Extremal triangle-free graphs with chromatic number at least four

Sijie Ren, Jian Wang, Shipeng Wang +1

Let $G$ be an $n$-vertex triangle-free graph. The celebrated Mantel's theorem showed that $e(G)\le \lfloor \frac{n^2}{4}\rfloor$. In 1962, Erdős (together with Gallai), and indepen…