M2ASAI Research group (UPM)

arXiv SIAM Journal on Applied Mathematics We introduce a new iterative method to recover a real compact supported potential of the Schödinger operator from their fixed angle scattering data. The method combines a fixed point argument with a suitable approximation of the resolvent of the Schödinger operator by partial sums associated to its Born series. The main interest is that, unlike other iterative methods in the literature, each iteration is explicit (and therefore faster computationally) and a rigorous analytical result on the convergence of the iterations is proved. This result requires potentials with small norm in certain Sobolev spaces. As an application we show some numerical experiments that illustrate this convergence.

arXiv Siam Journal on Mathematical Analysis We prove that in dimension $n \ge 2$ the main singularities of a complex potential $q$ having a certain a priori regularity are contained in the Born approximation $q_\theta$ constructed from fixed angle scattering data. Moreover, ${q-q_\theta}$ can be up to one derivative more regular than $q$ in the Sobolev scale. In fact, this result is optimal. We construct a family of compactly supported and radial potentials for which it is not possible to have more than one derivative gain. Also, these functions show that for $n>3$, the maximum derivative gain can be very small for potentials in the Sobolev scale not having a certain a priori level of regularity which grows with the dimension.

arXiv Discrete and Continuous Dinamycal Systems We present a new strategy for proving the Ambrose conjecture, a global version of the Cartan local lemma. We introduce the concepts of linking curves, unequivocal sets and sutured manifolds, and show that any sutured manifold satisfies the Ambrose conjecture. We then prove that the set of sutured Riemannian manifolds contains a residual set of the metrics on a given smooth manifold of dimension 3.

arXiv American Journal of Mathematics The notion of the magnitude of a metric space was introduced by Leinster in [8] and developed in [10], [9], [11] and [16], but the magnitudes of familiar sets in Euclidean space are only understood in relatively few cases. In this paper we study the magnitudes of compact sets in Euclidean spaces. We first describe the asymptotics of the magnitude of such sets in both the small and large-scale regimes. We then consider the magnitudes of compact convex sets with nonempty interior in Euclidean spaces of odd dimension, and relate them to the boundary behaviour of solutions to certain naturally associated higher order elliptic boundary value problems in exterior domains. We carry out calculations leading to an algorithm for explicit evaluation of the magnitudes of balls, and this establishes the convex magnitude conjecture of Leinster and Willerton [9] in the special case of balls in dimension three. In general we show that the magnitude of an odd-dimensional ball is a rational function of its radius. In addition to Fourier-analytic and PDE techniques, the arguments also involve some combinatorial considerations.

arXiv Analysis and PDE We study the regularity of stationary and time-dependent solutions to strong perturbations of the free Schrödinger equation on two-dimensional flat tori. This is achieved by performing a second microlocalization related to the size of the perturbation and by analysing concentration and nonconcentration properties at this new scale. In particular, we show that sufficiently accurate quasimodes can only concentrate on the set of critical points of the average of the potential along geodesics.

arXiv SIAM Journal on Mathematical Analysis We study the inverse boundary value problem and the inverse scattering problem for a homogeneous and isotropic linear elastic medium in $\mathbb{R}^3$ with an inhomogeneous and anisotropic mass density given by an unknown symmetric matrix. We define the displacement to traction map associated to the boundary value problem and we prove the uniqueness of the mass density from the knowledge of this map. To do this we construct solutions to the corresponding conjugate Faddeev type equation and derive the existence of complex waves solutions to the Lamé equation, the analogue to complex geometrical optics solutions in the inverse conductivity problem. We also prove the uniqueness of the mass density in the inverse scattering problem by reducing this problem to an inverse boundary value problem.