Double solids, categories and non-rationality

Abstract

This paper suggests a new approach to questions of rationality of threefolds based on category theory. Following M. Ballard, D. Favero, L. Katzarkov (ArXiv:1012.0864) and D. Favero, L. Katzarkov (Noether--Lefschetz Spectra and Algebraic cycles, in preparation) we enhance constructions from A. Kuznetsov (arXiv:0904.4330) by introducing Noether--Lefschetz spectra --- an interplay between Orlov spectra (C. Oliva, Algebraic cycles and Hodge theory on generalized Reye congruences, Compos. Math. 92, No. 1 (1994) 1--22) and Hochschild homology. The main goal of this paper is to suggest a series of interesting examples where above techniques might apply. We start by constructing a sextic double solid $X$ with 35 nodes and torsion in $H^3(X,\mathbb Z)$. This is a novelty --- after the classical example of Artin and Mumford (1972), this is the second example of a Fano threefold with a torsion in the 3-rd integer homology group. In particular $X$ is non-rational. We consider other examples as well --- $V_{10}$ with 10 singular points and double covering of quadric ramified in octic with 20 nodal singular points. After analyzing the geometry of their Landau--Ginzburg models we suggest a general non-rationality picture based on Homological Mirror Symmetry and category theory.Comment: 24 pages, 12 figures. Dedicated to Vyacheslav Shokurov on the occasion of his 60th birthda