233arXiv version: fonts, pagination and layout may vary from GTM published versionThe Heegaard structure of Dehn filled manifolds
YOAV MORIAH ERIC SEDGWICKWe expect manifolds obtained by Dehn filling to inherit properties from the knot manifold. To what extent does that hold true for the Heegaard structure? We study four changes to the Heegaard structure that may occur after filling: (1) Heegaard genus decreases, (2) a new Heegaard surface is created, (3) a non-stabilized Heegaard surface destabilizes, and (4) two or more non-isotopic Heegaard surfaces become isotopic. We survey general results that give quite satisfactory restrictions to phenomena (1) and (2) and, in a parallel thread, give a complete classification of when all four phenomena occur when filling most torus knot exteriors. This latter thread yields sufficient (and perhaps necessary) conditions for the occurrence of phenomena (3) and (4).
57N10; 57M27
IntroductionLet X be a knot manifold, that is a compact, orientable and irreducible 3-manifold with a single torus boundary component. There are many results demonstrating that most of the manifolds obtained by filling inherit properties from the knot manifold. We would also expect the Heegaard structure of filled manifolds to be closely related to the Heegaard structure of the knot manifold. For example, it is easy to see that every Heegaard surface for the knot manifold is a Heegaard surface for each filled manifold. In particular, this implies that the Heegaard genus of X is an upper bound on the genus of each filled manifold. However, the Heegaard structure of a filled manifold can differ from that of the knot manifold. Here are four ways that this could occur:(1) Heegaard genus decreases. By a new Heegaard surface, we mean that a filled manifold contains a Heegaard surface that is not isotopic (in the filled manifold) to a Heegaard surface for the knot manifold X . When the genus decreases (1), the filled manifold has a Heegaard surface of lower genus than every Heegaard surface for X . Indeed, it is a new Heegaard surface. So, restricting (2) also restricts (1).In each of these cases, we would like to either demonstrate that the set of fillings for which the phenomenon occurs is special, for example finite, a line of slopes, and/or conclude that the Heegaard surface(s) in question are special in some regard, for example γ -primitive, padded, or boundary stabilized.In Section 5 we survey known work that gives quite satisfactory restrictions to phenomena (1) and (2). We also give an extended example: Dehn filling on a torus knot exterior, for which we have almost complete knowledge. We are able to completely specify the fillings for which each of these four phenomena occur. This also illustrates sufficient conditions for (3) and (4) to occur.
AcknowledgementsWe would like to thank Ryan Derby-Talbot, David Bachman, Jesse Johnson and Jennifer Schultens for helpful discussions. Also thanks to DePaul CTI and the Mathematics Department of the Technion for their hospitality.
Background 2.1 Dehn fillin...