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Published online by Cambridge University Press: 18 May 2009
Let R be a hyperbolic Riemann surface and W an open subset of R with ∂W piecewise analytic. Denote by the space of Dirichlet finite Tonelli functions on R and by π the harmonic projection of
. Consider the relative HD–class on W, HD(W;∂W) = {u∈
│ u │ W∈HD(W) and u │ R\W = 0}. The extremization operation μ
is the linear mapping of HD(W;∂W) into HD(R) defined by μ
. Since π preserves values of functions at the Royden harmonic boundary, the maximum principle implies that μ
is an order preserving injection and that Mμ
is an isometry with respect to the supremum norms.