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Regularity results for a nonlinear elliptic-parabolic system with oscillating coefficients

In this paper we study the initial boundary value problem for the system $\mbox{div}(σ(u)\nablaφ)=0,\ \ u_t-Δu=σ(u)|\nablaφ|^2$. This problem is known as the thermistor problem which models the electrical heating of conductors. Our assumptions on $σ(u)$ leave open the possibility that $\liminf_{u\rightarrow\infty}σ(u)=0$, while $\limsup_{u\rightarrow\infty}σ(u)$ is large. This means that $σ(u)$ can oscillate wildly between $0$ and a large positive number as $u\rightarrow \infty$. Thus our degeneracy is fundamentally different from the one that is present in porous medium type of equations. We obtain a weak solution $(u, φ)$ with $|\nabla φ|, |\nabla u|\in L^\infty$ by first establishing a uniform upper bound for $e^{\varepsilon u}$ for some small $\varepsilon$. This leads to an inequality in $\nablaφ$, from whence follows the regularity result. This approach enables us to avoid first proving the Hölder continuity of $φ$ in the space variables, which would have required that the elliptic coefficient $σ(u)$ be an $A_2$ weight. As it is known, the latter implies that $\lnσ(u)$ is "nearly bounded".