A finite forbidden family with superlinear surplus and non-join extremal graphs

Abstract

We give a common counterexample to two product-structure conjectures in extremal graph theory. More precisely, we construct a fixed nonempty finite family $\mathcal{L}$ with $p(\mathcal{L})=2$ such that, for some $c>0$, \[ \operatorname{ex}(n,\mathcal L)>t_2(n)+cn^{3/2} \] for every sufficiently large $n$. Nevertheless, at every such order there is an $\mathcal{L}$-extremal graph whose complement is connected and which therefore admits no decomposition as a join of two nonempty graphs. This superlinear surplus also forces the decomposition family of $\mathcal{L}$ to contain no forest. The construction uses endpoint-injective repair with a finite obstruction family admitting an exact extremal formula and equality classification.

9 pages. Added a link to the Lean 4 formalization