THOMÉE, Vidar. Estimates of the Friedrichs -Lewy type for a hyperbolic equation with three characteristics. Mathematica Scandinavica, Berkeley, CA, United States, v. 3, p. 115–123, 1955. DOI: 10.7146/math.scand.a-10432. Disponível em: https://journals.msp.org/mscand/article/view/2888. Acesso em: 8 sep. 2026.