Nedialkov, E., and N. Nenov. “Each 11-Vertex Graph Without 4-Cliques Has a Triangle-Free 2-Partition of Vertices”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 91, Dec. 1999, pp. 127-4, https://annual.uni-sofia.bg/index.php/fmi/article/view/275.