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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*{bordism ring} [[!redirects cobordism ring]] \hypertarget{context}{}\subsubsection*{{Context}}\label{context} \hypertarget{manifolds_and_cobordisms}{}\paragraph*{{Manifolds and cobordisms}}\label{manifolds_and_cobordisms} [[!include manifolds and cobordisms - contents]] \hypertarget{cobordism_theory}{}\paragraph*{{Cobordism theory}}\label{cobordism_theory} [[!include cobordism 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{RelationToCohomotopy}{Relation to cohomotopy}\dotfill \pageref*{RelationToCohomotopy} \linebreak \noindent\hyperlink{examples}{Examples}\dotfill \pageref*{examples} \linebreak \noindent\hyperlink{FramedCobordism}{Framed cobordism}\dotfill \pageref*{FramedCobordism} \linebreak \noindent\hyperlink{OrientedCobordismRing}{Oriented cobordism}\dotfill \pageref*{OrientedCobordismRing} \linebreak \noindent\hyperlink{references}{References}\dotfill \pageref*{references} \linebreak \hypertarget{idea}{}\subsection*{{Idea}}\label{idea} The \textbf{(co)bordism ring} $\Omega_*=\oplus_{n\geq 0}\Omega_n$ is the [[graded ring]] whose \begin{itemize}% \item degree $n$ elements are classes of $n$-[[dimension|dimensional]] [[smooth manifolds]] modulo [[cobordism]]; \item product operation is given by the [[Cartesian product]] of manifolds; \item addition operation is given by the [[disjoint union]] of manifolds. \end{itemize} Instead of bare manifolds one may consider manifolds with extra structure, such as [[orientation]], [[spin structure]], [[string structure]], etc. and accordingly there is \begin{itemize}% \item the \emph{oriented cobordism ring} $\Omega^{SO}_*$, \item the \emph{spin cobordism ring} $\Omega^{Spin}_*$, \end{itemize} etc. In this general context the bare cobordism ring is also denoted $\Omega^O_*$ or $\Omega^{un}_*$, for emphasis. A ring [[homomorphism]] out of the cobordism ring is a (multiplicative) \emph{[[genus]]}. More generally, for $X$ a fixed [[manifold]] there is a relative cobordism ring $\Omega_\bullet(X)$ whose \begin{itemize}% \item elements are classes modulo cobordism over $X$ of manifolds equipped with [[continuous functions]] to $X$ (``singular manifolds''); \item multiplication of $[f_1 \colon \Sigma_1 \to X]$ with $[f_2 \colon \Sigma_2 \to X]$ is given by \emph{transversal intersection} $\Sigma_1 \cap_X \Sigma_2$ over $X$: perturb $f_1$ such $(f_1',f_2)$ becomes a [[transversal maps]] and then form the [[pullback]] $\Sigma_1 \times_{(f_1',f_2)} \Sigma_2$ in [[Diff]]. \end{itemize} This product is graded in that it satisfies the \textbf{dimension formula} \begin{displaymath} (dim X - dim \Sigma_1) + (dim X - dim \Sigma_2) = dim X - dim (\Sigma_1 \cap_X \Sigma_2) \end{displaymath} hence \begin{displaymath} dim (\Sigma_1 \cap_X \Sigma_2 ) = (dim \Sigma_1 + dim \Sigma_2) - dim X \,. \end{displaymath} Still more generally, this may be considered for $\Sigma$ being [[manifolds with boundary]]. Then $\Omega(X,A)$ for $(X,A)$ a [[CW pair]] is the ring of cobordism classes, relative boundary, of singular manifolds $\Sigma \to X$ such that the [[boundary]] of $\Sigma$ lands in in $A$. The resulting [[functor]] \begin{displaymath} (X,A) \mapsto \Omega^G_\bullet(X,A) \end{displaymath} constitutes a [[generalized homology theory]] (see e.g. \hyperlink{Buchstaber}{Buchstaber, II.8}). Accordingly this is called \emph{[[bordism homology theory]]}. The [[spectrum]] that represents this under the [[Brown representability theorem]] is the universal [[Thom spectra]] $M G$ (e.g. [[MO]] for $G=O$ or [[MU]] for $G = U$), which canonically is a [[ring spectrum]] under [[Whitney sum]] of [[universal vector bundles]]. Accordingly the (co-bordism ring) itself is equivalently the bordism homology groups of the point, hence the [[stable homotopy groups]] of the [[Thom spectrum]] (this is \emph{[[Thom's theorem]]}) \begin{displaymath} \Omega_\bullet^G \simeq \M G_\bullet(\ast) \simeq pi_\bullet(M G) \,. \end{displaymath} This remarkable relation between [[manifolds]] and [[stable homotopy theory]] is known as \emph{[[cobordism theory]]} (or ``Thom theory''). On general grounds this is equivalently the $M G$-[[generalized cohomology]] of the point ([[cobordism cohomology theory]]) \begin{displaymath} \Omega_\bullet^G \simeq M G^\bullet(\ast) \end{displaymath} which justifies calling $\Omega_\bullet^G$ both the ``bordism ring'' as well as the ``cobordism ring''. \hypertarget{properties}{}\subsection*{{Properties}}\label{properties} \hypertarget{RelationToCohomotopy}{}\subsubsection*{{Relation to cohomotopy}}\label{RelationToCohomotopy} Let $X$ be a [[smooth manifold]] of [[dimension]] $n \in \mathbb{N}$ and let $k \leq n$. Then the [[Pontryagin-Thom construction]] induces a [[bijection]] \begin{displaymath} [X, S^k] \overset{\simeq}{\longrightarrow} \Omega^{n-k}(X) \end{displaymath} from the [[cohomotopy]] sets of $X$ to the [[cobordism group]] of $(n-k)$-dimensional [[submanifolds]] with [[normal bundle|normal]] [[framed manifold|framing]] up to normally framed [[cobordism]]. In particular, the natural group structure on [[cobordism group]] (essentially given by [[disjoint union]] of submanifolds) this way induces a group structure on the cohomotopy sets. This is made explicit for instance in \hyperlink{Kosinski93}{Kosinski 93, chapter IX}. \hypertarget{examples}{}\subsection*{{Examples}}\label{examples} \hypertarget{FramedCobordism}{}\subsubsection*{{Framed cobordism}}\label{FramedCobordism} By [[Thom's theorem]], for any [[(B,f)-structure]] $\mathcal{B}$, there is an [[isomorphism]] (of [[commutative rings]]) \begin{displaymath} \Omega^{\mathcal{B}}_\bullet \overset{\simeq}{\longrightarrow} \pi_\bullet(M\mathcal{B}) \end{displaymath} from the [[cobordism ring]] of manifolds with stable normal $\mathcal{B}$-structure to the [[homotopy groups of a spectrum|homotopy groups]] of the universal $\mathcal{B}$-[[Thom spectrum]]. Now for $\mathcal{B} = Fr$ [[framing]] structure, then \begin{displaymath} M Fr \simeq \mathbb{S} \end{displaymath} is equivalently the [[sphere spectrum]]. Hence in this case [[Thom's theorem]] states that there is an isomorphism \begin{displaymath} \Omega^{fr}_\bullet \overset{\simeq}{\longrightarrow} \pi_\bullet(\mathbb{S}) \end{displaymath} between the framed cobordism ring and the [[stable homotopy groups of spheres]]. For discussion of computation of $\pi_\bullet(\mathbb{S})$ this way, see for instance (\hyperlink{WangXu10}{Wang-Xu 10, section 2}). For instance \begin{itemize}% \item $\Omega^{fr}_0 = \mathbb{Z}$ because there are two $k$-framings on a single point, corresponding to $\pi_0(O(k)) \simeq \mathbb{Z}_2$, the negative of a point with one framing is the point with the other framing, and so under disjoint union, the framed points form the group of integers; \item $\Omega^{fr}_1 = \mathbb{Z}_2$ because the only compact connected 1-manifold is the circle, there are two framings on the circle, corresponding to $\pi_1(O(k)) \simeq \mathbb{Z}_2$ and they are their own negatives. \end{itemize} \hypertarget{OrientedCobordismRing}{}\subsubsection*{{Oriented cobordism}}\label{OrientedCobordismRing} \begin{prop} \label{OrientedCobordismOverPoint}\hypertarget{OrientedCobordismOverPoint}{} The cobordism ring over the point for [[orientation|oriented]] manifolds starts out as \begin{tabular}{l|l|l|l|l|l|l|l|l|l|l} $k$&0&1&2&3&4&5&6&7&8&$\geq 9$\\ \hline $\Omega^{SO}_k$&$\mathbb{Z}$&0&0&0&$\mathbb{Z}$&$\mathbb{Z}_2$&0&0&$\mathbb{Z}\oplus \mathbb{Z}$&$\neq 0$\\ \end{tabular} \end{prop} see e.g. (\hyperlink{ManifoldAtlas}{ManifoldAtlas}) \begin{prop} \label{}\hypertarget{}{} For $X$ a [[CW-complex]] (for instance a [[manifold]]), then the oriented cobordism ring is expressed in terms of the [[ordinary homology]] $H_q(X,\Omega^{SO}_{p-q})$ of $X$ with [[coefficients]] in the cobordism ring over the point, prop. \ref{OrientedCobordismOverPoint}, as \begin{displaymath} \Omega_p^{SO}(X) = \oplus_{q = 0}^p H_q(X,\Omega_{p-q}^{SO}) \; mod\; odd \; torsion \,. \end{displaymath} \end{prop} e.g. \hyperlink{ConnorFloyd62}{Connor-Floyd 62, theorem 14.2} \hypertarget{references}{}\subsection*{{References}}\label{references} Textbook accounts include \begin{itemize}% \item [[Stanley Kochmann]], section 1.5 of \emph{[[Bordism, Stable Homotopy and Adams Spectral Sequences]]}, AMS 1996 \end{itemize} Lecture notes include \begin{itemize}% \item [[Victor Buchstaber]], \emph{Geometric cobordism theory} (\href{http://www.maths.manchester.ac.uk/~peter/MATH41101notes07.pdf}{pdf}) \item [[John Francis]] (notes by [[Owen Gwilliam]]), \emph{\href{http://math.northwestern.edu/~jnkf/classes/mflds/}{Topology of manifolds}}, \emph{Lecture 2: Cobordism} (\href{http://math.northwestern.edu/~jnkf/classes/mflds/2cobordism.pdf}{pdf}) \item [[Gerald Höhn]], \emph{Komplexe elliptische Geschlechter und $S^1$-\"a{}quivariante Kobordismustheorie} (german) (\href{http://arxiv.org/PS_cache/math/pdf/0405/0405232v1.pdf}{pdf}) \item [[Sander Kupers]], \emph{Oriented bordism: Calculation and application} (\href{http://web.stanford.edu/~kupers/orientedbordism.pdf}{pdf}) \end{itemize} Details for framed cobordism are spelled out in \begin{itemize}% \item [[Guozhen Wang]], Zhouli Xu, section 2 of \emph{A survey of computations of homotopy groups of Spheres and Cobordisms}, 2010 (\href{http://math.mit.edu/~guozhen/homotopy%20groups.pdf}{pdf}) \item [[Andrew Putman]], \emph{Homotopy groups of spheres and low-dimensional topology} (\href{http://www.math.rice.edu/~andyp/notes/HomotopySpheresLowDimTop.pdf}{pdf}) \end{itemize} The relation to [[cohomotopy]] is made explicit in \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}) \end{itemize} Further discussion of oriented cobordism includes \begin{itemize}% \item Manifold Atlas, \emph{\href{http://www.map.mpim-bonn.mpg.de/Oriented_bordism}{Oriented bordism}} \item P. E. Conner, E. E. Floyd, \emph{Differentiable periodic maps}, Bull. Amer. Math. Soc. Volume 68, Number 2 (1962), 76-86. (\href{http://projecteuclid.org/euclid.bams/1183524501}{Euclid}, \href{http://www.maths.ed.ac.uk/~aar/papers/cf1.pdf}{pdf}) \end{itemize} A historical review in the context of [[complex cobordism cohomology theory]]/[[Brown-Peterson theory]] is in \begin{itemize}% \item [[Doug Ravenel]], chapter 4, section 2 of \emph{[[Complex cobordism and stable homotopy groups of spheres]]} \end{itemize} On [[fibered cobordism groups]]: \begin{itemize}% \item Astey, Greenberg, Micha, Pastor, \emph{Some fibered cobordisms groups are not finitely generated} (\href{http://www.ams.org/journals/proc/1988-104-01/S0002-9939-1988-0958093-3/S0002-9939-1988-0958093-3.pdf}{pdf}) \end{itemize} Discussion of the $G$-equivariant complex coborism ring includes \begin{itemize}% \item G. Comezana and [[Peter May]], \emph{A completion theorem in complex cobordism}, in Equivariant Homotopy and Cohomology Theory, CBMS Regional conference series in Mathematics, American Mathematical Society Publications, Volume 91, Providence, 1996. \item [[Igor Kriz]], \emph{The $\mathbb{Z}/p$--equivariant complex cobordism ring}, from: ``Homotopy invariant algebraic structures (Baltimore, MD, 1998)'', Amer. Math. Soc. Providence, RI (1999) 217--223 \item [[Neil Strickland]], \emph{Complex cobordism of involutions}, Geom. Topol. 5 (2001) 335-345 (\href{http://arxiv.org/abs/math/0105020}{arXiv:math/0105020}) \item William Abrams, \emph{Equivariant complex cobordism}, 2013 (\href{http://deepblue.lib.umich.edu/bitstream/handle/2027.42/99796/abramwc_1.pdf}{pdf}) \end{itemize} [[!redirects bordism ring]] [[!redirects cobordism rings]] [[!redirects cobordism group]] [[!redirects cobordism groups]] \end{document}