Diagram genus, generators and applications

Abstract

We continue the study of the genus of knot diagrams, deriving a new description of generators using Hirasawa's algorithm. This description leads to good estimates on the maximal number of crossings of generators and allows us to complete their classification for knots of genus 4. As applications of the genus 4 classification, we establish non-triviality of the skein polynomial on $k$-almost positive knots for $k\le 4$, and of the Jones polynomial for $k\le 3$. For $k\le 4$, we classify the occurring achiral knots, and prove a trivializability result for $k$-almost positive unknot diagrams. This yields also estimates on the number of unknotting Reidemeister moves. We describe the positive knots of signature (up to) 4. Using a study of the skein polynomial, we prove the exactness of the Morton-Williams-Franks braid index inequality and the existence of a minimal string Bennequin surface for alternating knots up to genus 4. We also prove for such knots conjectures of Hoste and Fox about the roots and coefficients of the Alexander polynomial.