We prove the nonlinear fundamental convergence theorem for superharmonic functions on metric measure spaces. Our proof seems to be new even in the Euclidean setting. The proof uses direct methods in the calculus of variations and, in particular, avoids advanced tools from potential theory. We also provide a new proof for the fact that a Newtonian function has Lebesgue points outside a set of capacity zero, and give a sharp result on when superharmonic functions have -Lebesgue points everywhere.
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Anders Björn, Jana Björn, Mikko Parviainen, Lebesgue points and the fundamental convergence theorem for superharmonic functions on metric spaces. Rev. Mat. Iberoam. 26 (2010), no. 1, pp. 147–174DOI 10.4171/RMI/598