Categorical generic fiber (2111.00239v3)

Abstract: For smooth separated families with enough nice base schemes, we describe the abelian category of coherent sheaves on the generic fiber as a Serre quotient. When the family is a proper effectivization of a formal deformation, the Serre quotient is equivalent to the abelian category of coherent sheaves on the general fiber introduced by Huybrechts--Macr`i--Stellari. Passing to the derived category, we obtain a description of the derived category of the generic fiber as a Verdier quotient. It allows us to induce Fourier--Mukai transforms on the generic and geometric generic fibers from derived-equivalent smooth proper families. As an application, we provide new examples of nonbirational Calabi--Yau threefolds that are derived-equivalent. They consist of geometric generic fibers of smooth projective versal deformations of several known examples, which can be regarded as complex manifolds. The description as a Verdier quotient also allows us to lift any object along the projection. As another application, given two flat proper families of algebraic varieties over a common base, we prove that if their generic fibers are projective and derived-equivalent then there exists an open subset of the base over which the restrictions become derived-equivalent. This can be regarded as a categorical analogue of the fact that isomorphic generic fibers imply birational families in our setting.