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Boundary-incompressible surface : ウィキペディア英語版
Boundary-incompressible surface
In low-dimensional topology, a boundary-incompressible surface is a two-dimensional surface within a three-dimensional manifold whose topology cannot be made simpler by a certain type of operation known as boundary compression.
Suppose ''M'' is a 3-manifold with boundary. Suppose also that ''S'' is a compact surface with boundary that is properly embedded in ''M'',
meaning that the boundary of ''S'' is a subset of the boundary of ''M'' and the interior points of ''S'' are a subset of the interior points of ''M''.
A boundary-compressing disk for ''S'' in ''M'' is defined to be a disk ''D'' in ''M'' such that D \cap S = \alpha and D \cap \partial M = \beta are arcs in \partial D , with \alpha \cup \beta = \partial D , \alpha \cap \beta = \partial \alpha = \partial \beta , and \alpha is an essential arc in ''S'' ( \alpha does not cobound a disk in ''S'' with another arc in \partial S ).
The surface ''S'' is said to be boundary-compressible if either ''S'' is a disk that cobounds a ball with a disk in \partial M or there exists a boundary-compressing disk for ''S'' in ''M''. Otherwise, ''S'' is boundary-incompressible.
Alternatively, one can relax this definition by dropping the requirement that the surface be properly embedded. Suppose now that ''S'' is a compact surface (with boundary) embedded in the boundary of a 3-manifold ''M''. Suppose further that ''D'' is a properly embedded disk in ''M'' such that ''D'' intersects ''S'' in an essential arc (one that does not cobound a disk in ''S'' with another arc in \partial S ). Then ''D'' is called a boundary-compressing disk for ''S'' in ''M''. As above, ''S'' is said to be boundary-compressible if either ''S'' is a disk in \partial M or there exists a boundary-compressing disk for ''S'' in ''M''. Otherwise, ''S'' is boundary-incompressible.
For instance, if ''K'' is a trefoil knot embedded in the boundary of a solid torus ''V'' and ''S'' is the closure of a small annular neighborhood of ''K'' in \partial V , then ''S'' is not properly embedded in ''V'' since the interior of ''S'' is not contained in the interior of ''V''. However, ''S'' is embedded in \partial V and there does not exist a boundary-compressing disk for ''S'' in ''V'', so ''S'' is boundary-incompressible by the second definition.
== See also ==

* Incompressible surface

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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