Rational genus and Heegaard Floer homology

Abstract

Turaev defined a function on the first homology of a rational homology 3-sphere $Y$ as the minimal rational Seifert genus of all knots in this homology class. Ni and the first author discovered a lower bound of this function using the Heegaard Floer $d$-invariant and showed that Floer simple knots are rational Seifert genus minimizers. In this paper, we give a simple reproof of the above results. We then define a version of rational slice genus for knots in the product 4-manifold $Y\times I$ and investigate the analogous minimal genus problem. We prove the same lower bound in terms of the $d$-invariant formula and the same genus minimizers given by Floer simple knots.

24 pages, 2 figures. Notations are modified to avoid potential confusion with Hedden and Raoux's preprint arXiv:2308.16853