Minifolds and Phantoms

Abstract

A minifold is a smooth projective $n$-dimensional variety such that its bounded derived category of coherent sheaves $\D^b(X)$ admits a semi-orthogonal decomposition into an exceptional collection of $n+1$ exceptional objects. In this paper we classify minifolds of dimension $n\le 4$. We conjecture that the derived category of fake projective spaces have a similar semi-orthogonal decomposition into a collection of $n+1$ exceptional objects and a category with vanishing Hochschild homology. We prove this for fake projective planes with non-abelian automorphism group. We construct new examples of phantom categories with both Hochschild homology and Grothendieck group vanishing.

20 pages. New material in version 2: Theorem 1.2 proves Conjecture 3.1 in case of fake projective planes with non-abelian automorphism group, Proposition 3.10 provides new K-phantoms (admissible categories with vanishing Grothendieck group)