The Grassmann Space of a Planar Space

AbstractIn this paper we give a characterization of the Grassmann space of a planar space

The Degree of the Tangent and Secant Variety to a Projective Surface

In this paper we present a way of computing the degree of the secant (resp., tangent) variety of a smooth projective surface, under the assumption that the divisor giving the embedding in the projective space is $3$-very ample. This method exploits the link between these varieties and the Hilbert scheme $0$-dimensional subschemes of length $2$ of the surface.Comment: 20 pages; generalization of the previous version (from projective K3 surfaces to any projective surface) and improvement of the expositio

On the Grassmann space representing the lines of an affine space

AbstractIn 1982, Bichara and Mazzocca characterized the Grassmann space Gr(1,A) of the lines of an affine space A of dimension at least 3 over a skew-field K by means of the intersection properties of the three disjoint families Σ1,Σ2 and T of maximal singular subspaces of Gr(1,A). In this paper, we deal with the characterization of Gr(1,A) using only the family Σ=Σ1∪Σ2 of maximal singular subspaces

Orbital degeneracy loci and applications

Degeneracy loci of morphisms between vector bundles have been used in a wide variety of situations. We introduce a vast generalization of this notion, based on orbit closures of algebraic groups in their linear representations. A preferred class of our orbital degeneracy loci is characterized by a certain crepancy condition on the orbit closure, that allows to get some control on the canonical sheaf. This condition is fulfilled for Richardson nilpotent orbits, and also for partially decomposable skew-symmetric three-forms in six variables. In order to illustrate the efficiency and flexibility of our methods, we construct in both situations many Calabi--Yau manifolds of dimension three and four, as well as a few Fano varieties, including some new Fano fourfolds.Comment: To appear in Ann. Sc. Norm. Super. Pisa Cl. Sci. (5