Vg. Bagrov et Vv. Obukhov, COMPLETE SEPARATION OF VARIABLES IN THE FREE HAMILTON-JACOBI EQUATION, Theoretical and mathematical physics, 97(2), 1993, pp. 1275-1289
The theory of Stackel spaces is generalized to the case when the coord
inate system in which there is complete separation of variables in the
free Hamilton-Jacobi equation contains complex variables. Theorems wh
ich establish necessary and sufficient criteria of such spaces are pro
ved.