On a Poisson structure on the space of Stokes matrices

In this paper we study the map associating to a linear differential operator with rational coefficients its monodromy data. The operator has one regular and one irregular singularity of Poincare' rank 1. We compute the Poisson structure of the corresponding Monodromy Preserving Deformation Equations on the space of the monodromy data.Comment: 16 pages,Tex,to be published in IMR

Cox ring of the generic fiber

Given a surjective morphism $\pi\colon X\to Y$ of normal varieties satisfying some regularity hypotheses we prove how to recover a Cox ring of the generic fiber of $\pi$ from the Cox ring of $X$. As a corollary we show that in some cases it is also possible to recover the Cox ring of a very general fiber, and finally we give an application in the case of the blowing-up of a toric fiber space

A counterexample to a conjecture on linear systems on P^3

In this note we give a counterexample to a conjecture proposed by Ciliberto about special linear systems of P^n through multiple base points.Comment: 6 pages, LaTe

A conjecture on special linear systems of P^3

In this note we consider the behavior of linear systems of P^3 through fat points under a cubo-cubic Cremona transformation. This allows us to produce a class of special systems which we conjecture to be the only ones.Comment: 4 pages, LaTe

On multiples of divisors associated to Veronese embeddings with defective secant variety

In this note we consider multiples aD, where D is a divisor of the blow-up of P^n along points in general position which appears in the Alexander and Hirschowitz list of Veronese embeddings having defective secant varieties. In particular we show that there is such a D with h^1(X,D) > 0 and h^1(X,2D) = 0.Comment: 10 pages, LaTe

Elementary (-1)-curves of P^3

In this note we deal with rational curves in P^3 which are images of a line by means of a finite sequence of cubo-cubic Cremona transformations. We prove that these curves can always be obtained applying to the line a sequence of such transformations increasing at each step the degree of the curve. As a corollary we get a result about curves that can give speciality for linear systems of P^3.Comment: 12 pages, LaTe