The critical window for the classical Ramsey-Turán problem

<p>The first application of Szemerédi’s powerful regularity method was the following celebrated Ramsey-Turán result proved by Szemerédi in 1972: any <em>K</em> 4-free graph on <em>n</em> vertices with independence number <em>o</em>(<em>n</em>) has at most (18+o(1))n2 edges. Four years later, Bollobás and Erdős gave a surprising geometric construction, utilizing the isoperimetric inequality for the high dimensional sphere, of a <em>K</em> 4-free graph on <em>n</em> vertices with independence number <em>o</em>(<em>n</em>) and (18−o(1))n2 edges. Starting with Bollobás and Erdős in 1976, several problems have been asked on estimating the minimum possible independence number in the critical window, when the number of edges is about <em>n</em> 2/8. These problems have received considerable attention and remained one of the main open problems in this area. In this paper, we give nearly best-possible bounds, solving the various open problems concerning this critical window.</p>