AVAKUMOVIC, V. G. Über die Eigenwerte der Schwingungsgleichung. MATHEMATICA SCANDINAVICA, [S. l.], v. 4, p. 161–173, 1956. DOI: 10.7146/math.scand.a-10467. Disponível em: https://www.mscand.dk/article/view/10467. Acesso em: 3 jul. 2024.