Bruno Dewer | Searcharxiv

arXiv2024

Divisorial Mori contractions of submaximal length

A result due to Cho, Miyaoka, Shepherd-Barron [CMSB] and Kebekus [Ke] provides a numerical characterization of projective spaces. More recently, Dedieu and H\"oring [DH] gave a characterization of smooth quadrics based on similar arguments. As a relative version of [CMSB] and [Ke], H\"oring and Novelli proved in [HN] that the locus covered by positive-dimensional fibres in a Mori contraction of maximal length is a projective bundle up to birational modification. We change the length hypothesis and we prove that the exceptional locus of a divisorial Mori contraction of submaximal length is birational either to a projective bundle, or to a quadric bundle.

math.AG↗

arXiv2023

Extension of Gorenstein weighted projective 3-spaces and characterization of the primitive curves of their surface sections

We investigate the Gorenstein weighted projective spaces of dimension 3. Given such a space $\mathbf P$, our first focus is its maximal extension in its anticanonical model $\mathbf P \subset \mathbf P^{g+1}$, i.e., the variety $Y\subset \mathbf P^{g+1+r}$ of largest dimension such that $Y$ is not a cone and $\mathbf P$ is a linear section of $Y$. In [DS23] Thomas Dedieu and Edoardo Sernesi have computed the dimension of $Y$ by cohomological computations on the canonical curves inside $\mathbf P$. We give an explicit description of $Y$ in the cases where it was not known. Next, we examine the general anticanonical divisors of $\mathbf P$. These are K3 surfaces, not necessarily primitively polarized. We give a geometric characterization of the curve sections in their primitive polarization.

math.AG↗