|
We prove that, like the Seiberg–Witten monopole homology, the Heegaard Floer
homology for a three-manifold determines its Thurston norm. As a consequence, we
show that knot Floer homology detects the genus of a knot. This leads to new proofs
of certain results previously obtained using Seiberg–Witten monopole Floer homology
(in collaboration with Kronheimer and Mrowka). It also leads to a purely
Morse-theoretic interpretation of the genus of a knot. The method of proof shows
that the canonical element of Heegaard Floer homology associated to a weakly
symplectically fillable contact structure is non-trivial. In particular, for certain
three-manifolds, Heegaard Floer homology gives obstructions to the existence of taut
foliations.
|