Mathematical Research Letters
Volume 21 (2014)
Homological actions on sutured Floer homology
Pages: 1177 – 1197
We define the action of the homology group $H_1 (M, \partial M)$ on the sutured Floer homology $SFH(M,\gamma)$. It turns out that the contact invariant $EH(M, \gamma, \xi)$ is usually sent to zero by this action. This fact allows us to refine an earlier result proved by Ghiggini and the author. As a corollary, we classify knots in $\#^n(S^1 \times S^2)$ which have simple knot Floer homology groups: They are essentially the Borromean knots. This answers a question of Ozsváth.
In a different direction, we show that the only links in $S^3$ with simple knot Floer homology groups are the unlinks.