The amalgamation of high distance Heegaard splittings along tori

Let [Formula: see text] be a compact, connected, orientable closed 3-manifold. Let [Formula: see text] be a disjoint union of incompressible separating tori in [Formula: see text] which cut [Formula: see text] into the submanifolds [Formula: see text]. In the present paper, we show that the Heegaard genus [Formula: see text] of [Formula: see text] is given by [Formula: see text] provided that each [Formula: see text] has a Heegaard splitting [Formula: see text] with Hempel distance [Formula: see text].

Many Haken Heegaard splittings

We give a simple criterion for a Heegaard splitting to yield a Haken manifold. As a consequence, we construct many Haken manifolds, in particular homology spheres, with prescribed properties, namely Heegaard genus, Heegaard distance and Casson invariant. Along the way we give simpler and shorter proofs of the existence of splittings with specified Heegaard distance, originally proven by Ido–Jang–Kobayashi, of the existence of hyperbolic manifolds with prescribed Casson invariant, originally due to Lubotzky–Maher–Wu, and of a result about subsurface projections of disc sets (for which we even get better constants), originally due to Masur–Schleimer.

Minimal Heegaard genus of amalgamated 3-manifolds

Let [Formula: see text] [Formula: see text] be a perfect [Formula: see text]-manifold, [Formula: see text] be a component of [Formula: see text], [Formula: see text] be a homeomorphic map, [Formula: see text] and [Formula: see text]. In this paper, we show that if [Formula: see text] [Formula: see text] and [Formula: see text], then [Formula: see text]. As a corollary, if [Formula: see text] [Formula: see text] is a component of [Formula: see text], [Formula: see text] is a homeomorphic map, [Formula: see text], [Formula: see text], [Formula: see text] [Formula: see text] and [Formula: see text] [Formula: see text], then [Formula: see text].

Symmetric ribbon disks

We study the ribbon disks that arise from a symmetric union presentation of a ribbon knot. A natural notion of symmetric ribbon number rS(K) is introduced and compared with the classical ribbon number r(K). We show that the difference rS(K) - r(K) can be arbitrarily large by constructing an infinite family of ribbon knots Kn such that r(Kn) = 2 and rS(Kn) > n. The proof is based on a particularly simple description of symmetric unions in terms of certain band diagrams which leads to an upper bound for the Heegaard genus of their branched double covers.

Fundamental Group and Covering Properties of Hyperbolic Surgery Manifolds

We study a family of closed connected orientable 3-manifolds obtained by Dehn surgeries with rational coefficients along the oriented components of certain links. This family contains all the manifolds obtained by surgery along the (hyperbolic) 2-bridge knots. We find geometric presentations for the fundamental group of such manifolds and represent them as branched covering spaces. As a consequence, we prove that the surgery manifolds, arising from the hyperbolic 2-bridge knots, have Heegaard genus 2 and are 2-fold coverings of the 3-sphere branched over well-specified links.