TheInfoList

In
geometric topology In mathematics, geometric topology is the study of manifolds and Map (mathematics)#Maps as functions, maps between them, particularly embeddings of one manifold into another. History Geometric topology as an area distinct from algebraic topol ...
and differential topology, an (''n'' + 1)-dimensional ''W'' between ''n''-dimensional
manifold The real projective plane is a two-dimensional manifold that cannot be realized in three dimensions without self-intersection, shown here as Boy's surface. In mathematics, a manifold is a topological space that locally resembles Euclidean space ...
s ''M'' and ''N'' is an ''h''-cobordism (the ''h'' stands for homotopy equivalence) if the inclusion maps : $M \hookrightarrow W \quad\mbox\quad N \hookrightarrow W$ are homotopy equivalences. The ''h''-cobordism theorem gives sufficient conditions for an ''h''-cobordism to be trivial, i.e., to be C-isomorphic to the cylinder ''M'' × , 1 Here C refers to any of the categories of
smooth Smooth may refer to: Mathematics * Smooth function is a smooth function with compact support. In mathematical analysis, the smoothness of a function is a property measured by the number of continuous derivatives it has over some domain. ...
, piecewise linear, or topological manifolds. The theorem was first proved by Stephen Smale for which he received the
Fields Medal The Fields Medal is a prize awarded to two, three, or four mathematician A mathematician is someone who uses an extensive knowledge of mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity ...
and is a fundamental result in the theory of high-dimensional manifolds. For a start, it almost immediately proves the generalized Poincaré conjecture.

# Background

Before Smale proved this theorem, mathematicians became stuck while trying to understand manifolds of dimension 3 or 4, and assumed that the higher-dimensional cases were even harder. The ''h''-cobordism theorem showed that (simply connected) manifolds of dimension at least 5 are much easier than those of dimension 3 or 4. The proof of the theorem depends on the " Whitney trick" of Hassler Whitney, which geometrically untangles homologically-tangled spheres of complementary dimension in a manifold of dimension >4. An informal reason why manifolds of dimension 3 or 4 are unusually hard is that 4-manifold#Failure of the Whitney trick in dimension 4, the trick fails to work in lower dimensions, which have no room for untanglement.

# Precise statement of the ''h''-cobordism theorem

Let ''n'' be at least 5 and let ''W'' be a compact (''n'' + 1)-dimensional ''h''-cobordism between ''M'' and ''N'' in the category C=smooth manifold, Diff, piecewise linear manifold, PL, or topological manifold, Top such that ''W'', ''M'' and ''N'' are simply connected, then ''W'' is C-isomorphic to ''M'' × , 1 The isomorphism can be chosen to be the identity on ''M'' × . This means that the homotopy equivalence between M, W, and N is homotopic to a C-isomorphism.

# Lower dimensional versions

For ''n'' = 4, the ''h''-cobordism theorem is true topologically (proved by Michael Freedman using a 4-dimensional Whitney trick) but is false PL and smoothly (as shown by Simon Donaldson). For ''n'' = 3, the ''h''-cobordism theorem for smooth manifolds has not been proved and, due to the 3-dimensional Poincaré conjecture, is equivalent to the hard open question of whether the 4-sphere has non-standard smooth structures. For ''n'' = 2, the ''h''-cobordism theorem is equivalent to the Poincaré conjecture stated by Henri Poincaré, Poincaré in 1904 (one of the Millennium Prize Problems, Millennium Problems) and was proved by Grigori Perelman in a series of three papers in 2002 and 2003, where he follows Richard S. Hamilton's program using Ricci flow. For ''n'' = 1, the ''h''-cobordism theorem is vacuously true, since there is no closed simply-connected 1-dimensional manifold. For ''n'' = 0, the ''h''-cobordism theorem is trivially true: the interval is the only connected cobordism between connected 0-manifolds.

# A proof sketch

A Cobordism#Morse functions, Morse function $f:W\to\left[a,b\right]$ induces a handle decomposition of ''W'', i.e., if there is a single critical point of index ''k'' in $f^\left(\left[c,c\text{'}\right]\right)$, then the ascending cobordism $W_$ is obtained from $W_c$ by attaching a ''k''-handle. The goal of the proof is to find a handle decomposition with no handles at all so that integrating the non-zero gradient vector field of ''f'' gives the desired diffeomorphism to the trivial cobordism. This is achieved through a series of techniques. 1) Handle rearrangement First, we want to rearrange all handles by order so that lower order handles are attached first. The question is thus when can we slide an ''i''-handle off of a ''j''-handle? This can be done by a radial isotopy so long as the ''i'' attaching sphere and the ''j'' belt sphere do not intersect. We thus want $\left(i-1\right)+\left(n-j\right)\leq\dim\partial W-1=n-1$ which is equivalent to $i\leq j$. We then define the handle chain complex $\left(C_*,\partial_*\right)$ by letting $C_k$ be the free abelian group on the ''k''-handles and defining $\partial_k:C_k\to C_$ by sending a ''k''-handle $h_^k$ to $\sum_\beta \langle h_\alpha^k\mid h_\beta^\rangle h_\beta^$, where $\langle h_\alpha^k\mid h_\beta^\rangle$ is the intersection number of the ''k''-attaching sphere and the (''k'' − 1)-belt sphere. 2) Handle cancellation Next, we want to "cancel" handles. The idea is that attaching a ''k''-handle $h_\alpha^k$ might create a hole that can be filled in by attaching a (''k'' + 1)-handle $h_\beta^$. This would imply that $\partial_h_\beta^=\pm h_\alpha^k$ and so the $\left(\alpha,\beta\right)$ entry in the matrix of $\partial_$ would be $\pm 1$. However, when is this condition sufficient? That is, when can we geometrically cancel handles if this condition is true? The answer lies in carefully analyzing when the manifold remains simply-connected after removing the attaching and belt spheres in question, and finding an embedded disk using the Whitney trick. This analysis leads to the requirement that ''n'' must be at least 5. Moreover, during the proof one requires that the cobordism has no 0-,1-,''n''-, or (''n'' + 1)-handles which is obtained by the next technique. 3) Handle trading The idea of handle trading is to create a cancelling pair of (''k'' + 1)- and (''k'' + 2)-handles so that a given ''k''-handle cancels with the (''k'' + 1)-handle leaving behind the (''k'' + 2)-handle. To do this, consider the core of the ''k''-handle which is an element in $\pi_k\left(W,M\right)$. This group is trivial since ''W'' is an ''h''-cobordism. Thus, there is a disk $D^$ which we can fatten to a cancelling pair as desired, so long as we can embed this disk into the boundary of ''W''. This embedding exists if $\dim\partial W-1=n-1\geq 2\left(k+1\right)$. Since we are assuming ''n'' is at least 5 this means that ''k'' is either 0 or 1. Finally, by considering the negative of the given Morse function, −''f'', we can turn the handle decomposition upside down and also remove the ''n''- and (''n'' + 1)-handles as desired. 4) Handle sliding Finally, we want to make sure that doing row and column operations on $\partial_k$ corresponds to a geometric operation. Indeed, it isn't hard to show (best done by drawing a picture) that sliding a ''k''-handle $h_\alpha^k$ over another ''k''-handle $h_^k$ replaces $h_\alpha^k$ by $h_\alpha^k\pm h_\beta^k$ in the basis for $C_k$. The proof of the theorem now follows: the handle chain complex is exact since $H_*\left(W,M;\mathbb\right)=0$. Thus $C_k\cong \operatorname \partial_\oplus\operatorname \partial_$ since the $C_k$ are free. Then $\partial_k$, which is an integer matrix, restricts to an invertible morphism which can thus be diagonalized via elementary row operations (handle sliding) and must have only $\pm 1$ on the diagonal because it is invertible. Thus, all handles are paired with a single other cancelling handle yielding a decomposition with no handles.

# The ''s''-cobordism theorem

If the assumption that ''M'' and ''N'' are simply connected is dropped, ''h''-cobordisms need not be cylinders; the obstruction is exactly the Whitehead torsion τ (''W'', ''M'') of the inclusion $M \hookrightarrow W$. Precisely, the ''s''-cobordism theorem (the ''s'' stands for simple-homotopy equivalence), proved independently by Barry Mazur, John Stallings, and Dennis Barden, states (assumptions as above but where ''M'' and ''N'' need not be simply connected): : An ''h''-cobordism is a cylinder if and only if Whitehead torsion τ (''W'', ''M'') vanishes. The torsion vanishes if and only if the inclusion $M \hookrightarrow W$ is not just a homotopy equivalence, but a simple homotopy equivalence. Note that one need not assume that the other inclusion $N \hookrightarrow W$ is also a simple homotopy equivalence—that follows from the theorem. Categorically, ''h''-cobordisms form a groupoid. Then a finer statement of the ''s''-cobordism theorem is that the isomorphism classes of this groupoid (up to C-isomorphism of ''h''-cobordisms) are torsors for the respectiveNote that identifying the Whitehead groups of the various manifolds requires that one choose base points $m\in M, n\in N$ and a path in ''W'' connecting them. Whitehead torsion, Whitehead groups Wh(π), where $\pi \cong \pi_1\left(M\right) \cong \pi_1\left(W\right) \cong \pi_1\left(N\right).$