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Thom's theorem

Contents

This entry is about Thom’s theorem in cobordism theory. For the isomorphism in cohomology induced by Thom classes see at Thom isomorphism.

Contents

Idea

What is called Thom’s theorem or the Pontrjagin-Thom isomorphism (due to Thom 54, Pontrjagin 55) states that for a given universal G-structure the stable homotopy groups of the universal Thom spectrum MGM G, with its canonical ring spectrum structure form, the cobordism ring

Ω Gπ (MG) \Omega^G_\bullet \simeq \pi_\bullet(M G)

of manifolds with G-structure, and that this isomorphism is exhibited by the Pontryagin-Thom construction (see there).

More generally, for XX a topological space, then the group of GG-bordism classes of GG-manifolds in XX is isomorphic to the generalized homology of XX with coefficients in MGM G:

Ω G(X)π (MGX +)MG (X) \Omega^G_\bullet(X) \simeq \pi_\bullet( M G \wedge X_+) \simeq M G_\bullet(X)

Ingredients

GG-Structure on the Stable normal bundle

Definition

Given a smooth manifold XX of dimension nn and equipped with an embedding

i:X k i \;\colon\; X \hookrightarrow \mathbb{R}^k

for some kk \in \mathbb{N}, then the classifying map of its normal bundle is the function

g i:XGr kn( k)BO(kn) g_i \;\colon\; X \to Gr_{k-n}(\mathbb{R}^k) \hookrightarrow B O(k-n)

which sends xXx \in X to the normal of the tangent space

N xX=(T xX) k N_x X = (T_x X)^{\perp} \hookrightarrow \mathbb{R}^k

regarded as a point in G kn( k)G_{k-n}(\mathbb{R}^k).

The normal bundle of ii itself is the subbundle of the tangent bundle

T k k× k T \mathbb{R}^k \simeq \mathbb{R}^k \times \mathbb{R}^k

consisting of those vectors which are orthogonal to the tangent vectors of XX:

N i{xX,vT i(x) k|vi *T xXT i(x) k}. N_i \coloneqq \left\{ x\in X, v \in T_{i(x)}\mathbb{R}^k \;\vert\; v \,\perp\, i_\ast T_x X \subset T_{i(x)}\mathbb{R}^k \right\} \,.
Definition

A (B,f)(B,f)-structure is

  1. for each nn\in \mathbb{N} a pointed CW-complex B nTop CW */B_n \in Top_{CW}^{\ast/}

  2. equipped with a pointed Serre fibration

    B n f n BO(n) \array{ B_n \\ \downarrow^{\mathrlap{f_n}} \\ B O(n) }

    to the classifying space BO(n)B O(n) (def.);

  3. for all n 1n 2n_1 \leq n_2 a pointed continuous function

    g n 1,n 2:B n 1B n 2g_{n_1, n_2} \;\colon\; B_{n_1} \longrightarrow B_{n_2}

    which is the identity for n 1=n 2n_1 = n_2;

such that for all n 1n 2n_1 \leq n_2 \in \mathbb{N} these squares commute

B n 1 g n 1,n 2 B n 2 f n 1 f n 2 BO(n 1) BO(n 2), \array{ B_{n_1} &\overset{g_{n_1,n_2}}{\longrightarrow}& B_{n_2} \\ {}^{\mathllap{f_{n_1}}}\downarrow && \downarrow^{\mathrlap{f_{n_2}}} \\ B O(n_1) &\longrightarrow& B O(n_2) } \,,

where the bottom map is the canonical one from def. .

The (B,f)(B,f)-structure is multiplicative if it is moreover equipped with a system of maps μ n 1,n 2:B n 1×B n 2B n 1+n 2\mu_{n_1,n_2} \colon B_{n_1}\times B_{n_2} \to B_{n_1 + n_2} which cover the canonical multiplication maps (def.)

B n 1×B n 2 μ n 1,n 2 B n 1+n 2 f n 1×f n 2 f n 1+n 2 BO(n 1)×BO(n 2) BO(n 1+n 2) \array{ B_{n_1} \times B_{n_2} &\overset{\mu_{n_1, n_2}}{\longrightarrow}& B_{n_1 + n_2} \\ {}^{\mathllap{f_{n_1} \times f_{n_2}}}\downarrow && \downarrow^{\mathrlap{f_{n_1 + n_2}}} \\ B O(n_1) \times B O(n_2) &\longrightarrow& B O(n_1 + n_2) }

and which satisfy the evident associativity and unitality, for B 0=*B_0 = \ast the unit, and, finally, which commute with the maps gg in that all n 1,n 2,n 3n_1,n_2, n_3 \in \mathbb{N} these squares commute:

B n 1×B n 2 id×g n 2,n 2+n 3 B n 1×B n 2+n 3 μ n 1,n 2 μ n 1,n 2+n 3 B n 1+n 2 g n 1+n 2,n 1+n 2+n 3 B n 1+n 2+n 3 \array{ B_{n_1} \times B_{n_2} &\overset{id \times g_{n_2,n_2+n_3}}{\longrightarrow}& B_{n_1} \times B_{n_2 + n_3} \\ {}^{\mathllap{\mu_{n_1, n_2}}}\downarrow && \downarrow^{\mathrlap{\mu_{n_1,n_2 + n_3}}} \\ B_{n_1 + n_2} &\underset{g_{n_1+n_2, n_1 + n_2 + n_3}}{\longrightarrow}& B_{n_1 + n_2 + n_3} }

and

B n 1×B n 2 g n 1,n 1+n 3×id B n 1+n 3×B n 2 μ n 1,n 2 μ n 1+n 3,n 2 B n 1+n 2 g n 1+n 2,n 1+n 2+n 3 B n 1+n 2+n 3. \array{ B_{n_1} \times B_{n_2} &\overset{g_{n_1,n_1+n_3} \times id}{\longrightarrow}& B_{n_1+n_3} \times B_{n_2 } \\ {}^{\mathllap{\mu_{n_1, n_2}}}\downarrow && \downarrow^{\mathrlap{\mu_{n_1 + n_3 , n_2}}} \\ B_{n_1 + n_2} &\underset{g_{n_1+n_2, n_1 + n_2 + n_3}}{\longrightarrow}& B_{n_1 + n_2 + n_3} } \,.

Similarly, an S 2S^2-(B,f)(B,f)-structure is a compatible system

f 2n:B 2nBO(2n) f_{2n} \colon B_{2n} \longrightarrow B O(2n)

indexed only on the even natural numbers.

Generally, an S kS^k-(B,f)(B,f)-structure for kk \in \mathbb{N}, k1k \geq 1 is a compatible system

f kn:B knBO(kn) f_{k n} \colon B_{k n} \longrightarrow B O(k n)

for all nn \in \mathbb{N}, hence for all knkk n \in k \mathbb{N}.

We write V n V^\mathcal{B}_n for the universal vector bundle pulled back to the corresponding space of the (B,f)(B,f)-structure and with

V EO(n)×O(n) n (pb) B n f n BO(n) \array{ V^{\mathcal{B}} &\overset{}{\longrightarrow}& E O(n) \underset{O(n)}{\times} \mathbb{R}^n \\ \downarrow &(pb)& \downarrow \\ B_n &\underset{f_n}{\longrightarrow}& B O(n) }

and we write e n 1,n 2e_{n_1,n_2} for the maps of total space of vector bundles over the g n 1,n 2g_{n_1,n_2}:

V n 1 e n 1,n 2 V n 2 (pb) B n 1 g n 1,n 2 B n 2. \array{ V^{\mathcal{B}}_{n_1} &\overset{e_{n_1,n_2}}{\longrightarrow}& V^{\mathcal{B}}_{n_2} \\ \downarrow &(pb)& \downarrow \\ B_{n_1} &\underset{g_{n_1,n_2}}{\longrightarrow}& B_{n_2} } \,.
Example

Examples of (B,f)(B,f)-structures (def. ) include the following:

  1. B n=BO(n)B_n = B O(n) and f n=idf_n = id is orthogonal structure (or “no structure”);

  2. B n=EO(n)B_n = E O(n) and f nf_n the universal principal bundle-projection is framing-structure;

  3. B n=BSO(n)=EO(n)/SO(n)B_n = B SO(n) = E O(n)/SO(n) the classifying space of the special orthogonal group and f nf_n the canonical projection is orientation structure;

  4. B n=BSpin(n)=EO(n)/Spin(n)B_n = B Spin(n) = E O(n)/Spin(n) the classifying space of the spin group and f nf_n the canonical projection is spin structure.

Examples of S 2S^2-(B,f)(B,f)-structures (def. ) include

  1. B 2n=BU(n)=EO(2n)/U(n)B_{2n} = B U(n) = E O(2n)/U(n) the classifying space of the unitary group, and f 2nf_{2n} the canonical projection is almost complex structure (or rather: almost Hermitian structure).

  2. B 2n=BSp(2n)=EO(2n)/Sp(2n)B_{2n} = B Sp(2n) = E O(2n)/Sp(2n) the classifying space of the symplectic group, and f 2nf_{2n} the canonical projection is almost symplectic structure.

Examples of S 4S^4-(B,f)(B,f)-structures (def. ) include

  1. B 4n=BU (n)=EO(4n)/U (n)B_{4n} = B U_{\mathbb{H}}(n) = E O(4n)/U_{\mathbb{H}}(n) the classifying space of the quaternionic unitary group, and f 4nf_{4n} the canonical projection is almost quaternionic structure.
Definition

Given a smooth manifold XX of dimension nn, and given a (B,f)(B,f)-structure as in def. , then a (B,f)(B,f)-structure on the stable normal bundle of the manifold is an equivalence class of the following structure:

  1. an embedding i X:X ki_X \; \colon \; X \hookrightarrow \mathbb{R}^k for some kk \in \mathbb{N};

  2. a homotopy class of a lift g^\hat g of the classifying map gg of the normal bundle (def. )

    B kn g^ f kn X g BO(kn). \array{ && B_{k-n} \\ &{}^{\mathllap{\hat g}}\nearrow& \downarrow^{\mathrlap{f_{k-n}}} \\ X &\overset{g}{\longrightarrow}& B O(k-n) } \,.

The equivalence relation on such structures is to be that generated by the relation ((i X) 1,g^ 1)((i X) ,g^ 2)((i_{X})_1, \hat g_1) \sim ((i_{X})_,\hat g_2) if

  1. k 2k 1k_2 \geq k_1

  2. the second inclusion factors through the first as

    (i X) 2:X(i X) 1 k 1 k 2 (i_X)_2 \;\colon\; X \overset{(i_X)_1}{\hookrightarrow} \mathbb{R}^{k_1} \hookrightarrow \mathbb{R}^{k_2}
  3. the lift of the classifying map factors accordingly (as homotopy classes)

    g^ 2:Xg^ 1B k 1ng k 1n,k 2nB k 2n. \hat g_2 \;\colon\; X \overset{\hat g_1}{\longrightarrow} B_{k_1-n} \overset{g_{k_1-n,k_2-n}}{\longrightarrow} B_{k_2-n} \,.

Bordism

Throughout, let \mathcal{B} be a multiplicative (B,f)-structure (def.).

Definition

Write I[0,1]I \coloneqq [0,1] for the standard interval, regarded as a smooth manifold with boundary. For c +c \in \mathbb{R}_+ Consider its embedding

e:I 0 e \;\colon\; I \hookrightarrow \mathbb{R}\oplus \mathbb{R}_{\geq 0}

as the arc

e:tcos(πt)e 1+sin(πt)e 2, e \;\colon\; t \mapsto \cos(\pi t) \cdot e_1 + \sin(\pi t) \cdot e_2 \,,

where (e 1,e 2)(e_1, e_2) denotes the canonical linear basis of 2\mathbb{R}^2, and equipped with the structure of a manifold with normal framing structure (def.) by equipping it with the canonical framing

fr:tcos(πt)e 1+sin(πt)e 2 fr \;\colon\; t \mapsto \cos(\pi t) \cdot e_1 + \sin(\pi t) \cdot e_2

of its normal bundle.

Let now \mathcal{B} be a (B,f)-structure (def.). Then for Xi kX \overset{i}{\hookrightarrow}\mathbb{R}^k any embedded manifold with \mathcal{B}-structure g^:XB kn\hat g \colon X \to B_{k-n} on its normal bundle (def.), define its negative or orientation reversal (X,i,g^)-(X,i,\hat g) of (X,i,g^)(X,i, \hat g) to be the restriction of the structured manifold

(X×I(i,e) k+2,g^×fr) (X \times I \overset{(i,e)}{\hookrightarrow} \mathbb{R}^{k+2}, \hat g \times fr)

to t=1t = 1.

Definition

Two closed manifolds of dimension nn equipped with normal \mathcal{B}-structure (X 1,i 1,g^ 1)(X_1, i_1, \hat g_1) and (X 2,i 2,g^ 2)(X_2,i_2,\hat g_2) (def.) are called bordant if there exists a manifold with boundary WW of dimension n+1n+1 equipped with \mathcal{B}-strcuture (W,i W,g^ W)(W,i_W, \hat g_W) if its boundary with \mathcal{B}-structure restricted to that boundary is the disjoint union of X 1X_1 with the negative of X 2X_2, according to def.

(W,i W,g^ W)(X 1,i 1,g^ 1)(X 2,i 2,g^ 2). \partial(W,i_W,\hat g_W) \simeq (X_1, i_1, \hat g_1) \sqcup -(X_2, i_2, \hat g_2) \,.
Proposition

The relation of \mathcal{B}-bordism (def. ) is an equivalence relation.

Write Ω \Omega^\mathcal{B}_{\bullet} for the \mathbb{N}-graded set of \mathcal{B}-bordism classes of \mathcal{B}-manifolds.

Proposition

Under disjoint union of manifolds, then the set of \mathcal{B}-bordism equivalence classes of def. becomes an \mathbb{Z}-graded abelian group

Ω Ab \Omega^{\mathcal{B}}_\bullet \in Ab^{\mathbb{Z}}

(that happens to be concentrated in non-negative degrees). This is called the \mathcal{B}-bordism group.

Moreover, if the (B,f)-structure \mathcal{B} is multiplicative (def.), then Cartesian product of manifolds followed by the multiplicative composition operation of \mathcal{B}-structures makes the \mathcal{B}-bordism ring into a commutative ring, called the \mathcal{B}-bordism ring.

Ω CRing . \Omega^{\mathcal{B}}_\bullet \in CRing^{\mathbb{Z}} \,.

e.g. (Kochmann 96, prop. 1.5.3)

Thom spaces

Definition

Let XX be a topological space and let VXV \to X be a vector bundle over XX of rank nn, which is associated to an O(n)-principal bundle. Equivalently this means that VXV \to X is the pullback of the universal vector bundle E nBO(n)E_n \to B O(n) (def. ) over the classifying space. Since O(n)O(n) preserves the metric on n\mathbb{R}^n, by definition, such VV inherits the structure of a metric space-fiber bundle. With respect to this structure:

  1. the unit disk bundle D(V)XD(V) \to X is the subbundle of elements of norm 1\leq 1;

  2. the unit sphere bundle S(V)XS(V)\to X is the subbundle of elements of norm =1= 1;

    S(V)i VD(V)VS(V) \overset{i_V}{\hookrightarrow} D(V) \hookrightarrow V;

  3. the Thom space Th(V)Th(V) is the cofiber (formed in Top (prop.)) of i Vi_V

    Th(V)cofib(i V) Th(V) \coloneqq cofib(i_V)

    canonically regarded as a pointed topological space.

S(V) i V D(V) (po) * Th(V). \array{ S(V) &\overset{i_V}{\longrightarrow}& D(V) \\ \downarrow &(po)& \downarrow \\ \ast &\longrightarrow& Th(V) } \,.

If VXV \to X is a general real vector bundle, then there exists an isomorphism to an O(n)O(n)-associated bundle and the Thom space of VV is, up to based homeomorphism, that of this orthogonal bundle.

Remark

If the rank of VV is positive, then S(V)S(V) is non-empty and then the Thom space (def. ) is the quotient topological space

Th(V)D(V)/S(V). Th(V) \simeq D(V)/S(V) \,.

However, in the degenerate case that the rank of VV vanishes, hence the case that V=X× 0XV = X\times \mathbb{R}^0 \simeq X, then D(V)VXD(V) \simeq V \simeq X, but S(V)=S(V) = \emptyset. Hence now the pushout defining the cofiber is

i V X (po) * Th(V)X *, \array{ \emptyset &\overset{i_V}{\longrightarrow}& X \\ \downarrow &(po)& \downarrow \\ \ast &\longrightarrow& Th(V) \simeq X_* } \,,

which exhibits Th(V)Th(V) as the coproduct of XX with the point, hence as XX with a basepoint freely adjoined.

Th(X× 0)=Th(X)X +. Th(X \times \mathbb{R}^0) = Th(X) \simeq X_+ \,.
Proposition

Let V 1,V 2XV_1,V_2 \to X be two real vector bundles. Then the Thom space (def. ) of the direct sum of vector bundles V 1V 2XV_1 \oplus V_2 \to X is expressed in terms of the Thom space of the pullbacks V 2| D(V 1)V_2|_{D(V_1)} and V 2| S(V 1)V_2|_{S(V_1)} of V 2V_2 to the disk/sphere bundle of V 1V_1 as

Th(V 1V 2)Th(V 2| D(V 1))/Th(V 2| S(V 1)). Th(V_1 \oplus V_2) \simeq Th(V_2|_{D(V_1)})/Th(V_2|_{S(V_1)}) \,.
Proof

Notice that

  1. D(V 1V 2)D(V 2| IntD(V 1))S(V 1)D(V_1 \oplus V_2) \simeq D(V_2|_{Int D(V_1)}) \cup S(V_1);

  2. S(V 1V 2)S(V 2| IntD(V 1))IntD(V 2| S(V 1))S(V_1 \oplus V_2) \simeq S(V_2|_{Int D(V_1)}) \cup Int D(V_2|_{S(V_1)}).

(Since a point at radius rr in V 1V 2V_1 \oplus V_2 is a point of radius r 1rr_1 \leq r in V 2V_2 and a point of radius r 2r 1 2\sqrt{r^2 - r_1^2} in V 1V_1.)

Proposition

For VV a vector bundle then the Thom space (def. ) of nV\mathbb{R}^n \oplus V, the direct sum of vector bundles with the trivial rank nn vector bundle, is homeomorphic to the smash product of the Thom space of VV with the nn-sphere (the nn-fold reduced suspension).

Th( nV)S nTh(V)=Σ nTh(V). Th(\mathbb{R}^n \oplus V) \simeq S^n \wedge Th(V) = \Sigma^n Th(V) \,.
Proof

Apply prop. with V 1= nV_1 = \mathbb{R}^n and V 2=VV_2 = V. Since V 1V_1 is a trivial bundle, then

V 2| D(V 1)V 2×D n V_2|_{D(V_1)} \simeq V_2\times D^n

(as a bundle over X×D nX\times D^n) and similarly

V 2| S(V 1)V 2×S n. V_2|_{S(V_1)} \simeq V_2\times S^n \,.
Example

By prop. and remark the Thom space (def. ) of a trivial vector bundle of rank nn is the nn-fold suspension of the base space

Th(X× n) S nTh(X× 0) S n(X +). \begin{aligned} Th(X \times \mathbb{R}^n) & \simeq S^n \wedge Th(X\times \mathbb{R}^0) \\ & \simeq S^n \wedge (X_+) \end{aligned} \,.

Therefore a general Thom space may be thought of as a “twisted suspension”, with twist encoded by a vector bundle (or rather by its underlying spherical fibration). See at Thom spectrum – For infinity-module bundles for more on this.

Proposition

For V 1X 1V_1 \to X_1 and V 2X 2V_2 \to X_2 to vector bundles, let V 1V 2X 1×X 2V_1 \boxtimes V_2 \to X_1 \times X_2 be the direct sum of vector bundles of their pullbacks to X 1×X 2X_1 \times X_2. The corresponding Thom space (def. ) is the smash product of the individual Thom spaces:

Th(V 1V 2)Th(V 1)Th(V 2). Th(V_1 \boxtimes V_2) \simeq Th(V_1) \wedge Th(V_2) \,.

Universal Thom spectra MGM G

Proposition

For each nn \in \mathbb{N} the pullback of the rank-(n+1)(n+1) universal vector bundle to the classifying space of rank nn vector bundles is the direct sum of vector bundles of the rank nn universal vector bundle with the trivial rank-1 bundle: there is a pullback diagram of topological spaces of the form

(EO(n)×O(n) n) EO(n+1)×O(n+1) n+1 (pb) BO(n) BO(n+1), \array{ \mathbb{R}\oplus (E O(n)\underset{O(n)}{\times} \mathbb{R}^n) &\longrightarrow& E O(n+1) \underset{O(n+1)}{\times} \mathbb{R}^{n+1} \\ \downarrow &(pb)& \downarrow \\ B O(n) &\longrightarrow& B O(n+1) } \,,

where the bottom morphism is the canonical one (def.).

(e.g. Kochmann 96, p. 25)

Proof

For each kk \in \mathbb{N}, knk \geq n there is such a pullback of the canonical vector bundles over Grassmannians

{V n k,vV n,v n+1} {V n+1 k+1,vV n+1} Gr n( k) Gr n+1( k+1) \array{ \left\{ {V_{n}\subset \mathbb{R}^k, } \atop {v \in V_n, v_{n+1} \in \mathbb{R}} \right\} &\longrightarrow& \left\{ {V_{n+1} \subset \mathbb{R}^{k+1}}, \atop v \in V_{n+1} \right\} \\ \downarrow && \downarrow \\ Gr_n(\mathbb{R}^k) &\longrightarrow& Gr_{n+1}(\mathbb{R}^{k+1}) }

where the bottom morphism is the canonical inclusion (def.).

Now we claim that taking the colimit in each of the four corners of this system of pullback diagrams yields again a pullback diagram, and this proves the claim.

To see this, remember that we work in the category Top cgTop_{cg} of compactly generated topological spaces (def.). By their nature, we may test the universal property of a would-be pullback space already by mapping compact topological spaces into it. Now observe that all the inclusion maps in the four corners of this system of diagrams are relative cell complex inclusions, by prop. . Together this implies (via this lemma) that we may test the universal property of the colimiting square at finite stages. And so this implies the claim by the above fact that at each finite stage there is a pullback diagram.

Definition

The universal real Thom spectrum MOM O is the spectrum, which is represented by the sequential prespectrum (def.) whose nnth component space is the Thom space (def. )

(MO) nTh(EO(n)×O(n) n) (M O)_n \coloneqq Th(E O(n)\underset{O(n)}{\times}\mathbb{R}^n)

of the rank-nn universal vector bundle, and whose structure maps are the image under the Thom space functor Th()Th(-) of the top morphisms in prop. , via the homeomorphisms of prop. :

σ n:Σ(MO) nTh((EO(n)×O(n) n))Th(EO(n+1)×O(n+1) n+1)=(MO) n+1. \sigma_n \;\colon\; \Sigma (M O)_n \simeq Th(\mathbb{R}\oplus (E O(n)\underset{O(n)}{\times} \mathbb{R}^n)) \stackrel{}{\longrightarrow} Th(E O(n+1) \underset{O(n+1)}{\times} \mathbb{R}^{n+1}) = (M O)_{n+1} \,.

More generally, there are universal Thom spectra associated with any other tangent structure (“(B,f)]-structure?”), notably for the orthogonal group replaced by the special orthogonal groups SO(n)SO(n), or the spin groups Spin(n)Spin(n), or the string 2-group String(n)String(n), or the fivebrane 6-group Fivebrane(n)Fivebrane(n),…, or any level in the Whitehead tower of O(n)O(n). To any of these groups there corresponds a Thom spectrum (denoted, respectively, MSOM SO, MSpin, MStringM String, MFivebraneM Fivebrane, etc.), which is in turn related to oriented cobordism, spin cobordism, string cobordism, et cetera.:

Definition

Given a (B,f)-structure \mathcal{B} (def. ), write V n V^\mathcal{B}_n for the pullback of the universal vector bundle (def. ) to the corresponding space of the (B,f)(B,f)-structure and with

V VO(n)×O(n) n (pb) B n f n BO(n) \array{ V^{\mathcal{B}} &\overset{}{\longrightarrow}& V O(n) \underset{O(n)}{\times} \mathbb{R}^n \\ \downarrow &(pb)& \downarrow \\ B_n &\underset{f_n}{\longrightarrow}& B O(n) }

and we write e n 1,n 2e_{n_1,n_2} for the maps of total space of vector bundles over the g n 1,n 2g_{n_1,n_2}:

V n 1 e n 1,n 2 V n 2 (pb) B n 1 g n 1,n 2 B n 2. \array{ V^{\mathcal{B}}_{n_1} &\overset{e_{n_1,n_2}}{\longrightarrow}& V^{\mathcal{B}}_{n_2} \\ \downarrow &(pb)& \downarrow \\ B_{n_1} &\underset{g_{n_1,n_2}}{\longrightarrow}& B_{n_2} } \,.

Observe that the analog of prop. still holds:

Proposiiton

Given a (B,f)-structure \mathcal{B} (def. ), then the pullback of its rank-(n+1)(n+1) vector bundle V n+1 V^{\mathcal{B}}_{n+1} (def. ) along the map g n,n+1:B nB n+1g_{n,n+1} \colon B_n \to B_{n+1} is the direct sum of vector bundles of the rank-nn bundle V n V^{\mathcal{B}}_n with the trivial rank-1-bundle: there is a pullback square

V n e n,n+1 V n+1 (pb) B n g n,n+1 B n+1. \array{ \mathbb{R} \oplus V^{\mathcal{B}}_n &\overset{e_{n,n+1}}{\longrightarrow}& V^{\mathcal{B}}_{n+1} \\ \downarrow &(pb)& \downarrow \\ B_n &\underset{g_{n,n+1}}{\longrightarrow}& B_{n+1} } \,.
Proof

Unwinding the definitions, the pullback in question is

(g n,n+1) *V n+1 =(g n,n+1) *f n+1 *(EO(n+1)×O(n+1) n+1) (g n,n+1f n+1) *(EO(n+1)×O(n+1) n+1) (f ni n) *(EO(n+1)×O(n+1) n+1) f n *i n *(EO(n+1)×O(n+1) n+1) f n *((EO(n)×O(n) n)) V n, \begin{aligned} (g_{n,n+1})^\ast V^{\mathcal{B}}_{n+1} & = (g_{n,n+1})^\ast f_{n+1}^\ast (E O(n+1)\underset{O(n+1)}{\times} \mathbb{R}^{n+1}) \\ & \simeq (g_{n,n+1} \circ f_{n+1})^\ast (E O(n+1)\underset{O(n+1)}{\times} \mathbb{R}^{n+1}) \\ & \simeq ( f_n \circ i_n )^\ast (E O(n+1)\underset{O(n+1)}{\times} \mathbb{R}^{n+1}) \\ & \simeq f_n^\ast i_n^\ast (E O(n+1)\underset{O(n+1)}{\times} \mathbb{R}^{n+1}) \\ & \simeq f_n^\ast (\mathbb{R} \oplus (E O(n)\underset{O(n)}{\times} \mathbb{R}^{n})) \\ &\simeq \mathbb{R} \oplus V^{\mathcal{B}_n} \,, \end{aligned}

where the second but last step is due to prop. .

Definition

Given a (B,f)-structure \mathcal{B} (def. ), its universal Thom spectrum MM \mathcal{B} is, as a sequential prespectrum, given by component spaces being the Thom spaces (def. ) of the \mathcal{B}-associated vector bundles of def.

(M) nTh(V n ) (M \mathcal{B})_n \coloneqq Th(V^{\mathcal{B}}_n)

and with structure maps given via prop. by the top maps in prop. :

σ n:Σ(M) n=ΣTh(V n )Th(V n )Th(e n,n+1)Th(V n+1 )=(M) n+1. \sigma_n \;\colon\; \Sigma (M \mathcal{B})_n = \Sigma Th(V^{\mathcal{E}}_n) \simeq Th(\mathbb{R}\oplus V^{\mathcal{E}}_n) \overset{Th(e_{n,n+1})}{\longrightarrow} Th(V^{\mathcal{E}}_{n+1}) = (M \mathcal{B})_{n+1} \,.

Similarly for an S k(B,f)S^k-(B,f)-structure indexed on every kkth natural number (such as almost complex structure, almost quaternionic structure, example ), there is the corresponding Thom spectrum as a sequential S kS^k spectrum (def.).

If B n=BG nB_n = B G_n for some natural system of groups G nO(n)G_n \to O(n), then one usually writes MGM G for MM \mathcal{B}. For instance MSOM SO, MSpin, MU, MSp etc.

If the (B,f)(B,f)-structure is multiplicative (def. ), then the Thom spectrum MM \mathcal{B} canonical becomes a ring spectrum: the multiplication maps B n 1×B n 2B n 1+n 2B_{n_1} \times B_{n_2}\to B_{n_1 + n_2} are covered by maps of vector bundles

V n 1 V n 2 V n 1+n 2 V^{\mathcal{B}}_{n_1} \boxtimes V^{\mathcal{B}}_{n_2} \longrightarrow V^{\mathcal{B}}_{n_1 + n_2}

and under forming Thom spaces this yields (prop.) maps

(M) n 1(M) n 2(M) n 1+n 2 (M \mathcal{B})_{n_1} \wedge (M \mathcal{B})_{n_2} \longrightarrow (M \mathcal{B})_{n_1 + n_2}

which are associative by the associativity condition in a multiplicative (B,f)(B,f)-structure. The unit is

(M) 0=Th(V 0 )Th(*)S 0, (M \mathcal{B})_0 = Th(V^{\mathcal{B}}_0) \simeq Th(\ast) \simeq S^0 \,,

by remark .

Example

The universal Thom spectrum (def. ) for framing structure (exmpl.) is equivalently the sphere spectrum (def.)

M1𝕊. M 1 \simeq \mathbb{S} \,.

Because in this case B n*B_n \simeq \ast and so E n nE^{\mathcal{B}}_n \simeq \mathbb{R}^n, whence Th(E n )S nTh(E^{\mathcal{B}}_n) \simeq S^n.

Pontrjagin-Thom construction

Definition

For XX a smooth manifold and i:X ki \colon X \hookrightarrow \mathbb{R}^k an embedding, then a tubular neighbourhood of XX is a subset of the form

τ iX{x k|d(x,i(X))<ϵ} \tau_i X \coloneqq \left\{ x \in \mathbb{R}^k \;\vert\; d(x,i(X)) \lt \epsilon \right\}

for some ϵ\epsilon \in \mathbb{R}, ϵ>0\epsilon \gt 0, small enough such that the map

N iXτ iX N_i X \longrightarrow \tau_i X

from the normal bundle (def. ) given by

(i(x),v)(i(x),ϵ(1e |v|)v) (i(x),v) \mapsto (i(x), \epsilon (1-e^{- {\vert v\vert}}) v )

is a diffeomorphism.

Proposition

(tubular neighbourhood theorem)

For every embedding of smooth manifolds, there exists a tubular neighbourhood according to def. .

Remark

Given an embedding i:X ki \colon X \hookrightarrow \mathbb{R}^k with a tubuluar neighbourhood τ iXhookrigtharrow k\tau_i X \hookrigtharrow \mathbb{R}^k (def. ) then by construction:

  1. the Thom space (def. ) of the normal bundle (def. ) is homeomorphic to the quotient topological space of the topological closure of the tubular neighbourhood by its boundary:

    Th(N i(X))τ i(X)¯/τ i(X)¯Th(N_i(X)) \simeq \overline{ \tau_i(X)}/\partial \overline{\tau_i(X)};

  2. there exists a continous function

    kτ i(X)¯/τ i(X)¯ \mathbb{R}^k \longrightarrow \overline{ \tau_i(X)}/\partial \overline{\tau_i(X)}

    which is the identity on τ i(X) k\tau_i(X)\subset \mathbb{R}^k and is constant on the basepoint of the quotient on all other points.

Definition

For XX a smooth manifold of dimension nn and for i:X ki \colon X \hookrightarrow \mathbb{R}^k an embedding, then the Pontrjagin-Thom collapse map is, for any choice of tubular neighbourhood τ i(X) k\tau_i(X)\subset \mathbb{R}^k (def. ) the composite map of pointed topological spaces

S k( k) *τ i(X)¯/τ i(X)¯Th(N iX) S^k \overset{\simeq}{\to} (\mathbb{R}^k)^\ast \longrightarrow \overline{ \tau_i(X)}/\partial \overline{\tau_i(X)} \overset{\simeq}{\to} Th(N_i X)

where the first map identifies the k-sphere as the one-point compactification of k\mathbb{R}^k; and where the second and third maps are those of remark .

The Pontrjagin-Thom construction is the further composite

ξ i:S kTh(N iX)Th(e i)Th(EO(kn)×O(kn) kn)(MO) kn \xi_i \;\colon\; S^k \longrightarrow Th(N_i X) \overset{Th(e_i)}{\longrightarrow} Th( E O(k-n) \underset{O(k-n)}{\times} \mathbb{R}^{k-n} ) \simeq (M O)_{k-n}

with the image under the Thom space construction of the morphism of vector bundles

ν e i EO(kn)×O(kn) kn (pb) X g i BO(kn) \array{ \nu &\overset{e_i}{\longrightarrow}& E O(k-n)\underset{O(k-n)}{\times} \mathbb{R}^{k-n} \\ \downarrow &(pb)& \downarrow \\ X &\underset{g_i}{\longrightarrow}& B O(k-n) }

induced by the classifying map g ig_i of the normal bundle (def. ).

This defines an element

[S n+(kn)ξ i(MO) kn]π nMO [S^{n+(k-n)} \overset{\xi_i}{\to} (M O)_{k-n}] \in \pi_{n} M O

in the nnth stable homotopy group (def.) of the Thom spectrum MOM O (def. ).

More generally, for XX a smooth manifold with normal (B,f)-structure (X,i,g^ i)(X,i,\hat g_i) according to def. , then its Pontrjagin-Thom construction is the composite

ξ i:S kTh(N iX)Th(e^ i)Th(V kn )(M) kn \xi_i \;\colon\; S^k \longrightarrow Th(N_i X) \overset{Th(\hat e_i)}{\longrightarrow} Th( V^{\mathcal{B}}_{k-n} ) \simeq (M \mathcal{B})_{k-n}

with

ν e^ i V kn (pb) X g^ i BO(kn). \array{ \nu &\overset{\hat e_i}{\longrightarrow}& V^{\mathcal{B}}_{k-n} \\ \downarrow &(pb)& \downarrow \\ X &\underset{\hat g_i}{\longrightarrow}& B O(k-n) } \,.
Proposition

The Pontrjagin-Thom construction (def. ) respects the equivalence classes entering the definition of manifolds with stable normal \mathcal{B}-structure (def. ) hence descends to a function (of sets)

ξ:{n-manifoldswithstablenormal-structure}π n(M). \xi \;\colon\; \left\{ {n\text{-}manifolds\;with\;stable} \atop {normal\;\mathcal{B}\text{-}structure} \right\} \longrightarrow \pi_n(M\mathcal{B}) \,.
Proof

It is clear that the homotopies of classifying maps of \mathcal{B}-structures that are devided out in def. map to homotopies of representatives of stable homotopy groups. What needs to be shown is that the construction respects the enlargement of the embedding spaces.

Given a embedded manifold Xi k 1X \overset{i}{\hookrightarrow}\mathbb{R}^{k_1} with normal \mathcal{B}-structure

B k 1n g^ i f kn X g i BO(k 1n) \array{ && B_{k_1-n} \\ & {}^{\mathllap{\hat g_i}}\nearrow & \downarrow^{\mathrlap{f_{k-n}}} \\ X &\underset{g_i}{\longrightarrow}& B O(k_1-n) }

write

α:S n+(k 1n)Th(E k 1n) \alpha \;\colon\; S^{n+(k_1-n)} \overset{}{\longrightarrow} Th(E^{\mathcal{B}_{k_1-n}})

for its image under the Pontrjagin-Thom construction (def. ). Now given k 2k_2 \in \mathbb{N}, consider the induced embedding Xi k 1 k 1+k 2X \overset{i}{\hookrightarrow} \mathbb{R}^{k_1}\hookrightarrow \mathbb{R}^{k_1 + k_2} with normal \mathcal{B}-structure given by the composite

B k 1n g k 1n,k 1+k 2n B k 1+k 2n g^ i f k 1n×f k 2 f k 1+k 2n X g i BO(k 1n) BO(k 1+k 2n). \array{ && B_{k_1-n} &\overset{g_{k_1-n, k_1+ k_2 -n}}{\longrightarrow}& B_{k_1 + k_2-n} \\ & {}^{\mathllap{\hat g_i}}\nearrow & \downarrow^{\mathrlap{f_{k_1 - n} \times f_{k_2}}} && \downarrow^{\mathrlap{f_{k_1 + k_2-n}}} \\ X &\underset{g_i}{\longrightarrow}& B O(k_1-n) &\longrightarrow& B O(k_1 + k_2-n) } \,.

By prop. and using the pasting law for pullbacks, the classifying map g^ i\hat g'_i for the enlarged normal bundle sits in a diagram of the form

(ν i k 2) (e^ iid) (V k 1n k 2) e k 1n,k 1+k 2n V k 1+k 2n (pb) (pb) X g^ i B k 1n g k 1n,k 1+k 2n B k 1+k 2n. \array{ (\nu_i \oplus \mathbb{R}^{k_2}) &\overset{(\hat e_i \oplus id)}{\longrightarrow}& (V^{\mathcal{B}}_{k_1-n} \oplus \mathbb{R}^{k_2}) &\overset{e_{k_1-n,k_1+k_2-n}}{\longrightarrow}& V^{\mathcal{B}}_{k_1 + k_2 - n} \\ \downarrow &(pb)& \downarrow &(pb)& \downarrow \\ X &\underset{\hat g_i}{\longrightarrow}& B_{k_1-n} &\underset{g_{k_1-n, k_1 + k_2 - n}}{\longrightarrow}& B_{k_1 +k_2 - n} } \,.

Hence the Pontrjagin-Thom construction for the enlarged embedding space is (using prop. ) the composite

α k 2:S n+(k 1+k 2n)Th( k 2)S n+(k 1n)Th( k 2)Th(ν i)Th(id)Th(e^ i)Th( k 2)Th(E k 1n ))Th(e k 1n,k 1+k 2n)Th(V k 1+k 2n ). \alpha_{k_2} \;\colon\; S^{n + (k_1+ k_2 - n)} \simeq Th(\mathbb{R}^{k_2}) \wedge S^{n + (k_1 - n)} \overset{}{\longrightarrow} Th(\mathbb{R}^{k_2}) \wedge Th(\nu_i) \overset{Th(id)\wedge Th(\hat e_i)}{\longrightarrow} Th(\mathbb{R}^{k_2}) \wedge Th(E^{\mathcal{B}}_{k_1-n})) \overset{Th(e_{k_1-n, k_1 + k_2 - n})}{\longrightarrow} Th(V^{\mathcal{B}}_{k_1 + k_2 - n}) \,.

The composite of the first two morphisms here is S k kαS^{k_k}\wedge \alpha, while last morphism Th(e^ k 1n,k 1+k 2n)Th(\hat e_{k_1-n,k_1+k_2-n}) is the structure map in the Thom spectrum (by def. ):

α k 2:S k 2S n+(k 1n)S k 2αS k 2Th(E k 1+k 2n )σ k 1n,k 1+k 2n MTh(V k 1+k 2n ) \alpha_{k_2} \;\colon\; S^{k_2} \wedge S^{n + (k_1 - n)} \overset{S^{k_2} \wedge \alpha}{\longrightarrow} S^{k_2} \wedge Th(E^{\mathcal{B}}_{k_1 + k_2 - n}) \overset{\sigma^{M \mathcal{B}}_{k_1-n,k_1 + k_2 - n} }{\longrightarrow} Th(V^{\mathcal{B}}_{k_1+k_2 - n})

This manifestly identifies α k 2\alpha_{k_2} as being the image of α\alpha under the component map in the sequential colimit that defines the stable homotopy groups (def.). Therefore α\alpha and α k 2\alpha_{k_2}, for all k 2k_2 \in \mathbb{N}, represent the same element in π (M)\pi_{\bullet}(M \mathcal{B}).

Thom’s theorem

Recall that the Pontrjagin-Thom construction (def.) associates to an embbeded manifold (X,i,g^)(X,i,\hat g) with normal \mathcal{B}-structure (def.) an element in the stable homotopy group π dim(X)(M)\pi_{dim(X)}(M \mathcal{B}) of the universal \mathcal{B}-Thom spectrum in degree the dimension of that manifold.

Lemma

For \mathcal{B} be a multiplicative (B,f)-structure (def.), the \mathcal{B}-Pontrjagin-Thom construction (def.) is compatible with all the relations involved to yield a graded ring homomorphism

ξ:Ω π (M) \xi \;\colon\; \Omega^{\mathcal{B}}_\bullet \longrightarrow \pi_\bullet(M \mathcal{B})

from the \mathcal{B}-bordism ring (def. ) to the stable homotopy groups of the universal \mathcal{B}-Thom spectrum equipped with the ring structure induced from the canonical ring spectrum structure (def.).

Proof

By prop. the underlying function of sets is well-defined before dividing out the bordism relation (def. ). To descend this further to a function out of the set underlying the bordism ring, we need to see that the Pontrjagin-Thom construction respects the bordism relation. But the definition of bordism is just so as to exhibit under ξ\xi a left homotopy of representatives of homotopy groups.

Next we need to show that it is

  1. a group homomorphism;

  2. a ring homomorphism.

Regarding the first point:

The element 0 in the cobordism group is represented by the empty manifold. It is clear that the Pontrjagin-Thom construction takes this to the trivial stable homotopy now.

Given two nn-manifolds with \mathcal{B}-structure, we may consider an embedding of their disjoint union into some k\mathbb{R}^{k} such that the tubular neighbourhoods of the two direct summands do not intersect. There is then a map from two copies of the k-cube, glued at one face

k k1 k k \Box^k \underset{\Box^{k-1}}{\sqcup} \Box^k \longrightarrow \mathbb{R}^k

such that the first manifold with its tubular neighbourhood sits inside the image of the first cube, while the second manifold with its tubular neighbourhood sits indide the second cube. After applying the Pontryagin-Thom construction to this setup, each cube separately maps to the image under ξ\xi of the respective manifold, while the union of the two cubes manifestly maps to the sum of the resulting elements of homotopy groups, by the very definition of the group operation in the homotopy groups (def.). This shows that ξ\xi is a group homomorphism.

Regarding the second point:

The element 1 in the cobordism ring is represented by the manifold which is the point. Without restriction we may consoder this as embedded into 0\mathbb{R}^0, by the identity map. The corresponding normal bundle is of rank 0 and hence (by remark ) its Thom space is S 0S^0, the 0-sphere. Also V 0 V^{\mathcal{B}}_0 is the rank-0 vector bundle over the point, and hence (M) 0S 0(M \mathcal{B})_0 \simeq S^0 (by def. ) and so ξ(*):(S 0S 0)\xi(\ast) \colon (S^0 \overset{\simeq}{\to} S^0) indeed represents the unit element in π (M)\pi_\bullet(M\mathcal{B}).

Finally regarding respect for the ring product structure: for two manifolds with stable normal \mathcal{B}-structure, represented by embeddings into k i\mathbb{R}^{k_i}, then the normal bundle of the embedding of their Cartesian product is the direct sum of vector bundles of the separate normal bundles bulled back to the product manifold. In the notation of prop. there is a diagram of the form

ν 1ν 2 e^ 1e^ 2 V n 1 V n 2 κ n 1,n 2 V n 1+n 2 (pb) (pb) X 1×X 2 g^ 1×g^ 2 B k 1n 1×B k 2n 2 μ k 1n 1,k 2n 2 B k 1+k 2n 1n 2. \array{ \nu_1 \boxtimes \nu_2 &\overset{\hat e_1 \boxtimes \hat e_2}{\longrightarrow}& V^{\mathcal{B}}_{n_1} \boxtimes V^{\mathcal{B}}_{n_2} &\overset{\kappa_{n_1,n_2}}{\longrightarrow}& V^{\mathcal{B}}_{n_1 + n_2} \\ \downarrow &(pb)& \downarrow &(pb)& \downarrow \\ X_1 \times X_2 &\underset{\hat g_1 \times \hat g_2}{\longrightarrow}& B_{k_1-n_1} \times B_{k_2-n_2} &\underset{\mu_{k_1-n_1,k_2-n_2}}{\longrightarrow}& B_{k_1 + k_2 - n_1 - n_2} } \,.

To the Pontrjagin-Thom construction of the product manifold is by definition the top composite in the diagram

S n 1+n 2+(k 1+k 2n 1n 2) Th(ν 1ν 2) Th(e^ 1e^ 2) Th(V k 1n 1 V k 2n 2 ) κ k 1n 1,k 2n 2 Th(V k 1+k 2n 1n 2 ) = S n 1+(k 1n 1)S n 2+(k 2n 2) Th(ν 1)Th(ν 2) Th(e^ 1)Th(e^ 2) Th(V 1 )Th(V 2 ) κ k 1n 1,k 2n 2 Th(V k 1+k 2n 1n 2 ), \array{ S^{n_1 +n_2 + (k_1 + k_2 - n_1 - n_2)} &\overset{}{\longrightarrow}& Th(\nu_1 \boxtimes \nu_2) &\overset{Th(\hat e_1 \boxtimes \hat e_2)}{\longrightarrow}& Th(V^{\mathcal{B}}_{k_1-n_1} \boxtimes V^{\mathcal{B}}_{k_2-n_2}) &\overset{\kappa_{k_1-n_1, k_2-n_2}}{\longrightarrow}& Th(V^{\mathcal{B}}_{k_1 + k_2 - n_1 - n_2}) \\ {}^{\mathllap{\simeq}}\downarrow && \downarrow^{\mathrlap{\simeq}} && \downarrow^{\mathrlap{\simeq}} && \downarrow^{\mathrlap{=}} \\ S^{n_1 + (k_1 - n_1)} \wedge S^{n_2 + (k_2 - n_2)} &\overset{}{\longrightarrow}& Th(\nu_1) \wedge Th(\nu_2) &\overset{Th(\hat e_1)\wedge Th(\hat e_2)}{\longrightarrow}& Th(V^{\mathcal{B}}_1) \wedge Th(V^{\mathcal{B}}_2) &\overset{\kappa_{k_1-n_1, k_2-n_2}}{\longrightarrow}& Th(V^{\mathcal{B}}_{k_1 + k_2 - n_1 - n_2}) } \,,

which hence is equivalently the bottom composite, which in turn manifestly represents the product of the separate PT constructions in π (M)\pi_\bullet(M\mathcal{B}).

Theorem

The ring homomorphsim in lemma is an isomorphism.

Due to (Thom 54, Pontrjagin 55). See for instance (Kochmann 96, theorem 1.5.10).

Proof idea

Observe that given the result α:S n+(kn)Th(V kn)\alpha \colon S^{n+(k-n)} \to Th(V_{k-n}) of the Pontrjagin-Thom construction map, the original manifold Xi kX \overset{i}{\hookrightarrow} \mathbb{R}^k may be recovered as this pullback:

X i S n+(kn) g i (pb) α BO(kn) Th(V kn BO). \array{ X &\overset{i}{\longrightarrow}& S^{n + (k-n)} \\ {}^{\mathllap{g_i}}\downarrow &(pb)& \downarrow^{\mathrlap{\alpha}} \\ B O(k-n) &\longrightarrow& Th(V^{B O}_{k-n}) } \,.

To see this more explicitly, break it up into pieces:

X X + S n+(kn) (pb) (pb) X X +Th(X) Th(0) Th(ν i) (pb) (pb) B kn (B kn) +Th(B kn) Th(0) Th(V kn ) (pb) (pb) BO(kn) (BO(kn)) +Th(BO(kn)) Th(V kn BO). \array{ X &\longrightarrow& X_+ &\hookrightarrow& S^{n + (k-n)} \\ \downarrow &(pb)& \downarrow &(pb)& \downarrow \\ X &\longrightarrow& X_+ \simeq Th(X) &\overset{Th(0)}{\longrightarrow}& Th(\nu_i) \\ \downarrow &(pb)& \downarrow &(pb)& \downarrow \\ B_{k-n} &\longrightarrow& (B_{k-n})_+ \simeq Th(B_{k-n}) &\underset{Th(0)}{\longrightarrow}& Th(V^{\mathcal{B}}_{k-n}) \\ \downarrow &(pb)& \downarrow &(pb)& \downarrow \\ B O(k-n) &\longrightarrow& (B O(k-n))_+ \simeq Th(B O(k-n)) &\longrightarrow& Th(V^{B O}_{k-n}) } \,.

Moreover, since the n-spheres are compact topological spaces, and since the classifying space BO(n)B O(n), and hence its universal Thom space, is a sequential colimit over relative cell complex inclusions, the right vertical map factors through some finite stage (by this lemma), the manifold XX is equivalently recovered as a pullback of the form

X S n+(kn) g i (pb) Gr kn( k) i Th(V kn( k)×O(kn) kn). \array{ X &\longrightarrow& S^{n + (k-n)} \\ {}^{\mathllap{g_i}}\downarrow &(pb)& \downarrow \\ Gr_{k-n}(\mathbb{R}^k) &\overset{i}{\longrightarrow}& Th( V_{k-n}(\mathbb{R}^k) \underset{O(k-n)}{\times} \mathbb{R}^{k-n}) } \,.

(Recall that V kn V^{\mathcal{B}}_{k-n} is our notation for the universal vector bundle with \mathcal{B}-structure, while V kn( k)V_{k-n}(\mathbb{R}^k) denotes a Stiefel manifold.)

The idea of the proof now is to use this property as the blueprint of the construction of an inverse ζ\zeta to ξ\xi: given an element in π n(M)\pi_{n}(M \mathcal{B}) represented by a map as on the right of the above diagram, try to define XX and the structure map g ig_i of its normal bundle as the pullback on the left.

The technical problem to be overcome is that for a general continuous function as on the right, the pullback has no reason to be a smooth manifold, and for two reasons:

  1. the map S n+(kn)Th(V kn)S^{n+(k-n)} \to Th(V_{k-n}) may not be smooth around the image of ii;

  2. even if it is smooth around the image of ii, it may not be transversal to ii, and the intersection of two non-transversal smooth functions is in general still not a smooth manifold.

The heart of the proof is in showing that for any α\alpha there are small homotopies relating it to an α\alpha' that is both smooth around the image of ii and transversal to ii.

The first condition is guaranteed by Sard's theorem, the second by Thom's transversality theorem.

(…)

References

Due to

  • René Thom, Quelques propriétés globales des variétés

    différentiables_ Comment. Math. Helv. 28, (1954). 17-86

  • Lev Pontrjagin, 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)

Reviews include

Further lecture notes include

Last revised on May 12, 2017 at 05:46:42. See the history of this page for a list of all contributions to it.