On the conditions for definiteness of energy and charge
Loide, Rein-Karl
;
Kõiv, Madis
Journal of physics. A. Mathematical and general
1983
/
p. 2353-2361
https://www.ester.ee/record=b2318274*est
https://iopscience.iop.org/article/10.1088/0305-4470/16/11/009/pdf