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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 and are arcs in , with , , and is an essential arc in ''S'' ( does not cobound a disk in ''S'' with another arc in ). The surface ''S'' is said to be boundary-compressible if either ''S'' is a disk that cobounds a ball with a disk in 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 ). 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 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 , 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 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)』 ■ウィキペディアで「Boundary-incompressible surface」の詳細全文を読む スポンサード リンク
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