Fixed-energy harmonic functions

Abstract

We study the map from conductances to edge energies for harmonic functions on finite graphs with Dirichlet boundary conditions. We prove that for any compatible acyclic orientation and choice of energies there is a unique choice of conductances such that the associated harmonic function realizes those orientations and energies. We call the associated function enharmonic. For rational energies and boundary data the Galois group of ℚtr (the totally real algebraic numbers) over ℚ permutes the enharmonic functions, acting on the set of compatible acyclic orientations. A consequence is the non-tileability of certain polygons by rational-area rectangles. For planar graphs there is an enharmonic conjugate function; together these form the real and imaginary parts of a "fixed energy" analytic function. In the planar scaling limit for ℤ2 (and the fixed south/west orientation), these functions satisfy a nonlinear analog of the Cauchy-Riemann equations, namely uxvy = 1 uyvx = -1: We give an analog of the Riemann mapping theorem for these functions, as well as a variational approach to finding solutions in both the discrete and continuous settings.