B-Bordism

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1 Introduction

In this article we recall the definition of the bordism groups of closed manifolds, ususally with extra topological structure: orientation, spin-structure, weak complex structure etc.

We specify the extra structure by a fibration $\xi : B \to BO$${{Stub}} == Introduction == <wikitex>; In this article we recall the definition of the bordism groups of closed manifolds, ususally with extra topological structure: orientation, spin-structure, weak complex structure etc. We specify the extra structure by a fibration $\xi : B \to BO$ where $BO$ denotes the classifying space of the stable orthogonal group and $B$ is homotopy equivalent to a CW complex of finite type. For example the fibrations $BSO \to BO$ and $BU \to BO$ induced by the canonical homomorphisms $SO \to O$ and $U \to O$ are used for respectively oriented and complex bordism. Abusing notation, one writes $B$ for the fibraion $\xi$: for each $B$ and $n \geq 0$ there is a bordism group of closed n-manifolds with $B$ structure denoted $$\Omega_n^B ~~\text{or}~~ \Omega_n(B).$$ If $B = BG$ is the classifying space of the stable group $G$ then on often writes $$ \Omega_n^G ~~\text{or}~~ \Omega_n(G).$$ </wikitex> == Stable vector bundles == <wikitex>; We recall the defintion of a stable vector bundle from {{cite|Kreck&Lück2005|18.10}} which requires some notation. Let $\underline{\Rr^k}$ denote the trivial rank k bundle $Z \times \Rr^k$: the space $Z$ will be clear from context. Let $\{ X_k \}$ be a sequence $X_0 \to X_1 \to X_2 \to \dots$ of inclusions $j_k : X_k \to X_{k+1}$ of CW-complexes. A stable vector bundle $E$ over $\{ X_k \}$ is a sequence of rank k-vector bundles $\xi_k \to X_k$ and a sequence of bundle maps $$ \bar j_k : \xi_k \oplus \underline{\Rr} \to \xi_{k+1}$$ where $\bar j_k$ covers the inclusion $j_k$. An isomorphism of stable vector bundles over $\{ X \}$, $\theta : \{ (\xi_k\, \bar j_k) \} \cong \{\xi_k', \bar j_k'\}$ is a sequence of bundle ismorphisms $\theta_j : \xi_j \cong \xi_k'$ for $j \geq N$, $N$ some integer, which are compatible with $\bar j_k$ and $\bar j_k'$. Here are some important examples: * A rank $j$-vector $V \to X$ over a CW complex $X$ defines a stable vector bundle $\{ V \}$ by setting $X_k = X$ fixed for all $k$, $\xi_k = 0$ for $k < j$ and $\xi_k = E \oplus \underline{\Rr}^{j-k}$ for $k\geq j$ with $\bar j_k$ the obvious inclusions. * If $M$ is a compact n-manifold then $M$ embedds into $\Rr^{2n+k}$ for $k \geq 0$ and for $k \geq 1$ two such embeddings are isotopic. The normal bundle of any such embedding defines a stable vector bundle over $M$ as in the first example and there is a canonical isomorphicm between any two such stable bundles given by isotopies between the embeddings. We write $\nu_M$ for the stable normal bundle of $M$. Note that by defintion $\nu_M$ is a stable inverse to the tangent bundle of $M$: $$ \nu_M \oplus TM \cong \{ \underline{\Rr} \}.$$ * The universal bundle $\gamma \to BO$ is the stable bundle with $X_k = BO(k)$, $\xi_k$ the universal $k$-plane bundle and $\bar j_k$ the classifymap of $\xi_k \oplus \underline{\Rr}$. * If $E = \{ \xi_k, \bar j_k \}$ is a stable vector bundle over $\{ Y_k \}$ and $f$ is a sequence of maps $f_k : X_k \to Y_k$ compatible with inclusions then $\{ f_k^* \xi_k, f_k^* \bar j_k \}$ is a stable vector bundle over $\{X_k\}$ called the pull-back bundle and denoted $f^*E$. </wikitex> == B-manifolds and B-bordism == <wikitex>; Let $E = \{ \xi_k, X_k \}$ be a stable vector bundle. A normal $E$-manifold is a triple $(M, f, \alpha)$ consisting of a compact manifold $M$, a map sequence of maps $f_k : M \to X_k$, and a stable bundle isomorphism $\alpha : f^* E \cong \nu_M$. Recall that $\xi: B \to BO$ is a fibration. We next define normal $B$-manifolds and normal $B$-bordism: more details are given in {{cite|Stong1968}}[Chapter II] and {{cite|Kreck1999}}[Section 1]. If $W$ is a compact smooth manifold then $\nu_W$ is classified by a map, also denoted $\nu_M$, $\nu_W : W \to BO$: $\nu_W^*\gamma \cong \nu_W$. A $B$-structure on $W$ is a map $\bar \nu: W \to B$ lifting $\nu_W$ through $B$: $$ \xymatrix{ & B \ar[d]^{\gamma} \ W \ar[r]_{\nu_W}\ar[ur]^{\bar \nu} & BO.} $$ A $B$-manifold is a pair $(W, \bar \nu)$ where $\bar \nu$ is a $B$-structure on $W$. If $W$ has boundary $\partial W = M_0 \sqcup M_1$, a disjoint union of two closed $(n-1)$-manifolds with inclusions $i_\epsilon : M_\epsilon \to W$, $\epsilon = 0, 1$, then $W$ is a $B$-bordism between the $B$-manifolds $(M_0, \bar \nu \circ i_0)$ and $(M, \bar \nu \circ i_1)$. </wikitex> == The Pontrjagin Thom isomorphism == <wikitex> $$ \Omega_n^B \cong \pi_n^S(MB)$$ </wikitex> == Spectral sequences == <wikitex> $$ H_p(B; \pi_q^S) \Longrightarrow \Omega_{p+q}^B$$ </wikitex> == References == {{#RefList:}} [[Category:Theory]]\xi : B \to BO$ where $BO$$BO$ denotes the classifying space of the stable orthogonal group and $B$$B$ is homotopy equivalent to a CW complex of finite type. For example the fibrations $BSO \to BO$$BSO \to BO$ and $BU \to BO$$BU \to BO$ induced by the canonical homomorphisms $SO \to O$$SO \to O$ and $U \to O$$U \to O$ are used for respectively oriented and complex bordism.

Abusing notation, one writes $B$$B$ for the fibraion $\xi$$\xi$: for each $B$$B$ and $n \geq 0$$n \geq 0$ there is a bordism group of closed n-manifolds with $B$$B$ structure denoted

$\displaystyle \Omega_n^B ~~\text{or}~~ \Omega_n(B).$ $$\displaystyle \Omega_n^B ~~\text{or}~~ \Omega_n(B).$$

If $B = BG$$B = BG$ is the classifying space of the stable group $G$$G$ then on often writes

$\displaystyle \Omega_n^G ~~\text{or}~~ \Omega_n(G).$ $$\displaystyle \Omega_n^G ~~\text{or}~~ \Omega_n(G).$$

2 Stable vector bundles

We recall the defintion of a stable vector bundle from [Kreck&Lück2005, 18.10] which requires some notation. Let $\underline{\Rr^k}$$\underline{\Rr^k}$ denote the trivial rank k bundle $Z \times \Rr^k$$Z \times \Rr^k$: the space $Z$$Z$ will be clear from context. Let $\{ X_k \}$$\{ X_k \}$ be a sequence $X_0 \to X_1 \to X_2 \to \dots$$X_0 \to X_1 \to X_2 \to \dots$ of inclusions $j_k : X_k \to X_{k+1}$$j_k : X_k \to X_{k+1}$ of CW-complexes. A stable vector bundle $E$$E$ over $\{ X_k \}$$\{ X_k \}$ is a sequence of rank k-vector bundles $\xi_k \to X_k$$\xi_k \to X_k$ and a sequence of bundle maps

$\displaystyle \bar j_k : \xi_k \oplus \underline{\Rr} \to \xi_{k+1}$ $$\displaystyle \bar j_k : \xi_k \oplus \underline{\Rr} \to \xi_{k+1}$$

where $\bar j_k$$\bar j_k$ covers the inclusion $j_k$$j_k$. An isomorphism of stable vector bundles over $\{ X \}$$\{ X \}$, $\theta : \{ (\xi_k\, \bar j_k) \} \cong \{\xi_k', \bar j_k'\}$$\theta : \{ (\xi_k\, \bar j_k) \} \cong \{\xi_k', \bar j_k'\}$ is a sequence of bundle ismorphisms $\theta_j : \xi_j \cong \xi_k'$$\theta_j : \xi_j \cong \xi_k'$ for $j \geq N$$j \geq N$, $N$$N$ some integer, which are compatible with $\bar j_k$$\bar j_k$ and $\bar j_k'$$\bar j_k'$.

Here are some important examples:

• A rank $j$$j$-vector $V \to X$$V \to X$ over a CW complex $X$$X$ defines a stable vector bundle $\{ V \}$$\{ V \}$ by setting $X_k = X$$X_k = X$ fixed for all $k$$k$, $\xi_k = 0$$\xi_k = 0$ for $k < j$$k < j$ and $\xi_k = E \oplus \underline{\Rr}^{j-k}$$\xi_k = E \oplus \underline{\Rr}^{j-k}$ for $k\geq j$$k\geq j$ with $\bar j_k$$\bar j_k$ the obvious inclusions.
• If $M$$M$ is a compact n-manifold then $M$$M$ embedds into $\Rr^{2n+k}$$\Rr^{2n+k}$ for $k \geq 0$$k \geq 0$ and for $k \geq 1$$k \geq 1$ two such embeddings are isotopic. The normal bundle of any such embedding defines a stable vector bundle over $M$$M$ as in the first example and there is a canonical isomorphicm between any two such stable bundles given by isotopies between the embeddings. We write $\nu_M$$\nu_M$ for the stable normal bundle of $M$$M$. Note that by defintion $\nu_M$$\nu_M$ is a stable inverse to the tangent bundle of $M$$M$:

$\displaystyle \nu_M \oplus TM \cong \{ \underline{\Rr} \}.$ $$\displaystyle \nu_M \oplus TM \cong \{ \underline{\Rr} \}.$$

• The universal bundle $\gamma \to BO$$\gamma \to BO$ is the stable bundle with $X_k = BO(k)$$X_k = BO(k)$, $\xi_k$$\xi_k$ the universal $k$$k$-plane bundle and $\bar j_k$$\bar j_k$ the classifymap of $\xi_k \oplus \underline{\Rr}$$\xi_k \oplus \underline{\Rr}$.
• If $E = \{ \xi_k, \bar j_k \}$$E = \{ \xi_k, \bar j_k \}$ is a stable vector bundle over $\{ Y_k \}$$\{ Y_k \}$ and $f$$f$ is a sequence of maps $f_k : X_k \to Y_k$$f_k : X_k \to Y_k$ compatible with inclusions then $\{ f_k^* \xi_k, f_k^* \bar j_k \}$$\{ f_k^* \xi_k, f_k^* \bar j_k \}$ is a stable vector bundle over $\{X_k\}$$\{X_k\}$ called the pull-back bundle and denoted $f^*E$$f^*E$.

3 B-manifolds and B-bordism

Let $E = \{ \xi_k, X_k \}$$E = \{ \xi_k, X_k \}$ be a stable vector bundle. A normal $E$$E$-manifold is a triple $(M, f, \alpha)$$(M, f, \alpha)$ consisting of a compact manifold $M$$M$, a map sequence of maps $f_k : M \to X_k$$f_k : M \to X_k$, and a stable bundle isomorphism $\alpha : f^* E \cong \nu_M$$\alpha : f^* E \cong \nu_M$.

Recall that $\xi: B \to BO$$\xi: B \to BO$ is a fibration. We next define normal $B$$B$-manifolds and normal $B$$B$-bordism: more details are given in [Stong1968][Chapter II] and [Kreck1999][Section 1]. If $W$$W$ is a compact smooth manifold then $\nu_W$$\nu_W$ is classified by a map, also denoted $\nu_M$$\nu_M$, $\nu_W : W \to BO$$\nu_W : W \to BO$: $\nu_W^*\gamma \cong \nu_W$$\nu_W^*\gamma \cong \nu_W$. A $B$$B$-structure on $W$$W$ is a map $\bar \nu: W \to B$$\bar \nu: W \to B$ lifting $\nu_W$$\nu_W$ through $B$$B$:

$\displaystyle \xymatrix{ & B \ar[d]^{\gamma} \\ W \ar[r]_{\nu_W}\ar[ur]^{\bar \nu} & BO.}$

A $B$$B$-manifold is a pair $(W, \bar \nu)$$(W, \bar \nu)$ where $\bar \nu$$\bar \nu$ is a $B$$B$-structure on $W$$W$. If $W$$W$ has boundary $\partial W = M_0 \sqcup M_1$$\partial W = M_0 \sqcup M_1$, a disjoint union of two closed $(n-1)$$(n-1)$-manifolds with inclusions $i_\epsilon : M_\epsilon \to W$$i_\epsilon : M_\epsilon \to W$, $\epsilon = 0, 1$$\epsilon = 0, 1$, then $W$$W$ is a $B$$B$-bordism between the $B$$B$-manifolds $(M_0, \bar \nu \circ i_0)$$(M_0, \bar \nu \circ i_0)$ and $(M, \bar \nu \circ i_1)$$(M, \bar \nu \circ i_1)$.

4 The Pontrjagin Thom isomorphism

$\displaystyle \Omega_n^B \cong \pi_n^S(MB)$ $$\displaystyle \Omega_n^B \cong \pi_n^S(MB)$$

5 Spectral sequences

$\displaystyle H_p(B; \pi_q^S) \Longrightarrow \Omega_{p+q}^B$ $$\displaystyle H_p(B; \pi_q^S) \Longrightarrow \Omega_{p+q}^B$$

6 References

• [Kreck&Lück2005] M. Kreck and W. Lück, The Novikov conjecture, Birkhäuser Verlag, Basel, 2005. MR2117411 (2005i:19003) Zbl 1058.19001
• [Kreck1999] M. Kreck, Surgery and duality, Ann. of Math. (2) 149 (1999), no.3, 707–754. MR1709301 (2001a:57051) Zbl 0935.57039
• [Stong1968] R. E. Stong, Notes on cobordism theory, Princeton University Press, Princeton, N.J., 1968. MR0248858 (40 #2108) Zbl 0277.57010