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For other uses of this term, see embedded (disambiguation).

In mathematics, an embedding (or imbedding) is one instance of some mathematical object contained within another instance, such as a group that is a subgroup.

Contents 1 Topology and geometry 1.1 General topology 1.2 Differential topology 1.3 Riemannian geometry 2 Algebra 2.1 Field theory 2.2 Universal algebra and model theory 3 Order theory and domain theory 4 Metric spaces 4.1 Normed spaces 5 Abstractly or categorically 6 See also 7 References

In general topology, an embedding is a homeomorphism onto its image. More explicitly, a map f : X → Y between topological spaces X and Y is an embedding if f yields a homeomorphism between X and f(X) (where f(X) carries the subspace topology inherited from Y). Intuitively then, the embedding f : X → Y lets us treat X as a subspace of Y. Every embedding is injective and continuous. Every map that is injective, continuous and either open or closed is an embedding; however there are also embeddings which are neither open nor closed. The latter happens if the image f(X) is neither an open set nor a closed set in Y.

For a given space X, the existence of an embedding X → Y is a topological invariant of X. This allows two spaces to be distinguished if one is able to be embedded into a space which the other is not.

In differential topology: Let M and N be smooth manifolds and be a smooth map, it is called an immersion if the derivative of f is everywhere injective. Then an embedding, or a smooth embedding, is defined to be an immersion which is an embedding in the above sense (i.e. homeomorphism onto its image).

In other words, an embedding is diffeomorphic to its image, and in particular the image of an embedding must be a submanifold. An immersion is a local embedding (i.e. for any point there is a neighborhood such that is an embedding.)

When the domain manifold is compact, the notion of a smooth embedding is equivalent to that of an injective immersion.

An important case is N=Rⁿ. The interest here is in how large n must be, in terms of the dimension m of M. The Whitney embedding theorem states that n = 2m is enough. For example the real projective plane of dimension 2 requires n = 4 for an embedding. An immersion of this surface is, however, possible in R³, and one example is Boy's surface—which has self-intersections. The Roman surface fails to be an immersion as it contains cross-caps.

An embedding is proper if it behaves well w.r.t. boundaries: one requires the map to be such that

• , and
• f(X) is transversal to in any point of .

The first condition is equivalent to having and . The second condition, roughly speaking, says that f(X) is not tangent to the boundary of Y.

In Riemannian geometry: Let (M,g) and (N,h) be Riemannian manifolds. An isometric embedding is a smooth embedding f : M → N which preserves the metric in the sense that g is equal to the pullback of h by f, i.e. g = f*h. Explicitly, for any two tangent vectors

we have

Analogously, isometric immersion is an immersion between Riemannian manifolds which preserves the Riemannian metrics.

Equivalently, an isometric embedding (immersion) is a smooth embedding (immersion) which preserves length of curves (cf. Nash embedding theorem).

In general, for an algebraic category C, an embedding between two C-algebraic structures X and Y is a C-morphism e:X→Y which is injective.

In field theory, an embedding of a field E in a field F is a ring homomorphism σ : E → F.

The kernel of σ is an ideal of E which cannot be the whole field E, because of the condition σ(1)=1. Furthermore, it is a well-known property of fields that their only ideals are the zero ideal and the whole field itself. Therefore, the kernel is 0, so any embedding of fields is a monomorphism. Moreover, E is isomorphic to the subfield σ(E) of F. This justifies the name embedding for an arbitrary homomorphism of fields.

If σ is a signature and A,B are σ-structures (also called σ-algebras in universal algebra or models in model theory), then a map is a σ-embedding iff all the following holds:

• h is injective,
• for every n-ary function symbol and we have ,
• for every n-ary relation symbol and we have iff

Here is a model theoretical notation equivalent to . In model theory there is also a stronger notion of elementary embedding.

In order theory, an embedding of partial orders is a function F from X to Y such that :

.

In domain theory, an additional requirement is :

is directed.

Based on an article from FOLDOC, used by permission.

A mapping of metric spaces is called an embedding (with distortion C > 0) if

for some constant L > 0.

An important special case is that of normed spaces; in this case it is natural to consider linear embeddings.

One of the basic questions that can be asked about a finite-dimensional normed space is, what is the maximal dimension k such that the Hilbert space can be linearly embedded into X with constant distortion?

The answer is given by Dvoretzky's theorem.

In category theory, it is not possible to define an embedding without additional structures on the base category. However, generally speaking, it is possible to define what properties should satisfy a class of embeddings in a given category.

In all cases, the class of embeddings should contain all isomorphisms. Most of the time, embeddings are required to be stable under composition and be monic. Other typical requirements are: any extremal monomorphism is an embedding and embeddings are stable under pullbacks.

A common property of embeddings is that embedded subobjects of an object, thought equivalent up to an isomorphism, form an ordered set. In this case, the category is said to be well powered with respect to the class of embeddings. This allows one to define new local structures on the category (such as a closure operator).

The kind of structures on a category allowing to define embeddings are:

• a concrete category structure, embeddings are then defined as the morphisms with injective underlying function satisfying an initiality condition
• a factorization system (E,M), embeddings are then defined as the morphisms in M (in this case, the category is often required to be well powered with respect to M).

In most cases, concrete categories have a factorization structure (E,M) where M is the class of embeddings defined by the concrete structure. This is the case of the majority of the examples given in this article.

As usual in category theory, there is a dual concept, known as quotient. All the preceding properties can be dualized.

• Inclusion map

• Adámek, Jiří; Horst Herrlich, George Strecker [1990] (2006). Abstract and Concrete Categories (The Joy of Cats).