20230526075738.370529 http://digar.nlib.ee/show/nlib-digar:12277 Estonia http://www.kirj.ee/proc_mathphys Proceedings of the Estonian Academy of Sciences. Physics. Mathematics Proceedings of the Estonian Academy of Sciences. Physics. Mathematics Proceedings of the Estonian Academy of Sciences. 1406-0086 1018-7650#ISSN Proceedings of the Estonian Academy of Sciences.