20230526075738.370529 1406-0086 Proceedings of the Estonian Academy of Sciences. Proceedings of the Estonian Academy of Sciences. http://www.digar.ee/id/nlib-digar:12277 Proceedings of the Estonian Academy of Sciences. Physics. Mathematics (Online) Estonia Proceedings of the Estonian Academy of Sciences. Physics. Mathematics (Online) https://kirj.ee/proceedings-of-the-estonian-academy-of-sciences-physics-mathematics-publications/?filter[year]=2007&filter[issue]=28&v=a57b8491d1d8