End (topology)

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In topology, a branch of mathematics, the ends of a topological space are, roughly speaking, the connected components of the “ideal boundary” of the space. That is, each end represents a topologically distinct way to move to infinity within the space. Adding a point at each end yields a compactification of the original space, known as the end compactification.

Definition

Let X be a topological space, and suppose that

K1K2K3 ⊂ · · ·

is an ascending sequence of compact subsets of X whose interiors cover X. Then X has one end for every sequence

U1U2U3 ⊃ · · ·,

where each Un is a connected component of X \ Kn. The number of ends does not depend on the specific sequence {Ki} of compact sets; there is a natural bijection between the sets of ends associated with any two such sequences.

Using this definition, a neighborhood of an end {Ui} is an open set V such that V ⊃ Un for some n. Such neighborhoods represent the neighborhoods of the corresponding point at infinity in the end compactification (this “compactification” isn’t always compact; the topological space X has to be connected and locally connected).

The definition of ends given above applies only to spaces X that possess an exhaustion by compact sets (that is, X must be hemicompact). However, it can be generalized as follows: let X be any topological space, and consider the direct system {Kα} of compact subsets of X and inclusion maps. There is a corresponding inverse systemπ0X \ Kα ) }, where π0(Y) denotes the set of connected components of a space Y, and each inclusion map Y → Z induces a function π0(Y) → π0(Z). Then set of ends of X is defined to be the inverse limit of this inverse system. Under this definition, the set of ends is a functor from the category of topological spaces to the category of sets. The original definition above represents the special case where the direct system of compact subsets has a cofinal sequence.

Examples

History

The notion of an end of a topological space was introduced by Template:Harvs.

Ends of graphs and groups

{{#invoke:main|main}} In infinite graph theory, an end is defined slightly differently, as an equivalence class of semi-infinite paths in the graph, or as a haven, a function mapping finite sets of vertices to connected components of their complements. However, for locally finite graphs (graphs in which each vertex has finite degree), the ends defined in this way correspond one-for-one with the ends of topological spaces defined from the graph Template:Harv.

The ends of a finitely generated group are defined to be the ends of the corresponding Cayley graph; this definition is insensitive to the choice of generating set. Every finitely-generated infinite group has either 1, 2, or infinitely many ends, and Stallings theorem about ends of groups provides a decomposition for groups with more than one end.

Ends of a CW complex

For a path connected CW-complex, the ends can be characterized as homotopy classes of proper maps , called rays in X: more precisely, if between the restriction -to the subset - of any two of these maps exists a proper homotopy we say that they are equivalent and they define an equivalence class of proper rays. This set is called an end of X.

References

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  • Ross Geoghegan, Topological methods in group theory, GTM-243 (2008), Springer ISBN 978-0-387-74611-1.
  • Peter Scott, Terry Wall, Topological methods in group theory, London Math. Soc. Lecture Note Ser., 36, Cambridge Univ. Press (1979) 137-203.