# Diffeomorphism

In mathematics, a **diffeomorphism** is an isomorphism of smooth manifolds. It is an invertible function that maps one differentiable manifold to another such that both the function and its inverse are differentiable.

If *U*, *V* are connected open subsets of **R**^{n} such that *V* is simply connected, a differentiable map *f* : *U* → *V* is a **diffeomorphism** if it is proper and if the differential *Df _{x}* :

**R**

^{n}→

**R**

^{n}is bijective (and hence a linear isomorphism) at each point

*x*in

*U*.

It is essential for *V* to be simply connected for the function *f* to be globally invertible (under the sole condition that its derivative be a bijective map at each point). For example, consider the "realification" of the complex square function

Thus, though *Df _{x}* is bijective at each point,

*f*is not invertible because it fails to be injective (e.g.

*f*(1, 0) = (1, 0) =

*f*(−1, 0)).

Diffeomorphisms are necessarily between manifolds of the same dimension. Imagine *f* going from dimension *n* to dimension *k*. If *n* < *k* then *Df _{x}* could never be surjective, and if

*n*>

*k*then

*Df*could never be injective. In both cases, therefore,

_{x}*Df*fails to be a bijection.

_{x}If *Df _{x}* is a bijection at

*x*then

*f*is said to be a local diffeomorphism (since, by continuity,

*Df*will also be bijective for all

_{y}*y*sufficiently close to

*x*).

Given a smooth map from dimension *n* to dimension *k*, if *Df* (or, locally, *Df _{x}*) is surjective,

*f*is said to be a submersion (or, locally, a "local submersion"); and if

*Df*(or, locally,

*Df*) is injective,

_{x}*f*is said to be an immersion (or, locally, a "local immersion").

A differentiable bijection is *not* necessarily a diffeomorphism. *f*(*x*) = *x*^{3}, for example, is not a diffeomorphism from **R** to itself because its derivative vanishes at 0 (and hence its inverse is not differentiable at 0). This is an example of a homeomorphism that is not a diffeomorphism.

When *f* is a map between *differentiable* manifolds, a diffeomorphic *f* is a stronger condition than a homeomorphic *f*. For a diffeomorphism, *f* and its inverse need to be differentiable; for a homeomorphism, *f* and its inverse need only be continuous. Every diffeomorphism is a homeomorphism, but not every homeomorphism is a diffeomorphism.

*f* : *M* → *N* is called a **diffeomorphism** if, in coordinate charts, it satisfies the definition above. More precisely: Pick any cover of *M* by compatible coordinate charts and do the same for *N*. Let φ and ψ be charts on, respectively, *M* and *N*, with *U* and *V* as, respectively, the images of φ and ψ. The map ψ*f*φ^{−1} : *U* → *V* is then a diffeomorphism as in the definition above, whenever *f*(φ^{−1}(U)) ⊆ ψ^{−1}(V).

Since any manifold can be locally parametrised, we can consider some explicit maps from **R**^{2} into **R**^{2}.

Let *M* be a differentiable manifold that is second-countable and Hausdorff. The **diffeomorphism group** of *M* is the group of all *C ^{r}* diffeomorphisms of

*M*to itself, denoted by Diff

^{r}(

*M*) or, when

*r*is understood, Diff(

*M*). This is a "large" group, in the sense that—provided

*M*is not zero-dimensional—it is not locally compact.

The diffeomorphism group has two natural topologies: *weak* and *strong* (Hirsch 1997). When the manifold is compact, these two topologies agree. The weak topology is always metrizable. When the manifold is not compact, the strong topology captures the behavior of functions "at infinity" and is not metrizable. It is, however, still Baire.

Fixing a Riemannian metric on *M*, the weak topology is the topology induced by the family of metrics

as *K* varies over compact subsets of *M*. Indeed, since *M* is σ-compact, there is a sequence of compact subsets *K*_{n} whose union is *M*. Then:

The diffeomorphism group equipped with its weak topology is locally homeomorphic to the space of *C ^{r}* vector fields (Leslie 1967). Over a compact subset of

*M*, this follows by fixing a Riemannian metric on

*M*and using the exponential map for that metric. If

*r*is finite and the manifold is compact, the space of vector fields is a Banach space. Moreover, the transition maps from one chart of this atlas to another are smooth, making the diffeomorphism group into a Banach manifold with smooth right translations; left translations and inversion are only continuous. If

*r*= ∞, the space of vector fields is a Fréchet space. Moreover, the transition maps are smooth, making the diffeomorphism group into a Fréchet manifold and even into a regular Fréchet Lie group. If the manifold is σ-compact and not compact the full diffeomorphism group is not locally contractible for any of the two topologies. One has to restrict the group by controlling the deviation from the identity near infinity to obtain a diffeomorphism group which is a manifold; see (Michor & Mumford 2013).

For a connected manifold *M*, the diffeomorphism group acts transitively on *M*. More generally, the diffeomorphism group acts transitively on the configuration space *C _{k}M*. If

*M*is at least two-dimensional, the diffeomorphism group acts transitively on the configuration space

*F*and the action on

_{k}M*M*is multiply transitive (Banyaga 1997, p. 29).

In 1926, Tibor Radó asked whether the harmonic extension of any homeomorphism or diffeomorphism of the unit circle to the unit disc yields a diffeomorphism on the open disc. An elegant proof was provided shortly afterwards by Hellmuth Kneser. In 1945, Gustave Choquet, apparently unaware of this result, produced a completely different proof.

The (orientation-preserving) diffeomorphism group of the circle is pathwise connected. This can be seen by noting that any such diffeomorphism can be lifted to a diffeomorphism *f* of the reals satisfying [*f*(*x* + 1) = *f*(*x*) + 1]; this space is convex and hence path-connected. A smooth, eventually constant path to the identity gives a second more elementary way of extending a diffeomorphism from the circle to the open unit disc (a special case of the Alexander trick). Moreover, the diffeomorphism group of the circle has the homotopy-type of the orthogonal group O(2).

The corresponding extension problem for diffeomorphisms of higher-dimensional spheres **S**^{n−1} was much studied in the 1950s and 1960s, with notable contributions from René Thom, John Milnor and Stephen Smale. An obstruction to such extensions is given by the finite abelian group Γ_{n}, the "group of twisted spheres", defined as the quotient of the abelian component group of the diffeomorphism group by the subgroup of classes extending to diffeomorphisms of the ball *B*^{n}.

For manifolds, the diffeomorphism group is usually not connected. Its component group is called the mapping class group. In dimension 2 (i.e. surfaces), the mapping class group is a finitely presented group generated by Dehn twists (Dehn, Lickorish, Hatcher).^{[citation needed]} Max Dehn and Jakob Nielsen showed that it can be identified with the outer automorphism group of the fundamental group of the surface.

William Thurston refined this analysis by classifying elements of the mapping class group into three types: those equivalent to a periodic diffeomorphism; those equivalent to a diffeomorphism leaving a simple closed curve invariant; and those equivalent to pseudo-Anosov diffeomorphisms. In the case of the torus **S**^{1} × **S**^{1} = **R**^{2}/**Z**^{2}, the mapping class group is simply the modular group SL(2, **Z**) and the classification becomes classical in terms of elliptic, parabolic and hyperbolic matrices. Thurston accomplished his classification by observing that the mapping class group acted naturally on a compactification of Teichmüller space; as this enlarged space was homeomorphic to a closed ball, the Brouwer fixed-point theorem became applicable. Smale conjectured that if *M* is an oriented smooth closed manifold, the identity component of the group of orientation-preserving diffeomorphisms is simple. This had first been proved for a product of circles by Michel Herman; it was proved in full generality by Thurston.

Unlike non-diffeomorphic homeomorphisms, it is relatively difficult to find a pair of homeomorphic manifolds that are not diffeomorphic. In dimensions 1, 2 and 3, any pair of homeomorphic smooth manifolds are diffeomorphic. In dimension 4 or greater, examples of homeomorphic but not diffeomorphic pairs have been found. The first such example was constructed by John Milnor in dimension 7. He constructed a smooth 7-dimensional manifold (called now Milnor's sphere) that is homeomorphic to the standard 7-sphere but not diffeomorphic to it. There are, in fact, 28 oriented diffeomorphism classes of manifolds homeomorphic to the 7-sphere (each of them is the total space of a fiber bundle over the 4-sphere with the 3-sphere as the fiber).

More unusual phenomena occur for 4-manifolds. In the early 1980s, a combination of results due to Simon Donaldson and Michael Freedman led to the discovery of exotic **R**^{4}s: there are uncountably many pairwise non-diffeomorphic open subsets of **R**^{4} each of which is homeomorphic to **R**^{4}, and also there are uncountably many pairwise non-diffeomorphic differentiable manifolds homeomorphic to **R**^{4} that do not embed smoothly in **R**^{4}.