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# Embedding

[em-bed-ing]
In mathematics, an embedding (or imbedding) is one instance of some mathematical structure contained within another instance, such as a group that is a subgroup.

When some object X is said to be embedded in another object Y, the embedding is given by some injective and structure-preserving map . The precise meaning of "structure-preserving" depends on the kind of mathematical structure of which X and Y are instances. In the terminology of category theory, a structure-preserving map is called a morphism.

The fact that a map is an embedding is often indicated by the use of a "hooked arrow", thus: .

Given X and Y, several different embeddings of X in Y may be possible. In many cases of interest there is a standard (or "canonical") embedding, like those of the natural numbers in the integers, the integers in the rational numbers, the rational numbers in the real numbers, and the real numbers in the complex numbers. In such cases it is common to identify the domain X with its image f(X) contained in Y, so that then .

## Topology and geometry

### General topology

In general topology, an embedding is a homeomorphism onto its image. More explicitly, a map f : XY 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 : XY 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.

### Differential topology

In differential topology: Let M and N be smooth manifolds and $f:Mto N$ 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 $xin M$ there is a neighborhood $xin Usubset M$ such that $f:Uto N$ 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 $f: X rightarrow Y$ to be such that

• $f\left(partial X\right) = f\left(X\right) cap partial Y$, and
• $f\left(X\right)$ is transversal to $partial Y$ in any point of $f\left(partial X\right)$.

The first condition is equivalent to having $f\left(partial X\right) subseteq partial Y$ and $f\left(X setminus partial X\right) subseteq Y setminus partial Y$. The second condition, roughly speaking, says that f(X) is not tangent to the boundary of Y.

### Riemannian geometry

In Riemannian geometry: Let (M,g) and (N,h) be Riemannian manifolds. An isometric embedding is a smooth embedding f : MN 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

$v,win T_x\left(M\right)$

we have

$g\left(v,w\right)=h\left(df\left(v\right),df\left(w\right)\right).,$

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).

## Algebra

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.

### Field theory

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

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.

### Universal algebra and model theory

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 $h:A to B$ is a σ-embedding iff all the following holds:

• $h$ is injective,
• for every $n$-ary function symbol $f insigma$ and $a_1,ldots,a_n in A^n,$ we have $h\left(f^A\left(a_1,ldots,a_n\right)\right)=f^B\left(h\left(a_1\right),ldots,h\left(a_n\right)\right)$,
• for every $n$-ary relation symbol $R insigma$ and $a_1,ldots,a_n in A^n,$ we have $A models R\left(a_1,ldots,a_n\right)$ iff $B models R\left(h\left(a_1\right),ldots,h\left(a_n\right)\right).$

Here $Amodels R \left(a_1,ldots,a_n\right)$ is a model theoretical notation equivalent to $\left(a_1,ldots,a_n\right)in R^A$. In model theory there is also a stronger notion of elementary embedding.

## Order theory and domain theory

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

$forall x_1,x_2in X: x_1leq x_2Leftrightarrow F\left(x_1\right)leq F\left(x_2\right)$.

In domain theory, an additional requirement is :

$forall yin Y:\left\{x: F\left(x\right)leq y\right\}$ is directed.

Based on an article from FOLDOC, Foldoc license.

## Metric spaces

A mapping $phi: X to Y$ of metric spaces is called an embedding (with distortion $C>0$) if

$L d_X\left(x, y\right) leq d_Y\left(phi\left(x\right), phi\left(y\right)\right) leq CLd_X\left(x,y\right)$
for some constant $L>0$.

### Normed spaces

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 $\left(X, | cdot |\right)$ is, what is the maximal dimension $k$ such that the Hilbert space $ell_2^k$ can be linearly embedded into $X$ with constant distortion?

The answer is given by Dvoretzky's theorem.

## Category theory

In category theory, it is not possible to define an embedding without additional structures on the base category. However, in all generality, 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 the class of all embedded subobjects of a given object, thought equivalent up to an isomorphism, is small, and thus an ordered set. In this case, the category is said to be well powered with respect to the class of embeddings. This allows 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 $\left(E,M\right)$, 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 $\left(E,M\right)$ 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.