On the Floer homology of plumbed three-manifolds

We calculate the Heegaard Floer homologies for three- manifolds obtained by plumbings of spheres specified by certain graphs. Our class of graphs is sufficiently large to describe, for example, all Seifert fibered rational homology spheres. These calculations can be used to determine also these groups for other three- manifolds, including the product of a circle with a genus two surface.