1. Lebesgue, H.: Sur le probléme de Dirichlet. Rend. Cir. Mat. Palermo, 24, 371–402 (1907)
2. Mizuta, Y.: Potential Theory in Euclidean Spaces, Gakkōtosyo, Tokyo, 1996
3. Lectures Notes in Math.;M. Vuorinen,1988
4. Futamura, T., Mizuta, Y.: Lindelöf theorems for monotone Sobolev functions. Ann. Acad. Sci. Fenn. Math., 28, 271–277 (2003)
5. Mizuta, Y.: Tangential limits of monotone Sobolev functions. Ann. Acad. Sci. Fenn. Ser. A. I. Math., 20, 315–326 (1995)