1. L. Ahlfors,Lectures on Quasiconformal Mappings, Van Nostrand, London, 1966.
2. G. David,Solutions de l’équation de Beltrami avec ‖Μ‖∞ = 1, Ann. Acad. Sci. Fenn. Ser. A I Math.13 (1988), 25–70.
3. O. Lehto and K. Virtanen,Quasiconformal Mappings in the Plane, Springer-Verlag, Berlin, 1973.
4. Lecture Notes in Math.;O. Lehto,1970
5. O. Lehto,Remarks on generalized Beltrami equations and conformai mappings, Proceedings of the Romanian-Finnish Seminar on Teichmüller Spaces and Quasiconformal Mappings, Brasov, Romania, 1969, Publishing House of the Academy of the Socialist Republic of Romania, Bucharest, 1971, pp. 203–214.