Vanishing cohomology and Betti bounds for complex projective hypersurfaces

Abstract

We employ the formalism of vanishing cycles and perverse sheaves to introduce and study the vanishing cohomology of complex projective hypersurfaces. As a consequence, we give upper bounds for the Betti numbers of projective hypersurfaces, generalizing those obtained by different methods by Dimca in the isolated singularities case, and by Siersma-Tib\u{a}r in the case of hypersurfaces with a $1$-dimensional singular locus. We also prove a supplement to the Lefschetz hyperplane theorem for hypersurfaces, which takes the dimension of the singular locus into account, and we use it to give a new proof of a result of Kato.Comment: v2: clarified the proof of Theorem 1.6 and fixed some typos; any comments are greatly appreciated! v3: added Corollary 1.6 and reorganized Sections 3.2 and 3.