In other words, the PMFG is a topological triangulation of the sphere. Only cliques of three and four
elements are permitted in the PMFG. Indeed, Kuratowski’s theorem (23) does not allow cliques with a number of elements
larger than four in a planar graph. Larger cliques can only be present in graphs with genus k > 0. The larger the value of k, the larger the number of elements r of the maximal allowed clique
[specifically r (7 1 48k)2; see ref. 24].