1. V. K. Dobrev, Invariant Differential Operators, Vol. 2: Quantum Groups, De Gruyter Studies in Mathematical Physics, vol. 39 (De Gruyter, Berlin, Boston, 2017).
2. A. Sudbery, J. Phys. A 23, L697–L704 (1990).
3. V. K. Dobrev, Rep. Math. Phys. 25, 159–181 (1988).
4. V. K. Dobrev, J. Phys. A 27, 4841–4857; J. Phys. A 27, 6633–6634 (1994).
5. V. K. Dobrev and P. Parashar, J. Phys. A 26, 6991–7002 (1993).