Bikov, A., and N. Nenov. “LOWER BOUNDING THE FOLKMAN NUMBERS $F_v(a_1, . . . , a_s;M − 1)$”. Ann. Sofia Univ. Fac. Math. And Inf., vol. 104, Dec. 2017, pp. 39-53, https://ftl5.uni-sofia.bg/index.php/fmi/article/view/36.