Differential technique for the covariant orbital angular momentum operators M. Matveev, A. Sarantsev, K. Semenov-Tian-Shansky and A. Semenova Eur. Phys. J. A, 54 6 (2018) 108 Published online: 26 June 2018 DOI: 10.1140/epja/i2018-12539-9