Communications in Analysis and Geometry

Volume 24 (2016)

Number 5

Immersed disks, slicing numbers and concordance unknotting numbers

Pages: 1107 – 1138



Brendan Owens (School of Mathematics and Statistics, University of Glasgow, Scotland, United Kingdom)

Sašo Strle (Faculty of Mathematics and Physics, University of Ljubljana, Slovenia)


We study three knot invariants related to smoothly immersed disks in the four-ball. These are the four-ball crossing number, which is the minimal number of normal double points of such a disk bounded by a given knot; the slicing number, which is the minimal number of crossing changes required to obtain a slice knot; and the concordance unknotting number, which is the minimal unknotting number in a smooth concordance class. Using Heegaard Floer homology we obtain bounds that can be used to determine two of these invariants for all prime knots with crossing number ten or less, and to determine the concordance unknotting number for all but thirteen of these knots. As a further application we obtain some new bounds on Gordian distance between torus knots. We also give a strengthened version of Ozsváth and Szabó’s obstruction to unknotting number one.

Published 6 March 2017