In
mathematics, a Banach manifold is a
manifold
In mathematics, a manifold is a topological space that locally resembles Euclidean space near each point. More precisely, an n-dimensional manifold, or ''n-manifold'' for short, is a topological space with the property that each point has a ...
modeled on
Banach spaces
In mathematics, more specifically in functional analysis, a Banach space (pronounced ) is a complete normed vector space. Thus, a Banach space is a vector space with a metric that allows the computation of vector length and distance between vect ...
. Thus it is a
topological space
In mathematics, a topological space is, roughly speaking, a geometrical space in which closeness is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set whose elements are called po ...
in which each point has a
neighbourhood homeomorphic to an
open set
In mathematics, open sets are a generalization of open intervals in the real line.
In a metric space (a set along with a distance defined between any two points), open sets are the sets that, with every point , contain all points that are ...
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 manifold In mathematics, in particular in nonlinear analysis, a Fréchet manifold is a topological space modeled on a Fréchet space in much the same way as a manifold is modeled on a Euclidean space.
More precisely, a Fréchet manifold consists of a Hausd ...
s, 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
be a
set
Set, The Set, SET or SETS may refer to:
Science, technology, and mathematics Mathematics
*Set (mathematics), a collection of elements
*Category of sets, the category whose objects and morphisms are sets and total functions, respectively
Electro ...
. An
atlas
An atlas is a collection of maps; it is typically a bundle of maps of Earth or of a region of Earth.
Atlases have traditionally been bound into book form, but today many atlases are in multimedia formats. In addition to presenting geographi ...
of class
on
is a collection of pairs (called
charts)
such that
# each
is a
subset of
and the
union
Union commonly refers to:
* Trade union, an organization of workers
* Union (set theory), in mathematics, a fundamental operation on sets
Union may also refer to:
Arts and entertainment
Music
* Union (band), an American rock group
** ''U ...
of the
is the whole of
;
# each
is a
bijection
In mathematics, a bijection, also known as a bijective function, one-to-one correspondence, or invertible function, is a function between the elements of two sets, where each element of one set is paired with exactly one element of the other ...
from
onto an
open subset of some Banach space
and for any indices
is open in
# the crossover map
#:
#:is an
-times continuously differentiable function for every
that is, the
th
Fréchet derivative
In mathematics, the Fréchet derivative is a derivative defined on normed spaces. Named after Maurice Fréchet, it is commonly used to generalize the derivative of a real-valued function of a single real variable to the case of a vector-valued ...
#:
#:exists and is a
continuous function with respect to the
-
norm
Naturally occurring radioactive materials (NORM) and technologically enhanced naturally occurring radioactive materials (TENORM) consist of materials, usually industrial wastes or by-products enriched with radioactive elements found in the envi ...
topology on subsets of
and the
operator norm topology on
One can then show that there is a unique
topology on
such that each
is open and each
is a
homeomorphism
In the mathematical field of topology, a homeomorphism, topological isomorphism, or bicontinuous function is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isomorph ...
. Very often, this topological space is assumed to be a
Hausdorff space
In topology and related branches of mathematics, a Hausdorff space ( , ), separated space or T2 space is a topological space where, for any two distinct points, there exist neighbourhoods of each which are disjoint from each other. Of the many ...
, but this is not necessary from the point of view of the formal definition.
If all the Banach spaces
are equal to the same space
the atlas is called an
-atlas. However, it is not ''
a priori'' necessary that the Banach spaces
be the same space, or even
isomorphic as
topological vector spaces. However, if two charts
and
are such that
and
have a non-empty
intersection, a quick examination of the
derivative of the crossover map
shows that
and
must indeed be isomorphic as topological vector spaces. Furthermore, the set of points
for which there is a chart
with
in
and
isomorphic to a given Banach space
is both open and
closed
Closed may refer to:
Mathematics
* Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set
* Closed set, a set which contains all its limit points
* Closed interval, ...
. Hence, one can without loss of generality assume that, on each
connected component of
the atlas is an
-atlas for some fixed
A new chart
is called compatible with a given atlas
if the crossover map
is an
-times continuously differentiable function for every
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
A
-manifold structure on
is then defined to be a choice of equivalence class of atlases on
of class
If all the Banach spaces
are isomorphic as topological vector spaces (which is guaranteed to be the case if
is
connected
Connected may refer to:
Film and television
* ''Connected'' (2008 film), a Hong Kong remake of the American movie ''Cellular''
* '' Connected: An Autoblogography About Love, Death & Technology'', a 2011 documentary film
* ''Connected'' (2015 TV ...
), then an equivalent atlas can be found for which they are all equal to some Banach space
is then called an
-manifold, or one says that
is modeled on
Examples
* If
is a Banach space, then
is a Banach manifold with an atlas containing a single, globally-defined chart (the
identity map).
* Similarly, if
is an open subset of some Banach space then
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
is homeomorphic to
or even an open subset of
However, in an infinite-dimensional setting, it is possible to classify "
well-behaved
In mathematics, when a mathematical phenomenon runs counter to some intuition, then the phenomenon is sometimes called pathological. On the other hand, if a phenomenon does not run counter to intuition,
it is sometimes called well-behaved. T ...
" Banach manifolds up to homeomorphism quite nicely. A 1969 theorem of David Henderson states that every infinite-dimensional,
separable,
metric Banach manifold
can be
embedded as an open subset of the infinite-dimensional, separable Hilbert space,
(up to linear isomorphism, there is only one such space, usually identified with
). 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
Thus, in the infinite-dimensional, separable, metric case, the "only" Banach manifolds are the open subsets of Hilbert space.
See also
*
*
*
*
*
References
*
*
*
*
{{DEFAULTSORT:Banach Manifold
Banach spaces
Differential geometry
Generalized manifolds
Manifolds
Nonlinear functional analysis
Structures on manifolds