Conformally plane metrics of nonnegative curvature

Yu. G. Reshetnyak, “Investigation of manifolds of bounded curvature by means of isothermal coordinates,” Izv. Sib. Otd. Akad. Nauk SSSR,10, 15–28 (1959). Yu. G. Reshetnyak, “Isothermal coordinates on manifolds of bounded curvature. I,” Sib. Mat. Zh.,1, No. 1, 88–116 (1960). Yu. G. Reshetnyak, “Isothermal coordinates on manifolds of bounded curvature. II,” Sib. Mat. Zh.,1, No. 2, 248–276 (1960). V. V. Slavskii, “Conformally plane metrics of bounded curvature on an n-dimensional sphere” in: Studies in the Geometry “in the Large” and Mathematical Analysis [in Russian], Vol. 9, Nauka, Novosibirsk (1987), pp. 193–199. V. M. Gol'dshtein and Yu. G. Reshetnyak, Introduction to the Theory of Functions with Generalized Derivatives and Quasiconformal Mappings [in Russian], Nauka, Moscow (1983). W. K. Hayman and P. B. Kennedy, Subharmonic Functions, Academic Press, London (1976). L. Hörmander, The Analysis of Linear Partial Differential Operators. I. Distribution Theory and Fourier Analysis, Springer, Berlin (1983). S. G. Mikhlin, Linear partial Differential Equations [in Russian], Vysshaya Shkola, Moscow (1977). L. V. Ahlfors, Möbius Transformations in Several Dimensions, University of Minnesota, School of Mathematics, Minneapolis, Minn. (1981).