I will talk about several uniqueness results for inverse problems. I will first review the classical Calderón problem. Then I will focus on the fractional Calderón problem and its evolutionary and nonlinear variants. The goal is to determine nonlinearities/coefficients in fractional equations from exterior partial measurements of the Dirichlet-to-Neumann map.
We provide new proofs based on the Myers--Steenrod theorem to confirm that travel time data, travel time difference data and the broken scattering relations determine a simple Riemannian metric on a disc up to the natural gauge of a boundary fixing diffeomorphism. Our method of the proof leads to a Lipschitz-type stability estimate for the first two data sets in the class of simple metrics. This is joint work with Joonas Ilmavirta and Teemu Saksala.