THOMÉE, V. Estimates of the Friedrichs -Lewy type for a hyperbolic equation with three characteristics. MATHEMATICA SCANDINAVICA, [S. l.], v. 3, p. 115–123, 1955. DOI: 10.7146/math.scand.a-10432. Disponível em: https://www.mscand.dk/article/view/10432. Acesso em: 6 may. 2024.