Submanifold

In mathematics, a submanifold of a manifold "M" is a subset "S" which itself has the structure of a manifold, and for which the inclusion map "S" → "M" satisfies certain properties. There are different types of submanifolds depending on exactly which properties are required. Different authors often have different definitions.

Formal definition

In the following we assume all manifolds are differentiable manifolds of class "C"^"r" for a fixed "r" ≥ 1, and all morphisms are differentiable of class "C"^"r".

Immersed submanifolds

An immersed submanifold of a manifold "M" is a subset "S" together with a topology and differential structure such that "S" is a manifold and the inclusion map "i" : "S" → "M" is an injective immersion.

Given any injective immersion "f" : "N" → "M" the image of "N" in "M" can be uniquely given the structure of an immersed submanifold so that "f" : "N" → "f"("N") is a diffeomorphism. It follows that immersed submanifolds are precisely the images of injective immersions.

The submanifold topology on an immersed submanifold need not be the relative topology inherited from "M". In general, it will be finer than the subspace topology (i.e. have more open sets).

Immersed submanifolds occur in the theory of Lie groups where Lie subgroups are naturally immersed submanifolds.

Embedded submanifolds

An embedded submanifold (also called a regular submanifold) is an immersed submanifold for which the inclusion map is a topological embedding. That is, the submanifold topology on "S" is the same as the subspace topology.

Given any embedding "f" : "N" → "M" of a manifold "N" in "M" the image "f"("N") naturally has the structure of an embedded submanifold. That is, embedded submanifolds are precisely the images of embeddings.

There is an intrinsic definition of an embedded submanifold which is often useful. Let "M" be an "n"-dimensional manifold, and let "k" be an integer such that 0 ≤ "k" ≤ "n". A "k"-dimensional embedded submanifold of "M" is a subspace "S" ⊂ "M" such that for every point "p" ∈ "S" there exists a chart ("U" ⊂ "M", φ : "U" → R^"n") containing "p" such that φ("S" ∩ "U") is the intersection of a "k"-dimensional plane with φ("U"). The pairs ("S" ∩ "U", φ|_{"S" ∩ "U"}) form an atlas for the differential structure on "S".

Other variations

There are some other variations of submanifolds used in the literature. Sharpe (1997) defines a type of submanifold which lies somewhere between an embedded submanifold and an immersed submanifold.

Properties

Given any immersed submanifold "S" of "M", the tangent space to a point "p" in "S" can naturally be thought of as a linear subspace of the tangent space to "p" in "M". This follows from the fact that the inclusion map is an immersion and provides an injection:$i\_\{ast\}:\; T\_p\; S\; o\; T\_p\; M.$

Suppose "S" is an immersed submanifold of "M". If the inclusion map "i" : "S" → "M" is closed then "S" is actually an embedded submanifold of "M". Conversely, if "S" is an embedded submanifold which is also a closed subset then the inclusion map is closed. The inclusion map "i" : "S" → "M" is closed if and only if it is a proper map (i.e. inverse images of compact sets are compact). If "i" is closed then "S" is called a closed embedded submanifold of "M". Closed embedded submanifolds form the nicest class of submanifolds.

ubmanifolds of Euclidean space

Manifolds are often "defined" as embedded submanifolds of Euclidean space R^"n", so this forms a very important special case. By the Whitney embedding theorem any second-countable smooth "n"-manifold can be smoothly embedded in R^2"n".

References

*cite book | first = John | last = Lee | year = 2003 | title = Introduction to Smooth Manifolds | series = Graduate Texts in Mathematics 218 | location = New York | publisher = Springer | id = ISBN 0-387-95495-3
*cite book | last = Sharpe | first = R. W. | title = Differential Geometry: Cartan's Generalization of Klein's Erlangen Program | publisher = Springer | location = New York | year=1997 | id=ISBN 0-387-94732-9