1. Damhus, T., Harnung, S. E., Schäffer, C. E.: paper I in this series
2. Same, paper II in the series
3. Damhus, T.: Linear Algebra Appl. 32, 125 (1980)
4. Sivardière, J.: Phys. Stat. Sol. (b) 107, 117 (1981)
5. Artmann, B.: Math.-Phys. Semesterber. (Univ. Giessen) 24, 20 (1977)