Forcing Graphs to be Forcing

Abstract: Sidorenko's conjecture states that the number of copies of any given bipartite graph in another graph of given density is asymptotically minimized by a random graph. The forcing conjecture further strengthens this, claiming that any minimizer in fact needs to be quasi-random. Here we extend the family of bipartite graphs for which the forcing conjecture is known to hold to include balanced blow-ups of Sidorenko graphs and subdivisions of Sidorenko graphs by a forcing graph. This partially generalizes results by Conlon et al. (2018) and Conlon and Lee (2021). We also show that the box product of a Sidorenko graph with an edge is forcing, partially generalizing results of Kim, Lee, and Lee (2016) and, in particular, showing that cubes are forcing. We achieve these results through algebraic arguments building on Razborov's flag algebra framework (2007). This approach additionally allows us to construct Sidorenko hypergraphs from known 2-uniform Sidorenko graphs and to study forcing pairs.