We study the Dirichlet problem for p-harmonic functions (and p-energy minimizers) in bounded domains in proper, pathconnected metric measure spaces equipped with a doubling measure and supporting a Poincaré inequality. The Dirichlet problem has previously been solved for Sobolev type boundary data, and we extend this result and solve the problem for all continuous boundary data. We study the regularity of boundary points and prove the Kellogg property, i.e. that the set of irregular boundary points has zero p-capacity.We also construct p-capacitary, p-singular and p-harmonic measures on the boundary. We show that they are all absolutely continuous with respect to the p-capacity. For p ¼ 2 we show that all the boundary measures are comparable and that the singular and harmonic measures coincide. We give an integral representation for the solution to the Dirichlet problem when p ¼ 2, enabling us to extend the solvability of the problem to L 1 boundary data in this case. Moreover, we give a trace result for Newtonian functions when p ¼ 2. Finally, we give an estimate for the Hausdor¤ dimension of the boundary of a bounded domain in Ahlfors Q-regular spaces.
Introduction
Recent developments in analysis on metric spaces include a study ofSobolev type spaces on metric measure spaces; see Hajłasz [10], Hajłasz-Koskela [11], Heinonen-Koskela [14], Cheeger [6], Franchi-Hajłasz-Koskela [7], Kilpeläinen-Kinnunen-Martio [22], Kinnunen-Latvala [23], Kinnunen-Martio [24], [25], Koskela-MacManus [27], Shanmugalingam [35], and the references therein. A construction of a Sobolev type space, called the Newtonian space, was given in [35] and subsequently studied in Shanmugalingam [36], Kinnunen-Shanmugalingam [26], Bjö rn [3], Bjö rn-MacManus-Shanmugalingam [4], Shanmugalingam [34], Holopainen-Shanmugalingam [17], and Kallunki-Shanmugalingam [21]. The Sobolev type space constructed by Cheeger in [6] yields the same space studied in [35], and enables us to use the deep results of [6]. Moreover the Sobolev type spaces introduced by Hajłasz [10] coincide with our spaces in the case when the metric space satisfies our general assumptions. Brought to you by | University of Connecticut Authenticated Download Date | 5/16/15 11:57 PM Brought to you by | University of Connecticut Authenticated Download Date | 5/16/15 11:57 PMExamples of metric measure spaces that have the above-mentioned properties include the Euclidean spaces (with or without weights), complete manifolds with negatively pinched curvature and positive injectivity radius, Carnot groups such as the Heisenberg group as well as other sub-Riemannian manifolds that arise as Gromov-Hausdor¤ limits of certain Riemannian manifolds, the graphs with uniformly bounded geometry studied by Holopainen and Soardi in [18] and [19], as well as the metric spaces constructed by Bourdon-Pajot [5] and by Laakso [28].Acknowledgement.