On the hidden mechanism behind non-uniqueness for the anisotropic Calderón problem with data on disjoint sets

International audienceWe show that there is generically non-uniqueness for the anisotropic Calderón problem at fixed frequency when the Dirichlet and Neumann data are measured on disjoint sets of the boundary of a given domain. More precisely, we first show that given a smooth compact connected Riemannian manifold with boundary (M, g) of dimension n ≥ 3, there exist in the conformal class of g an infinite number of Riemannian metrics˜gmetrics˜ metrics˜g such that their corresponding DN maps at a fixed frequency coincide when the Dirichlet data ΓD and Neumann data ΓN are measured on disjoint sets and satisfy ΓD ∪ ΓN = ∂M. The conformal factors that lead to these non-uniqueness results for the anisotropic Calderón problem satisfy a nonlinear elliptic PDE of Yamabe type on the original manifold (M, g) and are associated to a natural but subtle gauge invariance of the anisotropic Calderón problem with data on disjoint sets. We then construct a large class of counterexamples to uniqueness in dimension n ≥ 3 to the anisotropic Calderón problem at fixed frequency with data on disjoint sets and modulo this gauge invariance. This class consists in cylindrical Riemannian manifolds with boundary having two ends (meaning that the boundary has two connected components), equipped with a suitably chosen warped product metric