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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.
General topology 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 fX → 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. An embedding is proper if it behaves well w.r.t. boundaries: one requires the map to be such that 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. Differential geometry In differential geometry: Let M and N be smooth manifolds and be a smooth map, it is called an immersion if for any point the differential is injective (here denotes tangent space of at ). 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). When the manifold is compact, the notion of a smooth embedding is equivalent to that of an injective immersion. 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.) 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. The less restrictive condition of immersion applies to the Boy's surface—which has self-intersections. The Roman surface fails to be an immersion as it contains cross-caps. Riemannian geometry 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 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). Field theory 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. Domain theory In domain theory, an embedding of partial orders is in the function space X →Y such that Based on an article from FOLDOC, . See also | ||||||||
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