On problems of Erdős and Baumann-Briggs on minimising the density of $s$-cliques in graphs with forbidden subgraphs

Using flag algebras, we prove that the minimum density of $8$-cliques in a large graph without an independent set of size $3$ is $491411/268435456+o(1)$, thus resolving a new case of an old problem of Erdős [Magyar Tud. Akad. Mat. Kutató Int. Közl. 7 (1962) 459-464]. Also, we establish some other results of this type; for example, we show that the minimum $s$-clique density in a large graph with no independent set of size 3 nor an induced 5-cycle is $2^{1-s}+o(1)$ when $s=4,5,6$. For each of these results, we also describe the structure of all extremal and almost extremal graphs of large order $n$. These results are applied to give an asymptotic solution to a number of cases of the problem of Baumann and Briggs [Electronic J Comb 32 (2025) P1.22] which asks for the minimum number of $s$-cliques in an $n$-vertex graph in which every $k$-set spans a $t$-clique.

💡 Research Summary

The paper tackles two long‑standing extremal problems in graph theory by employing the flag‑algebra method together with large‑scale semidefinite programming. The first problem originates from a question posed by Erdős in 1962: given a large graph that contains no independent set of size three, what is the smallest possible density of 8‑cliques? By constructing an appropriate flag‑algebra framework—selecting 8‑vertex flags and a suitable set of 7‑vertex types—the authors translate the minimisation of the 8‑clique density into a semidefinite program (SDP). Using high‑performance GPU clusters and high‑precision arithmetic, they solve the SDP to obtain the exact asymptotic value \