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Conformal mapping for cavity inverse problem: an explicit reconstruction formula

In this paper, we address a classical case of the Calderón (or conductivity) inverse problem in dimension two. We aim to recover the location and the shape of a single cavity $ω$ (with boundary $γ$) contained in a domain $Ω$ (with boundary $Γ$) from the knowledge of the Dirichlet-to-Neumann (DtN) map $Λ_γ: f \longmapsto \partial_n u^f|_Γ$, where $u^f$ is harmonic in $Ω\setminus\overlineω$, $u^f|_Γ=f$ and $u^f|_γ=c^f$, $c^f$ being the constant such that $\int_γ\partial_n u^f\,{\rm d}s=0$. We obtain an explicit formula for the complex coefficients $a_m$ arising in the expression of the Riemann map $z\longmapsto a_1 z + a_0 + \sum_{m\leqslant -1} a_m z^{m}$ that conformally maps the exterior of the unit disk onto the exterior of $ω$. This formula is derived by using two ingredients: a new factorization result of the DtN map and the so-called generalized Pólia-Szegö tensors (GPST) of the cavity. As a byproduct of our analysis, we also prove the analytic dependence of the coefficients $a_m$ with respect to the DtN. Numerical results are provided to illustrate the efficiency and simplicity of the method.