1. Galindo, A.: J. Math. Phys.8, 768–774 (1967).
2. Michel, L.: Lectures on “Group Extensions and Quantum Mechanics” at the Istanbul Summer School 1962. London: Gordon and Breach, 1965. Nucl. Phys.57, 356–386 (1964).
3. Macaulay, R. A.: Trans. Am. Math. Soc.95, 530–553 (1960).
4. Chevalley, C., andS. Eilenberg: Trans. Am. Math. Soc.63, 85–124 (1948).
5. Cartan, H., andS. Eilenberg: Homological algebra. Princeton: Princeton Univ. Press 1956.