Scheme of lines on a family of 2-dimensional quadrics: geometry and derived category

Abstract

Given a generic family $Q$ of 2-dimensional quadrics over a smooth 3-dimensional base $Y$ we consider the relative Fano scheme $M$ of lines of it. The scheme $M$ has a structure of a generically conic bundle $M\to X$ over a double covering $X\to Y$ ramified in the degeneration locus of $Q\to Y$. The double covering $X\to Y$ is singular in a finite number of points (corresponding to the points $y\in Y$ such that the quadric $Q_y$ degenerates to a union of two planes), the fibers of $M$ over such points are unions of two planes intersecting in a point. The main result of the paper is a construction of a semiorthogonal decomposition for the derived category of coherent sheaves on $M$. This decomposition has three components, the first is the derived category of a small resolution $X^{+}$ of singularities of the double covering $X\to Y$, the second is a twisted resolution of singularities of $X$ (given by the sheaf of even parts of Clifford algebras on $Y$), and the third is generated by a completely orthogonal exceptional collection.

14 pages

References in corpus (1)

• On nodal Enriques surfaces and quartic double solids

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