1. P. Stäckel, Math. Ann., 49, 145–147 (1897).
2. B. N. Shapovalov, Sibir. Mat. Zh., 20, 1117–1130 (1979).
3. A. V. Shapovalov and A. I. Breev, Int. J. Geom. Methods, Mod. Phys., 15, No. 5, 1850085 (2018).
4. A. I. Breev and A. V. Shapovalov, J. Phys.: Conf. Ser., 670, Article No. 012015 (2016).
5. V. V. Obukhov, Stäckel Spaces in the Theory of Gravitation [in Russian], Tomsk State Pedagogical University Publishing House, Tomsk (2006).