²⁰ Bishop (1967); Fiedler (1967a); Golb and Fiedler (1955); Gottheil and Lauterbach (1969); Gottheil and Vielhaber (1966); Gruenfeld and Arbuthnot (1968); Sashkin, Taylor, and Tripathi (1974); Steiner and McDiarmid (1957); and Strickland (1967).