1. Longuet-Higgins HC (1950) J. Chem. Phys. 18: 265
2. Pauling L (1948) The nature of the chemical bond, 2nd ed, Cornell Univ. Press, Ithaca, New York
3. Wheland GW (1953) The theory of resonance and its application to organic chemistry, Wiley, New York
4. Robinson R (1966) Aromaticity, Intern. Symposium, Sheffield 1966, p 47
5. Clar E (1972) The aromatic sextet, Wiley, London