1. Ingold and Kidd, J. Chem. Soc., 984 (1933). Hughes and Ingold, ibid., 608 (1941).
2. Robinson, J. Chem. Soc., 220 (1941).
3. For non-mathematical summaries, see Coulson, Proc Roy. Soc. Edin., 61, 115 (1942). Hückel, Z. Electrochem., 43, 752, 827 (1937).
4. Jacobson, Ann., 428, 76 (1922).