A-D-E diagrams, Hodge--Tate hyperplane sections and semisimple quantum cohomology

Abstract

It is known that the semisimplicity of quantum cohomology implies the vanishing of off-diagonal Hodge numbers (Hodge--Tateness). We investigate which hyperplane sections of homogeneous varieties possess either of the two properties. We provide a new efficient criterion for non-semisimplicity of the small quantum cohomology ring of Fano manifolds that depends only on the Fano index and Betti numbers. We construct a bijection between Dynkin diagrams of types A, D or E, and complex Grassmannians with Hodge-Tate smooth hyperplane sections. By applying our criteria and using monodromy action, we completely characterize the semisimplicity of the small quantum cohomology of smooth hyperplane sections in the case of complex Grassmannians, and verify a conjecture of Benedetti and Perrin in the case of (co)adjoint Grassmannians.

arXiv commentAll but Appendix A are written by Sergey Galkin, Naichung Conan Leung, Changzheng Li and Rui Xiong; Appendix A is written by Pieter Belmans and Markus Reineke. The appendix is newly added, showing that an infinite family of Kronecker moduli has non-semisimple small quantum cohomology, as a nice application of the simple criterion in the main body