Gradient integrability and rigidity results for two-phase conductivities in dimension two

This paper deals with higher gradient integrability for $σ$-harmonic functions $u$ with discontinuous coefficients $σ$, i.e. weak solutions of $÷(σ\nabla u) = 0$. We focus on two-phase conductivities, and study the higher integrability of the corresponding gradient field $|\nabla u|$. The gradient field and its integrability clearly depend on the geometry, i.e., on the phases arrangement. We find the optimal integrability exponent of the gradient field corresponding to any pair $\{σ_1,σ_2\}$ of positive definite matrices, i.e., the worst among all possible microgeometries. We also show that it is attained by so-called exact solutions of the corresponding PDE. Furthermore, among all two-phase conductivities with fixed ellipticity, we characterize those that correspond to the worse integrability.

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