THOMÉE, Vidar. 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://tidsskrift.dk/math/article/view/10432. Acesso em: 14 aug. 2026.