Target Geometry in Prescribed-Value Schwarz Lemmas for Harmonic Maps

Abstract

We study harmonic maps \(F:\D\to G\) into bounded domains in real Hilbert spaces, prescribing \(F(0)\) and \(dF_0(\R^2)\) when \(dF_0\) is nonzero and conformal. We prove a target-independent identity for the Möbius-weighted Hilbert pairing. After normalization, the pairing is affine in \(\|dF_0\|\) with positive slope, yielding an exact equivalence between the derivative extremal problem and a boundary-pairing problem. For a round affine section, this gives a necessary and sufficient integral criterion for the Möbius parametrization to be extremal. A supporting-hyperplane condition guarantees the required integral inequality and characterizes equality. Examples show that roundness alone is insufficient and that the supporting condition is not necessary. For unit balls of real Hilbert spaces of dimension at least two, including infinite-dimensional spaces, we obtain the sharp prescribed-value Schwarz-Pick estimate, all equality cases, and quantitative \(L^2\) boundary stability. We also prove a Cayley-Klein contraction under pointwise distortion. For \(K\geq1\), let \(M_K\) denote the supremum of \(L_F(0)\) over harmonic maps \(F:\D\to\B_H\) satisfying \(F(0)=0\) and \(0<\ell_F(0)\leq L_F(0)\leq K\ell_F(0)\), where \(L_F(0)\) and \(\ell_F(0)\) are the maximal and minimal stretchings at the origin, respectively. We determine \(M_K\), identify the unique optimizing parameter, and characterize all extremals. The function \(K\mapsto M_K\) is strictly increasing, with \(M_1=1\) and \(M_K\to4/π\) as \(K\to\infty\).