• On the degree of minimal equation for first-order wave equationsLoide, Rein-KarlEesti NSV Teaduste Akadeemia toimetised. Füüsika. Matemaatika = Известия Академии наук Эстонской ССР. Физика. Математика = Proceedings of Academy of Sciences of the Estonian SSR. Physics. Mathematics1982 / lk. 434-436 https://www.ester.ee/record=b1264310*est