In mathematics, a Banach manifold is a manifold modeled on Banach spaces. Thus it is a topological space in which each point has a neighbourhood homeomorphic to an open set in a Banach space (a more involved and formal definition is given below). Banach manifolds are one possibility of extending manifolds to infinite dimensions.

A further generalisation is to Fréchet manifolds, replacing Banach spaces by Fréchet spaces. On the other hand, a Hilbert manifold is a special case of a Banach manifold in which the manifold is locally modeled on Hilbert spaces.

Definition

Let ${\displaystyle X}$ be a set. An atlas of class ${\displaystyle C^{r},}$ ${\displaystyle r\geq 0,}$ on ${\displaystyle X}$ is a collection of pairs (called charts) ${\displaystyle \left(U_{i},\varphi _{i}\right),}$ ${\displaystyle i\in I,}$ such that

1. each ${\displaystyle U_{i))$ is a subset of ${\displaystyle X}$ and the union of the ${\displaystyle U_{i))$ is the whole of ${\displaystyle X}$;
2. each ${\displaystyle \varphi _{i))$ is a bijection from ${\displaystyle U_{i))$ onto an open subset ${\displaystyle \varphi _{i}\left(U_{i}\right)}$ of some Banach space ${\displaystyle E_{i},}$ and for any indices ${\displaystyle i{\text{ and ))j,}$ ${\displaystyle \varphi _{i}\left(U_{i}\cap U_{j}\right)}$ is open in ${\displaystyle E_{i};}$
3. the crossover map
${\displaystyle \varphi _{j}\circ \varphi _{i}^{-1}:\varphi _{i}\left(U_{i}\cap U_{j}\right)\to \varphi _{j}\left(U_{i}\cap U_{j}\right)}$
is an ${\displaystyle r}$-times continuously differentiable function for every ${\displaystyle i,j\in I;}$ that is, the ${\displaystyle r}$th Fréchet derivative
${\displaystyle \mathrm {d} ^{r}\left(\varphi _{j}\circ \varphi _{i}^{-1}\right):\varphi _{i}\left(U_{i}\cap U_{j}\right)\to \mathrm {Lin} \left(E_{i}^{r};E_{j}\right)}$
exists and is a continuous function with respect to the ${\displaystyle E_{i))$-norm topology on subsets of ${\displaystyle E_{i))$ and the operator norm topology on ${\displaystyle \operatorname {Lin} \left(E_{i}^{r};E_{j}\right).}$

One can then show that there is a unique topology on ${\displaystyle X}$ such that each ${\displaystyle U_{i))$ is open and each ${\displaystyle \varphi _{i))$ is a homeomorphism. Very often, this topological space is assumed to be a Hausdorff space, but this is not necessary from the point of view of the formal definition.

If all the Banach spaces ${\displaystyle E_{i))$ are equal to the same space ${\displaystyle E,}$ the atlas is called an ${\displaystyle E}$-atlas. However, it is not a priori necessary that the Banach spaces ${\displaystyle E_{i))$ be the same space, or even isomorphic as topological vector spaces. However, if two charts ${\displaystyle \left(U_{i},\varphi _{i}\right)}$ and ${\displaystyle \left(U_{j},\varphi _{j}\right)}$ are such that ${\displaystyle U_{i))$ and ${\displaystyle U_{j))$ have a non-empty intersection, a quick examination of the derivative of the crossover map

${\displaystyle \varphi _{j}\circ \varphi _{i}^{-1}:\varphi _{i}\left(U_{i}\cap U_{j}\right)\to \varphi _{j}\left(U_{i}\cap U_{j}\right)}$
shows that ${\displaystyle E_{i))$ and ${\displaystyle E_{j))$ must indeed be isomorphic as topological vector spaces. Furthermore, the set of points ${\displaystyle x\in X}$ for which there is a chart ${\displaystyle \left(U_{i},\varphi _{i}\right)}$ with ${\displaystyle x}$ in ${\displaystyle U_{i))$ and ${\displaystyle E_{i))$ isomorphic to a given Banach space ${\displaystyle E}$ is both open and closed. Hence, one can without loss of generality assume that, on each connected component of ${\displaystyle X,}$ the atlas is an ${\displaystyle E}$-atlas for some fixed ${\displaystyle E.}$

A new chart ${\displaystyle (U,\varphi )}$ is called compatible with a given atlas ${\displaystyle \left\{\left(U_{i},\varphi _{i}\right):i\in I\right\))$ if the crossover map

${\displaystyle \varphi _{i}\circ \varphi ^{-1}:\varphi \left(U\cap U_{i}\right)\to \varphi _{i}\left(U\cap U_{i}\right)}$
is an ${\displaystyle r}$-times continuously differentiable function for every ${\displaystyle i\in I.}$ Two atlases are called compatible if every chart in one is compatible with the other atlas. Compatibility defines an equivalence relation on the class of all possible atlases on ${\displaystyle X.}$

A ${\displaystyle C^{r))$-manifold structure on ${\displaystyle X}$ is then defined to be a choice of equivalence class of atlases on ${\displaystyle X}$ of class ${\displaystyle C^{r}.}$ If all the Banach spaces ${\displaystyle E_{i))$ are isomorphic as topological vector spaces (which is guaranteed to be the case if ${\displaystyle X}$ is connected), then an equivalent atlas can be found for which they are all equal to some Banach space ${\displaystyle E.}$ ${\displaystyle X}$ is then called an ${\displaystyle E}$-manifold, or one says that ${\displaystyle X}$ is modeled on ${\displaystyle E.}$

Examples

Every Banach space can be canonically identified as a Banach manifold. If ${\displaystyle (X,\|\,\cdot \,\|)}$ is a Banach space, then ${\displaystyle X}$ is a Banach manifold with an atlas containing a single, globally-defined chart (the identity map).

Similarly, if ${\displaystyle U}$ is an open subset of some Banach space then ${\displaystyle U}$ is a Banach manifold. (See the classification theorem below.)

Classification up to homeomorphism

It is by no means true that a finite-dimensional manifold of dimension ${\displaystyle n}$ is globally homeomorphic to ${\displaystyle \mathbb {R} ^{n},}$ or even an open subset of ${\displaystyle \mathbb {R} ^{n}.}$ However, in an infinite-dimensional setting, it is possible to classify "well-behaved" Banach manifolds up to homeomorphism quite nicely. A 1969 theorem of David Henderson[1] states that every infinite-dimensional, separable, metric Banach manifold ${\displaystyle X}$ can be embedded as an open subset of the infinite-dimensional, separable Hilbert space, ${\displaystyle H}$ (up to linear isomorphism, there is only one such space, usually identified with ${\displaystyle \ell ^{2))$). In fact, Henderson's result is stronger: the same conclusion holds for any metric manifold modeled on a separable infinite-dimensional Fréchet space.

The embedding homeomorphism can be used as a global chart for ${\displaystyle X.}$ Thus, in the infinite-dimensional, separable, metric case, the "only" Banach manifolds are the open subsets of Hilbert space.

See also

• Banach bundle – vector bundle whose fibres form Banach spaces
• Differentiation in Fréchet spaces
• Finsler manifold – smooth manifold equipped with a Minkowski functional at each tangent space
• Fréchet manifold – topological space modeled on a Fréchet space in much the same way as a manifold is modeled on a Euclidean space
• Hilbert manifold – manifold modeled on Hilbert spaces; separable Hausdorff space in which each point has a neighborhood homeomorphic to an infinite dimensional Hilbert space

References

• Abraham, Ralph; Marsden, J. E.; Ratiu, Tudor (1988). Manifolds, Tensor Analysis, and Applications. New York: Springer. ISBN 0-387-96790-7.
• Anderson, R. D. (1969). "Strongly negligible sets in Fréchet manifolds" (PDF). Bulletin of the American Mathematical Society. American Mathematical Society (AMS). 75 (1): 64–67. doi:10.1090/s0002-9904-1969-12146-4. ISSN 0273-0979. S2CID 34049979.
• Anderson, R. D.; Schori, R. (1969). "Factors of infinite-dimensional manifolds" (PDF). Transactions of the American Mathematical Society. American Mathematical Society (AMS). 142: 315–330. doi:10.1090/s0002-9947-1969-0246327-5. ISSN 0002-9947.
• Henderson, David W. (1969). "Infinite-dimensional manifolds are open subsets of Hilbert space". Bull. Amer. Math. Soc. 75 (4): 759–762. doi:10.1090/S0002-9904-1969-12276-7. MR 0247634.
• Lang, Serge (1972). Differential manifolds. Reading, Mass.–London–Don Mills, Ont.: Addison-Wesley Publishing Co., Inc.
• Zeidler, Eberhard (1997). Nonlinear functional analysis and its Applications. Vol.4. Springer-Verlag New York Inc.