Reflect-Push Methods Part I: Two Dimensional Techniques

TL;DR

This paper introduces the reflect-push method, a geometric and combinatorial technique, to find all maximum weight downsets in product posets, generalizing and strengthening several isoperimetric and graph extremal results.

Contribution

The paper presents the reflect-push method as a new general approach for identifying all solutions to edge-isoperimetric problems in two dimensions and beyond, extending previous theorems and results.

Findings

All maximum weight downsets in two-dimensional product of chains are determined.

The reflect-push method generalizes Lindsay's edge-isoperimetric theorem and other extremal graph results.

Lexicographic solutions are shown to be optimal and unique in many higher-dimensional isoperimetric problems.

Abstract

We determine all maximum weight downsets in the product of two chains, where the weight function is a strictly increasing function of the rank. Many discrete isoperimetric problems can be reduced to the maximum weight downset problem. Our results generalize Lindsay's edge-isoperimetric theorem in two dimensions in several directions. They also imply and strengthen (in several directions) a result of Ahlswede and Katona concerning graphs with maximal number of adjacent pairs of edges. We find all optimal shifted graphs in the Ahlswede-Katona problem. Furthermore, the results of Ahlswede-Katona are extended to posets with a rank increasing and rank constant weight function. Our results also strengthen a special case of a recent result by Keough and Radcliffe concerning graphs with the fewest matchings. All of these results are achieved by applications of a key lemma that we call the reflect-push method. This method is geometric and combinatorial. Most of the literature on edge-isoperimetric inequalities focuses on finding a solution, and there are no general methods for finding all possible solutions. Our results give a general approach for finding all compressed solutions for the above edge-isoperimetric problems. By using the Ahlswede-Cai local-global principle, one can conclude that lexicographic solutions are optimal for many cases of higher dimensional isoperimetric problems. With this and our two dimensional results we can prove Lindsay's edge-isoperimetric inequality in any dimension. Furthermore, our results show that lexicographic solutions are the unique solutions for which compression techniques can be applied in this general setting.