Graph theory in higher dimensions

Lead Research Organisation: University of Warwick
Department Name: Mathematics

Abstract

Graph theory is a modern and highly active branch of mathematics with an increasingly important impact on other areas, including computer science, geometry, number theory, topology, probability, and statistical mechanics.

A mainstream trend in graph theory aims at generalising results from graphs to hypergraphs. Such gen- eralisations tend to be much harder than their graph analogues, or even provably impossible, and so despite the career-long efforts and deep machinery of generations of graph theorists, we will never be able to extend all of graph theory to hypergraphs. But it is definitely worth doing so for those statements likely to have an impact on other disciplines.

This proposal takes this viewpoint as a starting point. It departs from the mainstream in two ways. Firstly, the generalisations we seek are driven by concrete applications to other areas of mathematics highlighted by the objectives set below. Secondly, rather than a purely combinatorial approach to graphs and hypergraphs, we take a topological viewpoint: when graphs are viewed as 1-dimensional simplicial complexes, natural topological extensions of definitions and theorems to higher-dimensional cell-complexes -the alter ego of hypergraphs- suggest themselves.

One of the ambitions of this project is to advance the development of a unified theory of low-dimensional topological combinatorics that parallels the growth of graph theory into a discipline and has a long-lasting impact on other disciplines. To avoid the risk of getting lost in abstract theory building, concrete objectives are set out below to ensure that the theory grows into the right directions and bears fruit within the time-frame of the project. Planarity, and its higher-dimensional analogues, plays an important role throughout providing some common ground for the various objectives. These objectives are both important and timely, being strongly related to seminal recent advances.

Publications

10 25 50

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Agelos Georgakopoulos (2024) A full Halin grid theorem

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Agelos Georgakopoulos (2022) On graph classes with minor-universal elements in Arxiv

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Agelos Georgakopoulos. (2023) The excluded minors for embeddability into a compact surface in Arxiv

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Georgakopoulos A (2025) On graph classes with minor-universal elements in Journal of Combinatorial Theory, Series B

 
Description We have made substantial progress towards all the Objectives set out in the proposal, as well as several related questions. Here are some highlights:

With my former postdocs J. Danielson-Larrson and J. Haslegrave, we are currently completing a paper on Objective 1, setting the basic theory about the minimum spanning sphere in the d-dimensional complete simplicial complex on n vertices with randomly weighted facets.

With my PhD student George Kontogeorgiou we have established the following theorem envisaged under Objective 2:
A finite group admits a faithful action by homeomorphisms on the 3-dimensional sphere S^3, if and only if it has a Cayley complex embeddable equivariantly in S^3. We have also been able to extend this result to infinite groups acting appropriately on simply-connected non-compact 3-manifolds, and have obtained a very satisfactory characterization of those groups.

In joint work with the postdoc M. Winter funded by the project, we have been able to prove a conjecture of van der Holst about the excluded minors of the class of 4-flat graphs, i.e. graphs all of whose 2-complexes embed in R^4, a result closely related to Objective 3.

In a recent preprint with P. Papasoglu (Oxford), we introduce a notion of large scale minor as envisaged by Objective 4, set several related conjectures and obtain related results. This work has immediately triggered further research by many groups world-wide, and our paper has already attracted about 10 citations within a few months.
Exploitation Route Many other researchers are already using and citing our work.
Sectors Education