Structural models: an introduction to the theory of directed graphs [by] Frank Harary, Robert Z. Norman [and] Dorwin Cartwright. Harary, Frank. Norman, Robert Z. (Robert Zane), 1924- joint author. Cartwright, Dorwin, joint author. Social sciences Mathematical models. H61 .H287 311.26 HAS Bibliography: p. 392-395. New York, Wiley [1965] [1965] 1965 Text ix, 415 p. eng