Knots with unknotting number one and Heegaard Floer homology

Peter Ozsvath, Zoltan Szabo

Published 2004-01-30, updated 2005-03-15Version 2

We use Heegaard Floer homology to give obstructions to unknotting a knot with a single crossing change. These restrictions are particularly useful in the case where the knot in question is alternating. As an example, we use them to classify all knots with crossing number less than or equal to nine and unknotting number equal to one. We also classify alternating knots with ten crossings and unknotting number equal to one.