Ore, Oystein, 1899-1968. The four-color problem. [[Book] /] Oystein Ore. - New York : Academic Press, 1967. - xv, 259 p : illus ; 24 cm. - Pure and applied mathematics, a series of monographs and textbooks ; 27 . Bibliography: p. 249-253. All Ages. Subjects--Topical Terms: Four-color problem. Dewey Class. No.: 513.83 National Library of Medicine Call No.: / O F