citation Greenwood & Gleason (1955) constructed a 2-coloring of K_17 from the quadratic residues mod 17 (the Paley graph on 17 vertices) with no monochromatic K_4, and proved every 2-coloring of K_18 contains one. Survey DS1, section 2.1(b), attributes R(4,4)=18 to [GG]; there is a unique (4,4)-critical graph.