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: $f\; :\; X\; \backslash hookrightarrow\; Y.$ (On the other hand, this notation is sometimes reserved for inclusion maps.)
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 .

Topology and geometry

General topology

In general topology, an embedding is a homeomorphism onto its image. More explicitly, an injective continuous map $f\; :\; X\; \backslash to\; Y$ between topological spaces $X$ and $Y$ is a topological 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 $f\; :\; X\; \backslash to\; Y$ 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 $Y$, the existence of an embedding $X\; \backslash to\; Y$ is a topological invariant of $X$. This allows two spaces to be distinguished if one is able to be embedded in a space while the other is not.

Differential topology

In differential topology: Let $M$ and $N$ be smooth manifolds and $f:M\backslash to\; N$ be a smooth map. Then $f$ is called an immersion if its derivative is everywhere injective. An embedding, or a smooth embedding, is defined to be an injective immersion which is an embedding in the topological sense mentioned above (i.e. homeomorphism onto its image). In other words, the domain of 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 $x\backslash in\; M$ there is a neighborhood $x\backslash in\; U\backslash subset\; M$ such that $f:U\backslash to\; N$ 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 $N\; =\; \backslash mathbb^n$. The interest here is in how large $n$ must be for an embedding, in terms of the dimension $m$ of $M$. The Whitney embedding theorem states that $n\; =\; 2m$ is enough, and is the best possible linear bound. For example, the real projective space RP^{''m''} of dimension $m$, where $m$ is a power of two, requires $n\; =\; 2m$ for an embedding. However, this does not apply to immersions; for instance, RP^{2} can be immersed in $\backslash mathbb^3$ as is explicitly shown by 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 with respect to boundaries: one requires the map $f:\; X\; \backslash rightarrow\; Y$ to be such that
*$f(\backslash partial\; X)\; =\; f(X)\; \backslash cap\; \backslash partial\; Y$, and
*$f(X)$ is transverse to $\backslash partial\; Y$ in any point of $f(\backslash partial\; X)$.
The first condition is equivalent to having $f(\backslash partial\; X)\; \backslash subseteq\; \backslash partial\; Y$ and $f(X\; \backslash setminus\; \backslash partial\; X)\; \backslash subseteq\; Y\; \backslash setminus\; \backslash partial\; Y$. The second condition, roughly speaking, says that ''f''(''X'') is not tangent to the boundary of ''Y''.

Riemannian and pseudo-Riemannian geometry

In Riemannian geometry and pseudo-Riemannian geometry: Let (''M'', ''g'') and (''N'', ''h'') be Riemannian manifolds or more generally pseudo-Riemannian manifolds. An isometric embedding is a smooth embedding ''f'' : ''M'' → ''N'' which preserves the (pseudo-)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 $v,w\backslash in\; T\_x(M)$ we have :$g(v,w)=h(df(v),df(w)).$ Analogously, isometric immersion is an immersion between (pseudo)-Riemannian manifolds which preserves the (pseudo)-Riemannian metrics. Equivalently, in Riemannian geometry, an isometric embedding (immersion) is a smooth embedding (immersion) which preserves length of curves (cf. Nash embedding theorem).Nash J., ''The embedding problem for Riemannian manifolds,'' Ann. of Math. (2), 63 (1956), 20–63.

Algebra

In general, for an algebraic category ''C'', an embedding between two ''C''-algebraic structures ''X'' and ''Y'' is a ''C''-morphism that is injective.

Field theory

In field theory, an embedding of a field ''E'' in a field ''F'' is a ring homomorphism . The kernel of ''σ'' is an ideal of ''E'' which cannot be the whole field ''E'', because of the condition . 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. Hence, ''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 $h:A\; \backslash to\; B$ is a σ-embedding iff all of the following hold: * $h$ is injective, * for every $n$-ary function symbol $f\; \backslash in\backslash sigma$ and $a\_1,\backslash ldots,a\_n\; \backslash in\; A^n,$ we have $h(f^A(a\_1,\backslash ldots,a\_n))=f^B(h(a\_1),\backslash ldots,h(a\_n))$, * for every $n$-ary relation symbol $R\; \backslash in\backslash sigma$ and $a\_1,\backslash ldots,a\_n\; \backslash in\; A^n,$ we have $A\; \backslash models\; R(a\_1,\backslash ldots,a\_n)$ iff $B\; \backslash models\; R(h(a\_1),\backslash ldots,h(a\_n)).$ Here $A\backslash models\; R\; (a\_1,\backslash ldots,a\_n)$ is a model theoretical notation equivalent to $(a\_1,\backslash ldots,a\_n)\backslash in\; R^A$. In model theory there is also a stronger notion of elementary embedding.

Order theory and domain theory

In order theory, an embedding of partially ordered sets is a function ''F'' between partially ordered sets ''X'' and ''Y'' such that :$\backslash forall\; x\_1,x\_2\backslash in\; X:\; x\_1\backslash leq\; x\_2\; \backslash iff\; F(x\_1)\backslash leq\; F(x\_2).$ Injectivity of ''F'' follows quickly from this definition. In domain theory, an additional requirement is that :$\backslash forall\; y\backslash in\; Y:\backslash $ is directed.

Metric spaces

A mapping $\backslash phi:\; X\; \backslash to\; Y$ of metric spaces is called an ''embedding'' (with distortion $C>0$) if :$L\; d\_X(x,\; y)\; \backslash leq\; d\_Y(\backslash phi(x),\; \backslash phi(y))\; \backslash leq\; CLd\_X(x,y)$ 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 $(X,\; \backslash |\; \backslash cdot\; \backslash |)$ is, ''what is the maximal dimension $k$ such that the Hilbert space $\backslash ell\_2^k$ can be linearly embedded into $X$ with constant distortion?''
The answer is given by Dvoretzky's theorem.

Category theory

In category theory, there is no satisfactory and generally accepted definition of embeddings that is applicable in all categories. One would expect that all isomorphisms and all compositions of embeddings are embeddings, and that all embeddings are monomorphisms. Other typical requirements are: any extremal monomorphism is an embedding and embeddings are stable under pullbacks. Ideally the class of all embedded subobjects of a given object, up to isomorphism, should also be 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 defining new local structures in the category (such as a closure operator). In a concrete category, an embedding is a morphism ''ƒ'': ''A'' → ''B'' which is an injective function from the underlying set of ''A'' to the underlying set of ''B'' and is also an initial morphism in the following sense: If ''g'' is a function from the underlying set of an object ''C'' to the underlying set of ''A'', and if its composition with ''ƒ'' is a morphism ''ƒg'': ''C'' → ''B'', then ''g'' itself is a morphism. A factorization system for a category also gives rise to a notion of embedding. If (''E'', ''M'') is a factorization system, then the morphisms in ''M'' may be regarded as the embeddings, especially when the category is well powered with respect to ''M''. Concrete theories often have a factorization system in which ''M'' consists of the embeddings in the previous sense. 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. An embedding can also refer to an embedding functor.

See also

*Closed immersion *Cover *Dimension reduction *Immersion *Johnson–Lindenstrauss lemma *Submanifold *Subspace *Universal space

Notes

** References **

*
*
*
*
*
*
*
*
*
*
*
* .
*
* .

** External links **

*

Embedding of manifolds

on the Manifold Atlas {{set index article Category:Abstract algebra Category:Category theory Category:General topology Category:Differential topology Category:Functions and mappings Category:Maps of manifolds Category:Model theory Category:Order theory

Topology and geometry

General topology

In general topology, an embedding is a homeomorphism onto its image. More explicitly, an injective continuous map $f\; :\; X\; \backslash to\; Y$ between topological spaces $X$ and $Y$ is a topological 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 $f\; :\; X\; \backslash to\; Y$ 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 $Y$, the existence of an embedding $X\; \backslash to\; Y$ is a topological invariant of $X$. This allows two spaces to be distinguished if one is able to be embedded in a space while the other is not.

Differential topology

In differential topology: Let $M$ and $N$ be smooth manifolds and $f:M\backslash to\; N$ be a smooth map. Then $f$ is called an immersion if its derivative is everywhere injective. An embedding, or a smooth embedding, is defined to be an injective immersion which is an embedding in the topological sense mentioned above (i.e. homeomorphism onto its image). In other words, the domain of 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 $x\backslash in\; M$ there is a neighborhood $x\backslash in\; U\backslash subset\; M$ such that $f:U\backslash to\; N$ 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 $N\; =\; \backslash mathbb^n$. The interest here is in how large $n$ must be for an embedding, in terms of the dimension $m$ of $M$. The Whitney embedding theorem states that $n\; =\; 2m$ is enough, and is the best possible linear bound. For example, the real projective space RP

Riemannian and pseudo-Riemannian geometry

In Riemannian geometry and pseudo-Riemannian geometry: Let (''M'', ''g'') and (''N'', ''h'') be Riemannian manifolds or more generally pseudo-Riemannian manifolds. An isometric embedding is a smooth embedding ''f'' : ''M'' → ''N'' which preserves the (pseudo-)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 $v,w\backslash in\; T\_x(M)$ we have :$g(v,w)=h(df(v),df(w)).$ Analogously, isometric immersion is an immersion between (pseudo)-Riemannian manifolds which preserves the (pseudo)-Riemannian metrics. Equivalently, in Riemannian geometry, an isometric embedding (immersion) is a smooth embedding (immersion) which preserves length of curves (cf. Nash embedding theorem).Nash J., ''The embedding problem for Riemannian manifolds,'' Ann. of Math. (2), 63 (1956), 20–63.

Algebra

In general, for an algebraic category ''C'', an embedding between two ''C''-algebraic structures ''X'' and ''Y'' is a ''C''-morphism that is injective.

Field theory

In field theory, an embedding of a field ''E'' in a field ''F'' is a ring homomorphism . The kernel of ''σ'' is an ideal of ''E'' which cannot be the whole field ''E'', because of the condition . 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. Hence, ''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 $h:A\; \backslash to\; B$ is a σ-embedding iff all of the following hold: * $h$ is injective, * for every $n$-ary function symbol $f\; \backslash in\backslash sigma$ and $a\_1,\backslash ldots,a\_n\; \backslash in\; A^n,$ we have $h(f^A(a\_1,\backslash ldots,a\_n))=f^B(h(a\_1),\backslash ldots,h(a\_n))$, * for every $n$-ary relation symbol $R\; \backslash in\backslash sigma$ and $a\_1,\backslash ldots,a\_n\; \backslash in\; A^n,$ we have $A\; \backslash models\; R(a\_1,\backslash ldots,a\_n)$ iff $B\; \backslash models\; R(h(a\_1),\backslash ldots,h(a\_n)).$ Here $A\backslash models\; R\; (a\_1,\backslash ldots,a\_n)$ is a model theoretical notation equivalent to $(a\_1,\backslash ldots,a\_n)\backslash in\; R^A$. In model theory there is also a stronger notion of elementary embedding.

Order theory and domain theory

In order theory, an embedding of partially ordered sets is a function ''F'' between partially ordered sets ''X'' and ''Y'' such that :$\backslash forall\; x\_1,x\_2\backslash in\; X:\; x\_1\backslash leq\; x\_2\; \backslash iff\; F(x\_1)\backslash leq\; F(x\_2).$ Injectivity of ''F'' follows quickly from this definition. In domain theory, an additional requirement is that :$\backslash forall\; y\backslash in\; Y:\backslash $ is directed.

Metric spaces

A mapping $\backslash phi:\; X\; \backslash to\; Y$ of metric spaces is called an ''embedding'' (with distortion $C>0$) if :$L\; d\_X(x,\; y)\; \backslash leq\; d\_Y(\backslash phi(x),\; \backslash phi(y))\; \backslash leq\; CLd\_X(x,y)$ for some constant $L>0$.

Category theory

In category theory, there is no satisfactory and generally accepted definition of embeddings that is applicable in all categories. One would expect that all isomorphisms and all compositions of embeddings are embeddings, and that all embeddings are monomorphisms. Other typical requirements are: any extremal monomorphism is an embedding and embeddings are stable under pullbacks. Ideally the class of all embedded subobjects of a given object, up to isomorphism, should also be 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 defining new local structures in the category (such as a closure operator). In a concrete category, an embedding is a morphism ''ƒ'': ''A'' → ''B'' which is an injective function from the underlying set of ''A'' to the underlying set of ''B'' and is also an initial morphism in the following sense: If ''g'' is a function from the underlying set of an object ''C'' to the underlying set of ''A'', and if its composition with ''ƒ'' is a morphism ''ƒg'': ''C'' → ''B'', then ''g'' itself is a morphism. A factorization system for a category also gives rise to a notion of embedding. If (''E'', ''M'') is a factorization system, then the morphisms in ''M'' may be regarded as the embeddings, especially when the category is well powered with respect to ''M''. Concrete theories often have a factorization system in which ''M'' consists of the embeddings in the previous sense. 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. An embedding can also refer to an embedding functor.

See also

*Closed immersion *Cover *Dimension reduction *Immersion *Johnson–Lindenstrauss lemma *Submanifold *Subspace *Universal space

Notes

Embedding of manifolds

on the Manifold Atlas {{set index article Category:Abstract algebra Category:Category theory Category:General topology Category:Differential topology Category:Functions and mappings Category:Maps of manifolds Category:Model theory Category:Order theory