base point. Identify a sphere $S^{n+r}$ with a one-point compactification $\mathbf{R}^{n+r}\cup \{\infty\}$. Then the Pontrjagin-Thom construction is the map $S^{n+r}\to Th(N_i X)$ obtained by collapsing the complement of the interior of the unit disc bundle $D(N_i M^n)$ to the point corresponding to $S(N_i M^n)$ and by mapping each point of $D(N_i M^n)$ to itself. Thus to a framed manifold $M^n$ one associates the composition \begin{displaymath} S^{n+r}\to Th(N_i X)\cong \Sigma^r M^n_+\to S^r \end{displaymath} and its homotopy class defines an element in $\pi_{n+r}(S^r)$. \hypertarget{AbstractDefinitionInTermsOfDuality}{}\subsubsection*{{Abstract definition in terms of duality}}\label{AbstractDefinitionInTermsOfDuality} The following is a more abstract description of Pontryagin-Thom collapse in the [[stable homotopy theory]] of [[sphere spectrum]]-[[(∞,1)-module bundles]]. \begin{defn} \label{SpanierDualityOperation}\hypertarget{SpanierDualityOperation}{} Write \begin{displaymath} D \coloneqq (-)^\vee\circ \Sigma^\infty_+ \coloneqq L_{whe} Top \to \mathbb{S}Mod \end{displaymath} for the [[Spanier-Whitehead duality]] map which sends a [[topological space]] first to its [[suspension spectrum]] and then that to its [[dual object]] in the [[(∞,1)-category of spectra]]. \end{defn} (\hyperlink{ABG11}{ABG 11, def 10.3}). \begin{prop} \label{}\hypertarget{}{} For $X$ a [[compact manifold]], let $X \to \mathbb{R}^n$ be an [[embedding]] and write $S^n \to X^{\nu_n}$ for the classical [[Pontryagin-Thom collapse map]] for this situation, and write \begin{displaymath} \mathbb{S} \to X^{-T X} \end{displaymath} for the corresponding [[looping]] map from the [[sphere spectrum]] to the [[Thom spectrum]] of the negative [[tangent bundle]] of $X$. Then [[Atiyah duality]] produces an [[equivalence]] \begin{displaymath} X^{- T X} \simeq D X \end{displaymath} which identifies the [[Thom spectrum]] with the [[dual object]] of $\Sigma^\infty_+ X$ in $\mathbb{S} Mod$ and this constitutes a [[commuting diagram]] \begin{displaymath} \itexarray{ && X^{- T X} \\ & \nearrow & \downarrow^{\mathrlap{\simeq}} \\ \mathbb{S} &\underset{D(X \to \ast)}{\to}& D X } \end{displaymath} identifying the classical [[Pontryagin-Thom collapse map]] with the abstract [[dual morphism]] construction of prop. \ref{SpanierDualityOperation}. More generally, for $W \hookrightarrow X$ an [[embedding]] of [[manifolds]], then [[Atiyah duality]] identifies the [[Pontryagin-Thom collapse maps]] \begin{displaymath} \mathbb{S} \to X^{-T X} \to W^{- T W} \end{displaymath} with the abstract [[dual morphisms]] \begin{displaymath} \mathbb{S} \to D X \to D W \,. \end{displaymath} \end{prop} (\hyperlink{ABG11}{ABG 11, prop. 10.5}). \begin{remark} \label{}\hypertarget{}{} Given now $E \in CRing_\infty$ an [[E-∞ ring]], then the [[dual morphism]] $\mathbb{S} \to D X$ induces under [[smash product]] a similar Pontryagin-Thom collapse map, but now not in [[sphere spectrum]]-[[(∞,1)-modules]] but in $E$-[[(∞,1)-modules]]. \begin{displaymath} E \to D X \otimes_{\mathbb{S}} E \,. \end{displaymath} The image of this under the $E$-[[generalized cohomology theory|cohomology]] functor produces \begin{displaymath} [D X \otimes_{\mathbb{S}} E, E] \to E \,. \end{displaymath} If now one has a [[Thom isomorphism]] ($E$-[[orientation in generalized cohomology|orientation]]) $[D X \otimes_{\mathbb{S}} E, E] \simeq [X,E]$ that identifies the cohomology of the dual object with the original cohomology, then together with produces the [[Umkehr map]] \begin{displaymath} [X,E] \simeq [D X \otimes_{\mathbb{S}} E, E] \to E \end{displaymath} that pushes the $E$-cohomology of $X$ to the $E$-cohomology of the point. Analogously if instead of the terminal map $X \to \ast$ we start with a more general map $X \to Y$. More generally a [[Thom isomorphism]] may not exists, but $[D X \otimes_{\mathbb{S}} E, E]$ may still be equivalent to a [[twisted cohomology]]-variant $[X,E]_{\chi}$ of $[X,E]$, namely to $[\Gamma_X(\chi),E]$, where $\chi \colon \Pi(X) \to E Line \hookrightarrow E Mod$ is an ([[flat (∞,1)-bundle|flat]]) $E$-[[(∞,1)-module bundle]] on $X$ and and $\Gamma \simeq \underset{\to}{\lim}$ is the [[(∞,1)-colimit]] (the [[generalized Thom spectrum]] construction). In this case the above yields a \emph{[[twisted Umkehr map]]}. \end{remark} (\hyperlink{ABG10}{ABG 10, 9.1}) \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{general}{}\subsubsection*{{General}}\label{general} \begin{prop} \label{}\hypertarget{}{} For given $i$ all collapse maps for different choices of [[tubular neighbourhood]] $f$ are [[homotopy|homotopic]]. \end{prop} \begin{proof} By the fact that the space of [[tubular neighbourhood]]s (see there for details) is [[contractible]]. \end{proof} \hypertarget{relation_between_cohomotopy_and_cobordism}{}\subsubsection*{{Relation between cohomotopy and cobordism}}\label{relation_between_cohomotopy_and_cobordism} For $X$ a [[closed manifold|closed]] [[smooth manifold]] of [[dimension]] $D$, the [[Pontryagin-Thom construction]] (e.g. \hyperlink{Kosinski93}{Kosinski 93, IX.5}) identifies the [[set]] \begin{displaymath} SubMfd_{/bord}^{d}(X) \end{displaymath} of [[cobordism classes]] of [[closed manifold|closed]] and [[normally framed submanifolds]] $\Sigma \overset{\iota}{\hookrightarrow} X$ of [[dimension]] $d$ inside $X$ with the [[cohomotopy]] $\pi^{D-d}(X)$ of $X$ in degree $D- d$ \begin{displaymath} SubMfd_{/bord}^{d}(X) \underoverset{\simeq}{PT}{\longrightarrow} \pi^{D-d}(X) \,. \end{displaymath} (e.g. \hyperlink{Kosinski93}{Kosinski 93, IX Theorem (5.5)}) In particular, by this [[bijection]] the canonical [[group]] [[structure]] on [[cobordism groups]] in sufficiently high [[codimension]] (essentially given by [[disjoint union]] of [[submanifolds]]) this way induces a group structure on the cohomotopy sets in sufficiently high degree. \hypertarget{related_concepts}{}\subsection*{{Related concepts}}\label{related_concepts} \begin{itemize}% \item [[Thom's theorem]] \item [[Thom isomorphism]] \end{itemize} [[!include generalized fiber integration synonyms - table]] \hypertarget{references}{}\subsection*{{References}}\label{references} \begin{itemize}% \item [[René Thom]], \emph{Quelques propri\'e{}t\'e{}s globales des vari\'e{}t\'e{}s diff\'e{}rentiables} Comment. Math. Helv. 28, (1954). 17-86 (\href{http://www.digizeitschriften.de/dms/img/?PID=GDZPPN002056259}{digiz:GDZPPN002056259}) \item [[Lev Pontrjagin]], \emph{Smooth manifolds and their applications in Homotopy theory}, Trudy Mat. Inst. im Steklov, No 45, Izdat. Akad. Nauk. USSR, Moscow, 1955 (AMS Translation Series 2, Vol. 11, 1959) (\href{https://web.math.rochester.edu/people/faculty/doug/otherpapers/pont4.pdf}{pdf}, \href{https://www.worldscientific.com/doi/abs/10.1142/9789812772107_0001}{doi:10.1142/9789812772107\_0001}) \item [[John Milnor]], section 7 of \emph{Topology -- From the differentiable viewpoint}, 1965 (\href{http://teachingdm.unito.it/paginepersonali/sergio.console/Dispense/Milnor%20Topology%20from%20%23681EA.pdf}{pdf}) \item [[Stanley Kochmann]], section 1.5 of \emph{[[Bordism, Stable Homotopy and Adams Spectral Sequences]]}, AMS 1996 \item [[Yuli Rudyak]], \emph{In Thom spectra, Orientability and Cobordism}, Springer 1998 (\href{http://www.maths.ed.ac.uk/~aar/papers/rudyakthom.pdf}{pdf}) \item [[Cary Malkiewich]], \emph{Unoriented cobordism and MO}, 2011 (\href{http://math.uiuc.edu/~cmalkiew/cobordism.pdf}{pdf}) \end{itemize} An illustration is given on \href{http://www.math.wisc.edu/~gstgc/slides/Koytcheff.pdf#page=15}{slide 15} \begin{itemize}% \item [[Antoni Kosinski]], chapter IX of \emph{Differential manifolds}, Academic Press 1993 (\href{http://www.maths.ed.ac.uk/~v1ranick/papers/kosinski.pdf}{pdf}) \item [[Ralph Cohen]], John Klein, \emph{Umkehr Maps} (\href{http://arxiv.org/abs/0711.0540}{arXiv:0711.0540}) \item [[Victor Snaith]], \emph{Stable homotopy around the arf-Kervaire invariant}, Birkhauser 2009 \end{itemize} The general abstract formulation in [[stable homotopy theory]] is in sketched in section 9 of \begin{itemize}% \item [[Matthew Ando]], [[Andrew Blumberg]], [[David Gepner]], \emph{Twists of K-theory and TMF}, in Robert S. Doran, Greg Friedman, [[Jonathan Rosenberg]], \emph{Superstrings, Geometry, Topology, and $C^*$-algebras}, Proceedings of Symposia in Pure Mathematics \href{http://www.ams.org/bookstore-getitem/item=PSPUM-81}{vol 81}, American Mathematical Society (\href{http://arxiv.org/abs/1002.3004}{arXiv:1002.3004}) \end{itemize} and is in section 10 of \begin{itemize}% \item [[Matthew Ando]], [[Andrew Blumberg]], [[David Gepner]], \emph{Parametrized spectra, multiplicative Thom spectra, and the twisted Umkehr map} (\href{http://arxiv.org/abs/1112.2203}{arXiv:1112.2203}) \end{itemize} with an emphases on [[parameterized spectra]]. [[!redirects Pontrjagin-Thom construction]] [[!redirects Pontryagin-Thom construction]] [[!redirects Pontryagin-Thom collaps map]] [[!redirects Pontrjagin-Thom collaps map]] [[!redirects Pontrjagin-Thom collapse map]] [[!redirects Thom collapse map]] [[!redirects Thom collapse maps]] [[!redirects Thom collaps map]] [[!redirects Thom collaps maps]] [[!redirects Pontrjagin-Thom collapse]] [[!redirects Pontryagin-Thom collapse]] [[!redirects Thom collapse]] [[!redirects Pontryagin-Thom collapse map]] [[!redirects Pontryagin-Thom collapse maps]] [[!redirects Pontrjagin-Thom collapse maps]] [[!redirects Pontrjagin-Thom collapse construction]] [[!redirects Pontrjagin-Thom collapse constructions]] [[!redirects Pontryagin-Thom collapse construction]] [[!redirects Pontryagin-Thom collapse constructions]] \end{document}