On problems in extremal multigraph theory

Abstract: A multigraph G is said to be an (s,q)-graph if every s-set of vertices in G supports at most q edges (counting multiplicities). In this paper we consider the maximal sum and product of edge multiplicities in an (s,q)-graph on n vertices. These are multigraph analogues of a problem of Erd\H{o}s raised by F\"uredi and K\"undgen and Mubayi and Terry respectively, with applications to counting problems and extremal hypergraph theory. We make major progress, settling conjectures of Day, Falgas-Ravry and Treglown and of Falgas-Ravry, establishing intricate behaviour for both the sum and the product problems, and providing both a general picture and evidence that the problems may prove computationally intractable in general.