Sala Seminari (Dip. Matematica).
We will describe a general method for calculating DW-invariants of orientable 3-manifolds. The method is based on a relation between DW-invariants and Arf-invariants of special quadratic functions over Z_2 on the first cohomology group of the given manifold. As an example, we calculate DW-invariants of all orientable Seifert manifolds fibered over the 2-dimensional sphere.