Boundary-incompressible surface explained

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

References

• W. Jaco, Lectures on Three-Manifold Topology, volume 43 of CBMS Regional Conference Series in Mathematics. American Mathematical Society, Providence, R.I., 1980.
• T. Kobayashi, A construction of 3-manifolds whose homeomorphism classes of Heegaard splittings have polynomial growth, Osaka J. Math. 29 (1992), no. 4, 653 - 674. .