Deformations of the Fano scheme of a cubic

Abstract

We study the deformation theory of the Fano scheme $\mathrm{F}=\mathrm{F}(\mathrm{X})$ of lines on a cubic $\mathrm{X}$ of dimension $d$ with only finitely many singularities. By taking the relative Fano scheme, we define a morphism $\eta :{\mathcal{D}}_{\mathrm{X}}\to {\mathcal{D}}_{\mathrm{F}}$ of the local moduli functors associated to $\mathrm{X}$ and $\mathrm{F}$, respectively. We show that for $d⩾5$, $\eta$ yields an isomorphism on first-order deformations; in particular, $\eta$ is an isomorphism whenever ${\mathrm{H}}^0({\Theta }_{\mathrm{X}})=0$.

Final version, to appear in Mathematische Zeitschrift