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.

The notion of an end of a topological space was introduced by Hans Freudenthal (1931).

Definition

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Let   be a topological space, and suppose that

 

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

 

where each   is a connected component of  . The number of ends does not depend on the specific sequence   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   is an open set   such that   for some  . Such neighborhoods represent the neighborhoods of the corresponding point at infinity in the end compactification (this "compactification" is not always compact; the topological space X has to be connected and locally connected).

The definition of ends given above applies only to spaces   that possess an exhaustion by compact sets (that is,   must be hemicompact). However, it can be generalized as follows: let   be any topological space, and consider the direct system   of compact subsets of   and inclusion maps. There is a corresponding inverse system  , where   denotes the set of connected components of a space  , and each inclusion map   induces a function  . Then set of ends of   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, where morphisms are only proper continuous maps, to the category of sets. Explicitly, if   is a proper map and   is an end of   (i.e. each element   in the family is a connected component of   and they are compatible with maps induced by inclusions) then   is the family   where   ranges over compact subsets of Y and   is the map induced by   from   to  . Properness of   is used to ensure that each   is compact in  .

The original definition above represents the special case where the direct system of compact subsets has a cofinal sequence.

Examples

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  • The set of ends of any compact space is the empty set.
  • The real line   has two ends. For example, if we let Kn be the closed interval [−nn], then the two ends are the sequences of open sets Un = (n, ∞) and Vn = (−∞, −n). These ends are usually referred to as "infinity" and "minus infinity", respectively.
  • If n > 1, then Euclidean space   has only one end. This is because   has only one unbounded component for any compact set K.
  • More generally, if M is a compact manifold with boundary, then the number of ends of the interior of M is equal to the number of connected components of the boundary of M.
  • The union of n distinct rays emanating from the origin in   has n ends.
  • The infinite complete binary tree has uncountably many ends, corresponding to the uncountably many different descending paths starting at the root. (This can be seen by letting Kn be the complete binary tree of depth n.) These ends can be thought of as the "leaves" of the infinite tree. In the end compactification, the set of ends has the topology of a Cantor set.

Ends of graphs and groups

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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 (Diestel & Kühn 2003).

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

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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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  • Diestel, Reinhard; Kühn, Daniela (2003), "Graph-theoretical versus topological ends of graphs", Journal of Combinatorial Theory, Series B, 87 (1): 197–206, doi:10.1016/S0095-8956(02)00034-5, MR 1967888.
  • Freudenthal, Hans (1931), "Über die Enden topologischer Räume und Gruppen", Mathematische Zeitschrift, 33, Springer Berlin / Heidelberg: 692–713, doi:10.1007/BF01174375, ISSN 0025-5874, S2CID 120965216, Zbl 0002.05603
  • Ross Geoghegan, Topological methods in group theory, GTM-243 (2008), Springer ISBN 978-0-387-74611-1.
  • Scott, Peter; Wall, Terry; Wall, C. T. C. (1979). "Topological methods in group theory". Homological Group Theory. pp. 137–204. doi:10.1017/CBO9781107325449.007. ISBN 9781107325449.