Sizes of cross-intersecting families

Abstract

The size of cross-intersecting families ${\cal F}_1, \dots, {\cal F}_d$ is usually measured by the sum or the product of the sizes $|{\cal F}_i|$. These two natural size functions have different extremal structures, and different techniques have been developed for them. We introduce the cross-degree of a size function and show that the potential extremal structures of maximum cross-intersecting families are the same for all size functions of finite cross-degree, including the sum and the product. Our main tool is a strengthened form of the generating family method of Ahlswede--Khachatrian, based on standard generating families and on local modifications of cross-intersecting families, which we call mutations. It treats two completely different types of extremal structures in a unified way and, unlike the original method, needs shifting in only one step.