A notion of \Delta-multigenus for certain rank two ample vector bundles

A notion of "delta-genus" for ample vector bundles E of rank two on a smooth projective threefold X is defined as a couple of integers (\delta_1, \delta_2). This extends the classical definition holding for ample line bundles. Then pairs (X,E) with low \delta_1 and \delta_2 are classified under suitable additional assumptions on E.