1. S. L. Krushkal', Quasiconformal Mappings and Riemann Surfaces [in Russian], Nauka, Novosibirsk (1975).
2. R. S. Hamilton, ?Extremal quasiconformal mappings with prescribed boundary values,? Trans. Am. Math. Soc.,138, 399?406 (1969).
3. E. Reich and K. Strebel, ?Extremal plane quasiconformal mappings with given boundary values,? Bull. Am. Math. Soc.,79, 488?490 (1973).
4. V. G. Sheretov, ?On the theory of extremal quasiconformal mappings,? Mat. Sb.,107, No. 1, 146?158 (1978).
5. V. G. Sheretov, ?Criteria for extremality in a problem for quasiconformal mappings,? Mat. Zametki,39, No. 1, 14?23 (1986).