On the Euler characteristic of a relative hypersurface

TL;DR

This paper presents a general, base-independent formula for computing the Euler characteristic of a fibration of projective hypersurfaces, simplifying calculations and applicable to various fiber types and dimensions.

Contribution

It introduces a novel pushforward formula that reduces Euler characteristic computations to algebraic manipulations based on divisor classes, applicable to arbitrary base varieties.

Findings

Derived a general formula for Euler characteristics of hypersurface fibrations.

Provided a base-independent pushforward method for algebraic computations.

Applied the formula to explicit families of hypersurfaces with diverse fibers.

Abstract

We derive a general formula for the Euler characteristic of a fibration of projective hypersurfaces in terms of invariants of an arbitrary base variety. When the general fiber is an elliptic curve, such formulas have appeared in the physics literature in the context of calculating D-brane charge for M-/F-theory and type-IIB compactifications of string vacua. While there are various methods for computing Euler characteristics of algebraic varieties, we prove a base-independent pushforward formula which reduces the computation of the Euler characteristic of relative hypersurfaces to simple algebraic manipulations of rational expressions determined by its divisor class in a projective bundle. We illustrate our methods by applying them to an explicit family of relative hypersurfaces whose fibers are of arbitrary dimension and degree.