Piecewise Harmonic Subharmonic Functions and Positive Cauchy Transforms

We give a local characterization of the class of functions having positive distributional derivative with respect to z̄ that are almost everywhere equal to one of finitely many analytic functions and satisfy some mild nondegeneracy assumptions. As a consequence, we show that any subharmonic piecewise harmonic function satisfying these conditions locally coincides with the maximum of finitely many harmonic functions and we describe the topology of their level curves. These results are valid in a quite general setting as they assume no à priori conditions on the differentiable structure of the support of the associated Riesz measures. We also discuss applications to positive Cauchy transforms and we consider several examples and related problems.