1. See, e.g., Salem, L.: Molecular orbital theory of conjugated systems. New York: Benjamin 1966
2. Burkitt, F. H., Coulson, C. A., Longuet-Higgins, H. C.: Trans. Faraday Soc.47, 553 (1951);
3. Greenwood, H. H.: Trans. Faraday Soc.48, 585 (1952); Yvan, P.: J. Chim. Phys.49, 457 (1952)
4. Streitwieser, A.: Molecular orbital theory for organic chemists, §11.5. New York: Wiley 1962
5. For review on the application of graph theory in the molecular orbital theory of conjugated molecules see: Gutman, I., Trinajsti?, N.: Topics Curr. Chem.42, 49 (1973)