Topology Seminar

Speaker: 
Ben Ruppik (Max Planck Institute)
Topic: 
Casson-Whitney unknotting numbers of knotted 2-spheres in the 4-sphere

Abstract:
This is joint work with Jason Joseph, Michael Klug, Hannah Schwartz. Any smoothly knotted 2-sphere in the 4-sphere is regularly homotopic to the unknot. This means every 2-knot in $S^4$ can be obtained by first performing a number of (trivial) finger moves on the unknot, and then removing the resulting intersection points in pairs via Whitney moves (along possibly complicated discs). We define the Casson-Whitney number of $K$ as the minimal number of finger moves needed in such a process to arrive at $K$. In this talk we will relate this to the 1-handle stabilization number, look specifically at examples of ribbon and twist-spun 2-knots and give algebraic lower bounds coming from the fundamental group.

Event Date: 
November 5, 2020 - 2:30pm to 3:20pm
Location: 
Online
Calendar Category: 
Seminar
Seminar Category: 
Geometry and Topology