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Matti Lassas

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math.AP math-ph math.MP math.DG math.PR Machine Learning math.NA math.OC math.FA math.ST Statistics Theory eess.IV math.SP physics.optics math.GT Methodology Numerical Analysis

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preprint2026arXiv

Denoising data using convex relaxations

We study the problem of denoising observations $Y_i=X_i+Z_i$ , where the latent variables $X_i$ are sampled from a low-dimensional manifold in $\mathbb{R}^n$ and the noise variables $Z_i$ are isotropic Gaussian. We propose a convex-relaxation estimator that first reduces dimension by principal component analysis and then projects the observations onto the convex hull of the projected latent manifold. We construct a statistical oracle that estimates its supporting hyperplanes from empirical Gaussian tail probabilities of the noisy sample. Under a lower-mass condition on the latent distribution, we prove finite-sample guarantees for the oracle and derive error bounds for the resulting denoiser. The analysis combines risk bounds for least-squares projection under convex constraints with entropy bounds for convex hulls. We also verify the assumptions of the framework for a Cryo-Electron Microscopy observation model by establishing suitable covering number and Lipschitz estimates for the associated group action and imaging operators.

preprint2026arXiv

Function graph transformers universally approximate operators between function spaces

We study the approximation of nonlinear operators between function spaces by transformers. Our approach is to lift functions to measures supported on their graphs and leverage a recently introduced measure-theoretic view of transformers. A function $h$ is represented by its graph measure $γ_h$ , with finite tokens $\{(x_j,h(x_j))\}_{j=1}^N$ being its empirical approximations. We show that this framework elegantly models discretization refinement via convergence of measures and provides a natural setting for operator learning. Within this framework, we introduce function graph transformers, a graph-preserving subclass of measure-theoretic transformers that maps graph measures to graph measures, which is to say that outputs remain single-valued functions. Crucially, this additional structure does not reduce generality: we prove that the resulting graph-preserving maps can be approximated by finite compositions of standard softmax self-attention layers and pointwise MLPs, yielding universal approximation results for broad classes of nonlinear operators. Unlike existing theoretical approaches to operator learning with transformers, the measure-theoretic framework also accommodates regularized negative-order Sobolev inputs for which discretization invariance is particularly challenging, as well as query points on different output domains. Overall, function graph transformers provide a continuum viewpoint and mathematical toolkit for transformer-based operator learning, clarifying the roles of positional encodings, graph structure, regularization, and ensuring consistency across discretizations.

preprint2026arXiv

Lens rigidity in 2D: The reconstruction of a Riemann surface from its geodesic lengths

We address the question of whether a Riemannian manifold-with-boundary (M,g) in dimension two is uniquely determined from knowledge of the distances between points on its boundary. An affirmative answer is called boundary rigidity for (M,g); it is closely related to lens rigidity. The latter question originates in the problem of reconstructing the speed of sound in an unknown medium from measurements of the travel time of sound waves that are sent in and ultimately return to the boundary. We prove essentially optimal results on these rigidity questions: Our first result answers proves rigidity locally, near a convex portion of the boundary. Our second result proves rigidity globally, for manifolds with convex boundary, in the absence of trapping (closed geodesics), thus confirming a conjecture of Uhlmann. Our final result proves the optimal reconstruction for convex boundaries even in the presence of trapping, showing rigidity up to outermost trapped geodesics. Our results thus extend the classical work of Pestov and Uhlmann on rigidity of simple 2-manifolds, as well as the many prior results on injectivity of the X-ray transform, which address linearized versions of the rigidity problem. \par Our method is to treat the (non-linear) rigidity problem directly, where we simultaneously re-cast the lens data as generalized Riemannian circles, and obtain rigidity for these ``pseudo-circles'', by studying a system of equations that we show these objects must satisfy. The rigidity we obtain ultimately is proven via {novel} estimates that are reminiscent of energy-type estimates for hyperbolic equations.

preprint2024arXiv

On the lack of external response of a nonlinear medium in the second-harmonic generation process

This paper concerns the scattering problem for a nonlinear medium of compact support, $D$ , with second-harmonic generation. Such a medium, when probed with monochromatic light beams at frequency $ω$ , generates additional waves at frequency $2ω$ . The response of the medium is governed by a system of two coupled semilinear partial differential equations for the electric fields at frequency $ω$ and $2ω$ . We investigate whether there are situations in which the generated $2ω$ wave is localized inside $D$ , that is, the nonlinear interaction of the medium with the probing wave is invisible to an outside observer. This leads to the analysis of a semilinear elliptic system formulated in $D$ with non-standard boundary conditions. The analysis presented here sets up a mathematical framework needed to investigate a multitude of questions related to nonlinear scattering with second-harmonic generation.

preprint2023arXiv

Inverse problems for discrete heat equations and random walks for a class of graphs

We study the inverse problem of determining a finite weighted graph $(X,E)$ from the source-to-solution map on a vertex subset $B\subset X$ for heat equations on graphs, where the time variable can be either discrete or continuous. We prove that this problem is equivalent to the discrete version of the inverse interior spectral problem, provided that there does not exist a nonzero eigenfunction of the weighted graph Laplacian vanishing identically on $B$ . In particular, we consider inverse problems for discrete-time random walks on finite graphs. We show that under a novel geometric condition (called the Two-Points Condition), the graph structure and the transition matrix of the random walk can be uniquely recovered from the distributions of the first passing times on $B$ , or from the observation on $B$ of one realization of the random walk.

preprint2022arXiv

An inverse problem for a semi-linear wave equation: a numerical study

We consider an inverse problem of recovering a potential associated to a semi-linear wave equation with a quadratic nonlinearity in $1 + 1$ dimensions. We develop a numerical scheme to determine the potential from a noisy Dirichlet-to-Neumann map on the lateral boundary. The scheme is based on the recent higher order linearization method [20]. We also present an approach to numerically estimating two-dimensional derivatives of noisy data via Tikhonov regularization. The methods are tested using synthetic noisy measurements of the Dirichlet-to-Neumann map. Various examples of reconstructions of the potential functions are given.

preprint2022arXiv

An inverse problem for the Riemannian minimal surface equation

In this paper we consider determining a minimal surface embedded in a Riemannian manifold $Σ\times \mathbb{R}$ . We show that if $Σ$ is a two dimensional Riemannian manifold with boundary, then the knowledge of the associated Dirichlet-to-Neumann map for the minimal surface equation determine $Σ$ up to an isometry.

preprint2022arXiv

Deep learning architectures for nonlinear operator functions and nonlinear inverse problems

We develop a theoretical analysis for special neural network architectures, termed operator recurrent neural networks, for approximating nonlinear functions whose inputs are linear operators. Such functions commonly arise in solution algorithms for inverse boundary value problems. Traditional neural networks treat input data as vectors, and thus they do not effectively capture the multiplicative structure associated with the linear operators that correspond to the data in such inverse problems. We therefore introduce a new family that resembles a standard neural network architecture, but where the input data acts multiplicatively on vectors. Motivated by compact operators appearing in boundary control and the analysis of inverse boundary value problems for the wave equation, we promote structure and sparsity in selected weight matrices in the network. After describing this architecture, we study its representation properties as well as its approximation properties. We furthermore show that an explicit regularization can be introduced that can be derived from the mathematical analysis of the mentioned inverse problems, and which leads to certain guarantees on the generalization properties. We observe that the sparsity of the weight matrices improves the generalization estimates. Lastly, we discuss how operator recurrent networks can be viewed as a deep learning analogue to deterministic algorithms such as boundary control for reconstructing the unknown wavespeed in the acoustic wave equation from boundary measurements.

preprint2022arXiv

Fitting a manifold of large reach to noisy data

Let ${\mathcal M}\subset {\mathbb R}^n$ be a $C^2$ -smooth compact submanifold of dimension $d$ . Assume that the volume of ${\mathcal M}$ is at most $V$ and the reach (i.e. the normal injectivity radius) of ${\mathcal M}$ is greater than $τ$ . Moreover, let $μ$ be a probability measure on ${\mathcal M}$ whose density on ${\mathcal M}$ is a strictly positive Lipschitz-smooth function. Let $x_j\in {\mathcal M}$ , $j=1,2,\dots,N$ be $N$ independent random samples from distribution $μ$ . Also, let $ξ_j$ , $j=1,2,\dots, N$ be independent random samples from a Gaussian random variable in ${\mathbb R}^n$ having covariance $σ^2I$ , where $σ$ is less than a certain specified function of $d, V$ and $τ$ . We assume that we are given the data points $y_j=x_j+ξ_j,$ $j=1,2,\dots,N$ , modelling random points of ${\mathcal M}$ with measurement noise. We develop an algorithm which produces from these data, with high probability, a $d$ dimensional submanifold ${\mathcal M}_o\subset {\mathbb R}^n$ whose Hausdorff distance to ${\mathcal M}$ is less than $Cdσ^2/τ$ and whose reach is greater than $cτ/d^6$ with universal constants $C,c > 0$ . The number $N$ of random samples required depends almost linearly on $n$ , polynomially on $σ^{-1}$ and exponentially on $d$ .

preprint2022arXiv

Inverse problems for locally perturbed lattices -- Discrete Hamiltonian and quantum graph

We consider the inverse scattering problems for two types of Schrödinger operators on locally perturbed periodic lattices. For the discrete Hamiltonian, the knowledge of the S-matrix for all energies determines the graph structure and the coefficients of the Hamiltonian. For locally perturbed equilateral metric graphs, the knowledge of the S-matrix for all energies determines the graph structure.

preprint2022arXiv

Learning a microlocal prior for limited-angle tomography

Limited-angle tomography is a highly ill-posed linear inverse problem. It arises in many applications, such as digital breast tomosynthesis. Reconstructions from limited-angle data typically suffer from severe stretching of features along the central direction of projections, leading to poor separation between slices perpendicular to the central direction. A new method is introduced, based on machine learning and geometry, producing an estimate for interfaces between regions of different X-ray attenuation. The estimate can be presented on top of the reconstruction, indicating more reliably the true form and extent of features. The method uses directional edge detection, implemented using complex wavelets and enhanced with morphological operations. By using machine learning, the visible part of the wavefront set is first extracted and then extended to the full domain, filling in the parts of the wavefront set that would otherwise be hidden due to the lack of measurement directions.

preprint2022arXiv

Universal Joint Approximation of Manifolds and Densities by Simple Injective Flows

We study approximation of probability measures supported on $n$ -dimensional manifolds embedded in $\mathbb{R}^m$ by injective flows -- neural networks composed of invertible flows and injective layers. We show that in general, injective flows between $\mathbb{R}^n$ and $\mathbb{R}^m$ universally approximate measures supported on images of extendable embeddings, which are a subset of standard embeddings: when the embedding dimension m is small, topological obstructions may preclude certain manifolds as admissible targets. When the embedding dimension is sufficiently large, $m \ge 3n+1$ , we use an argument from algebraic topology known as the clean trick to prove that the topological obstructions vanish and injective flows universally approximate any differentiable embedding. Along the way we show that the studied injective flows admit efficient projections on the range, and that their optimality can be established "in reverse," resolving a conjecture made in Brehmer and Cranmer 2020.

preprint2021arXiv

A foliated and reversible Finsler manifold is determined by its broken scattering relation

The broken scattering relation consists of the total lengths of broken geodesics that start from the boundary, change direction once inside the manifold, and propagate to the boundary. We show that if two reversible Finsler manifolds satisfying a convex foliation condition have the same broken scattering relation, then they are isometric. This implies that some anisotropic material parameters of the Earth can be in principle reconstructed from single scattering measurements at the surface.

preprint2021arXiv

Random tree Besov priors -- Towards fractal imaging

We propose alternatives to Bayesian a priori distributions that are frequently used in the study of inverse problems. Our aim is to construct priors that have similar good edge-preserving properties as total variation or Mumford-Shah priors but correspond to well defined infinite-dimensional random variables, and can be approximated by finite-dimensional random variables. We introduce a new wavelet-based model, where the non zero coefficient are chosen in a systematic way so that prior draws have certain fractal behaviour. We show that realisations of this new prior take values in some Besov spaces and have singularities only on a small set $τ$ that has a certain Hausdorff dimension. We also introduce an efficient algorithm for calculating the MAP estimator, arising from the the new prior, in denoising problem.

preprint2021arXiv

X-Ray Transform in Asymptotically Conic Spaces

In this article, we study the properties of the geodesic X-ray transform for asymptotically Euclidean or conic Riemannian metrics and show injectivity under non-trapping and no conjugate point assumptions. We also define a notion of lens data for such metrics and study the associated inverse problem.

preprint2020arXiv

Construction of artificial point sources for a linear wave equation in unknown medium

We study the wave equation on a bounded domain of $\mathbb R^m$ and on a compact Riemannian manifold $M$ with boundary. We assume that the coefficients of the wave equation are unknown but that we are given the hyperbolic Neumann-to-Dirichlet map $Λ$ that corresponds to the physical measurements on the boundary. Using the knowledge of $Λ$ we construct a sequence of Neumann boundary values so that at a time $T$ the corresponding waves converge to zero while the time derivative of the waves converge to a delta distribution. Such waves are called an artificial point source. The convergence of the wave takes place in the function spaces naturally related to the energy of the wave. We apply the results for inverse problems and demonstrate the focusing of the waves numerically in the 1-dimensional case.

preprint2020arXiv

Deep neural networks for inverse problems with pseudodifferential operators: an application to limited-angle tomography

We propose a novel convolutional neural network (CNN), called $Ψ$ DONet, designed for learning pseudodifferential operators ( $Ψ$ DOs) in the context of linear inverse problems. Our starting point is the Iterative Soft Thresholding Algorithm (ISTA), a well-known algorithm to solve sparsity-promoting minimization problems. We show that, under rather general assumptions on the forward operator, the unfolded iterations of ISTA can be interpreted as the successive layers of a CNN, which in turn provides fairly general network architectures that, for a specific choice of the parameters involved, allow to reproduce ISTA, or a perturbation of ISTA for which we can bound the coefficients of the filters. Our case study is the limited-angle X-ray transform and its application to limited-angle computed tomography (LA-CT). In particular, we prove that, in the case of LA-CT, the operations of upscaling, downscaling and convolution, which characterize our $Ψ$ DONet and most deep learning schemes, can be exactly determined by combining the convolutional nature of the limited angle X-ray transform and basic properties defining an orthogonal wavelet system. We test two different implementations of $Ψ$ DONet on simulated data from limited-angle geometry, generated from the ellipse data set. Both implementations provide equally good and noteworthy preliminary results, showing the potential of the approach we propose and paving the way to applying the same idea to other convolutional operators which are $Ψ$ DOs or Fourier integral operators.

preprint2020arXiv

Inverse scattering on non-compact manifolds with general metric

The problems we address in this paper are the spectral theory and the inverse problems associated with Laplacians on non-compact Riemannian manifolds and more general manifolds admitting conic singularities. In particular, we study the inverse scattering problem where one observes the asymptotic behavior of the solutions of the Helmholtz equation on the manifold. These observations are analogous to Heisenberg's scattering matrix in quantum mechanics. We then show that the knowledge of the scattering matrix determines the topology and the metric of the manifold. In the paper we develop a unified approach to consider scattering problems on manifolds that can have very different type of infinities, such as regular hyperbolic ends, cusps, and cylindrical ends related to models encountered in the study of wave guides. We allow the manifold to have also conic singularities. Due to this, the studied class of manifolds include orbifolds. Such non-smooth structures arise in the study of the stability of inverse problems and of the geometrical collapse.

preprint2020arXiv

Uniqueness and stability of an inverse problem for a semi-linear wave equation

We consider the recovery of a potential associated with a semi-linear wave equation on $\mathbb{R}^{n+1}$ , $n\geq 1$ . We show a Hölder stability estimate for the recovery of an unknown potential $a$ of the wave equation $\square u +a u^m=0$ from its Dirichlet-to-Neumann map. We show that an unknown potential $a(x,t)$ , supported in $Ω\times[t_1,t_2]$ , of the wave equation $\square u +a u^m=0$ can be recovered in a Hölder stable way from the map $u|_{\partial Ω\times [0,T]}\mapsto \langleψ,\partial_νu|_{\partial Ω\times [0,T]}\rangle_{L^2(\partial Ω\times [0,T])}$ . This data is equivalent to the inner product of the Dirichlet-to-Neumann map with a measurement function $ψ$ . We also prove similar stability result for the recovery of $a$ when there is noise added to the boundary data. The method we use is constructive and it is based on the higher order linearization. As a consequence, we also get a uniqueness result. We also give a detailed presentation of the forward problem for the equation $\square u +a u^m=0$ .

preprint2019arXiv

Reconstruction and stability in Gel'fand's inverse interior spectral problem

Assume that $M$ is a compact Riemannian manifold of bounded geometry given by restrictions on its diameter, Ricci curvature and injectivity radius. Assume we are given, with some error, the first eigenvalues of the Laplacian $Δ_g$ on $M$ as well as the corresponding eigenfunctions restricted on an open set in $M$ . We then construct a stable approximation to the manifold $(M,g)$ . Namely, we construct a metric space and a Riemannian manifold which differ, in a proper sense, just a little from $M$ when the above data are given with a small error. We give an explicit $\log\log$ -type stability estimate on how the constructed manifold and the metric on it depend on the errors in the given data. Moreover a similar stability estimate is derived for the Gel'fand's inverse problem. The proof is based on methods from geometric convergence, a quantitative stability estimate for the unique continuation and a new version of the geometric Boundary Control method.

preprint2019arXiv

The Light Ray transform on Lorentzian manifolds

We study the weighted light ray transform $L$ of integrating functions on a Lorentzian manifold over lightlike geodesics. We analyze $L$ as a Fourier Integral Operator and show that if there are no conjugate points, one can recover the spacelike singularities of a function $f$ from its the weighted light ray transform $Lf$ by a suitable filtered back-projection.

preprint2016arXiv

Analysis of regularized inversion of data corrupted by white Gaussian noise

Tikhonov regularization is studied in the case of linear pseudodifferential operator as the forward map and additive white Gaussian noise as the measurement error. The measurement model for an unknown function $u(x)$ is \begin{eqnarray*} m(x) = Au(x) + δ\hspace{.2mm}\varepsilon(x), \end{eqnarray*} where $δ>0$ is the noise magnitude. If $\varepsilon$ was an $L^2$ -function, Tikhonov regularization gives an estimate \begin{eqnarray*} T_α(m) = \text{argmin}_{u\in H^r}\big\{\|A u-m\|_{L^2}^2+ α\|u\|_{H^r}^2 \big\}\end{eqnarray*} for $u$ where $α=α(δ)$ is the regularization parameter. Here penalization of the Sobolev norm $\|u\|_{H^r}$ covers the cases of standard Tikhonov regularization ( $r=0$ ) and first derivative penalty ( $r=1$ ). Realizations of white Gaussian noise are almost never in $L^2$ , but do belong to $H^s$ with probability one if $s<0$ is small enough. A modification of Tikhonov regularization theory is presented, covering the case of white Gaussian measurement noise. Furthermore, the convergence of regularized reconstructions to the correct solution as $δ\rightarrow 0$ is proven in appropriate function spaces using microlocal analysis. The convergence of the related finite-dimensional problems to the infinite-dimensional problem is also analysed.

preprint2016arXiv

Correlation based passive imaging with a white noise source

Passive imaging refers to problems where waves generated by unknown sources are recorded and used to image the medium through which they travel. The sources are typically modelled as a random variable and it is assumed that some statistical information is available. In this paper we study the stochastic wave equation $\partial_t^2 u - Δ_g u = χW$ , where $W$ is a random variable with the white noise statistics on ${\mathbb R}^{1+n}$ , $n \ge 3$ , $χ$ is a smooth function vanishing for negative times and outside a compact set in space, and $Δ_g$ is the Laplace-Beltrami operator associated to a smooth non-trapping Riemannian metric tensor $g$ on ${\mathbb R}^n$ . The metric tensor $g$ models the medium to be imaged, and we assume that it coincides with the Euclidean metric outside a compact set. We consider the empirical correlations on an open set $\mathcal X \subset {\mathbb R}^n$ , $C_T(t_1, x_1, t_2, x_2) = \frac 1 T \int_0^T u(t_1+s,x_1) u(t_2+s,x_2) ds, \quad t_1,t_2>0,\ x_1,x_2\in \mathcal X,$ for $T>0$ . Supposing that $χ$ is non-zero on $\mathcal X$ and constant in time after $t > 1$ , we show that in the limit $T \to \infty$ , the data $C_T$ becomes statistically stable, that is, independent of the realization of $W$ . Our main result is that, with probability one, this limit determines the Riemannian manifold $({\mathbb R}^n,g)$ up to an isometry. To our knowledge, this is the first result showing that a medium can be determined in a passive imaging setting, without assuming a separation of scales.

preprint2016arXiv

Inverse problems for semilinear wave equations on Lorentzian manifolds

We consider inverse problems in space-time $(M, g)$ , a $4$ -dimensional Lorentzian manifold. For semilinear wave equations $\square_g u + H(x, u) = f$ , where $\square_g$ denotes the usual Laplace-Beltrami operator, we prove that the source-to-solution map $L: f \rightarrow u|_V$ , where $V$ is a neighborhood of a time-like geodesic $μ$ , determines the topological, differentiable structure and the conformal class of the metric of the space-time in the maximal set where waves can propagate from $μ$ and return back. Moreover, on a given space-time $(M, g)$ , the source-to-solution map determines some coefficients of the Taylor expansion of $H$ in $u$ .

preprint2016arXiv

Inverse scattering for a random potential

In this paper we consider an inverse problem for the $n$ -dimensional random Schrödinger equation $(Δ-q+k^2)u = 0$ . We study the scattering of plane waves in the presence of a potential $q$ which is assumed to be a Gaussian random function such that its covariance is described by a pseudodifferential operator. Our main result is as follows: given the backscattered far field, obtained from a single realization of the random potential $q$ , we uniquely determine the principal symbol of the covariance operator of $q$ . Especially, for $n=3$ this result is obtained for the full non-linear inverse backscattering problem. Finally, we present a physical scaling regime where the method is of practical importance.

preprint2016arXiv

Posterior consistency and convergence rates for Bayesian inversion with hypoelliptic operators

Bayesian approach to inverse problems is studied in the case where the forward map is a linear hypoelliptic pseudodifferential operator and measurement error is additive white Gaussian noise. The measurement model for an unknown Gaussian random variable $U(x,ω)$ is \begin{eqnarray*} M(y,ω) = A(U(x,ω) )+ δ\hspace{.2mm}\mathcal{E}(y,ω), \end{eqnarray*} where $A$ is a finitely many times smoothing linear hypoelliptic operator and $δ>0$ is the noise magnitude. The covariance operator $C_U$ of $U$ is $2r$ times smoothing, self-adjoint, injective and elliptic pseudodifferential operator. If $\mathcal{E}$ was taking values in $L^2$ then in Gaussian case solving the conditional mean (and maximum a posteriori) estimate is linked to solving the minimisation problem \begin{eqnarray*} T_δ(M) = \text{argmin}_{u\in H^r} \big\{\|A u-m\|_{L^2}^2+ δ^2\|C_U^{-1/2}u\|_{L^2}^2 \big\}. \end{eqnarray*} However, Gaussian white noise does not take values in $L^2$ but in $H^{-s}$ where $s>0$ is big enough. A modification of the above approach to solve the inverse problem is presented, covering the case of white Gaussian measurement noise. Furthermore, the convergence of conditional mean estimate to the correct solution as $δ\rightarrow 0$ is proven in appropriate function spaces using microlocal analysis. Also the contraction of the confidence regions is studied.

preprint2016arXiv

The Calderón problem for the conformal Laplacian

We consider a conformally invariant version of the Calderón problem, where the objective is to determine the conformal class of a Riemannian manifold with boundary from the Dirichlet-to-Neumann map for the conformal Laplacian. The main result states that a locally conformally real-analytic manifold in dimensions $\geq 3$ can be determined in this way, giving a positive answer to an earlier conjecture by Lassas and Uhlmann (2001). The proof proceeds as in the standard Calderón problem on a real-analytic Riemannian manifold, but new features appear due to the conformal structure. In particular, we introduce a new coordinate system that replaces harmonic coordinates when determining the conformal class in a neighborhood of the boundary.

preprint2015arXiv

A Direct Reconstruction Method for Anisotropic Electrical Impedance Tomography

A novel computational, non-iterative and noise-robust reconstruction method is introduced for the planar anisotropic inverse conductivity problem. The method is based on bypassing the unstable step of the reconstruction of the values of the isothermal coordinates on the boundary of the domain. Non-uniqueness of the inverse problem is dealt with by recovering the unique isotropic conductivity that can be achieved as a deformation of the measured anisotropic conductivity by \emph{isothermal coordinates}. The method shows how isotropic D-bar reconstruction methods have produced reasonable and informative reconstructions even when used on EIT data known to come from anisotropic media, and when the boundary shape is not known precisely. Furthermore, the results pave the way for regularized anisotropic EIT. Key aspects of the approach involve D-bar methods and inverse scattering theory, complex geometrical optics solutions, and quasi-conformal mapping techniques.

preprint2015arXiv

Determination of the Spacetime from Local Time Measurements

We consider an inverse problem for a Lorentzian spacetime $(M,g)$ , and show that time measurements, that is, the knowledge of the Lorentzian time separation function on a submanifold $Σ\subset M$ determine the $C^\infty$ -jet of the metric in the Fermi coordinates associated to $Σ$ . We use this result to study the global determination of the spacetime $(M,g)$ when it has a real-analytic structure or is stationary and satisfies the Einstein-scalar field equations. In addition to this, we require that $(M,g)$ is geodesically complete modulo scalar curvature singularities. The results are Lorentzian counterparts of extensively studied inverse problems in Riemannian geometry - the determination of the jet of the metric and the boundary rigidity problem. We give also counterexamples in cases when the assumptions are not valid, and discuss inverse problems in general relativity.

preprint2015arXiv

Multi-resolution parameter choice method for total variation regularized tomography

A computational method is introduced for choosing the regularization parameter for total variation (TV) regularization. The approach is based on computing reconstructions at a few different resolutions and various values of regularization parameter. The chosen parameter is the smallest one resulting in approximately discretization-invariant TV norms of the reconstructions. The method is tested with X-ray tomography data measured from a walnut and compared to the S-curve method. The proposed method seems to automatically adapt to the desired resolution and noise level, and it yields useful results in the tests. The results are comparable to those of the S-curve method; however, the S-curve method needs a priori information about the sparsity of the unknown, while the proposed method does not need any a priori information (apart from the choice of a desired resolution). Mathematical analysis is presented for (partial) understanding of the properties of the proposed parameter choice method. It is rigorously proven that the TV norms of the reconstructions converge with any choice of regularization parameter.

preprint2015arXiv

On Absence and Existence of the Anomalous Localized Resonance without the Quasi-static Approximation

The paper considers the transmission problems for Helmholtz equation with bodies that have negative material parameters. Such material parameters are used to model metals on optical frequencies and so-called metamaterials. As the absorption of the materials in the model tends to zero the fields may blow up. When the speed of the blow up is suitable, this is called the Anomalous Localized Reconance (ALR). In this paper we study this phenomenon and formulate a new condition, the weak Anomalous Localized Reconance (w-ALR), where the speed of the blow up of fields may be slower. Using this concept, we can study the blow up of fields in the presence of negative material parameters without the commonly used quasi-static approximation. We give simple geometric conditions under which w-ALR or ALR may, or may not appear. In particular, we show that in a case of a curved layer of negative material with a strictly convex boundary neither ALR nor w-ALR appears with non-zero frequencies (i.e. in the dynamic range) in dimensions $d\ge 3$ . In the case when the boundary of the negative material contains a flat subset we show that the w-ALR always happens with some point sources in dimensions $d\ge 2$ . These results, together with the earlier results of Milton et al. ( [22, 23]) and Ammari et al. ([2]) show that for strictly convex bodies ALR may appear only for bodies so small that the quasi-static approximation is realistic. This gives limits for size of the objects for which invisibility cloaking methods based on ALR may be used.

preprint2015arXiv

On the inverse problem of finding cosmic strings and other topological defects

We consider how microlocal methods developed for tomographic problems can be used to detect singularities of the Lorentzian metric of the Universe using measurements of the Cosmic Microwave Background radiation. The physical model we study is mathematically rigorous but highly idealized.

preprint2015arXiv

Positive-energy D-bar method for acoustic tomography: a computational study

A new computational method for reconstructing a potential from the Dirichlet-to-Neumann map at positive energy is developed. The method is based on D-bar techniques and it works in absence of exceptional points -- in particular, if the potential is small enough compared to the energy. Numerical tests reveal exceptional points for perturbed, radial potentials. Reconstructions for several potentials are computed using simulated Dirichlet-to-Neumann maps with and without added noise. The new reconstruction method is shown to work well for energy values between $10^{-5}$ and $5$ , smaller values giving better results.

preprint2015arXiv

Regularization strategy for inverse problem for 1+1 dimensional wave equation

An inverse boundary value problem for a 1+1 dimensional wave equation with wave speed $c(x)$ is considered. We give a regularisation strategy for inverting the map $\mathcal A:c\mapsto Λ,$ where $Λ$ is the hyperbolic Neumann-to-Dirichlet map corresponding to the wave speed $c$ . More precisely, we consider the case when we are given a perturbation of the Neumann-to-Dirichlet map $\tilde Λ=Λ+\mathcal E$ , where $\mathcal E$ corresponds to the measurement errors, and reconstruct an approximate wave speed $\tilde c$ . We emphasize that $\tilde Λ$ may not not be in the range of the map $\mathcal A$ . We show that the reconstructed wave speed $\tilde c$ satisfies $\| \tilde c-c\|_{L^\infty}<C \|E\|^{1/18}$ . Our regularization strategy is based on a new formula to compute $c$ from $Λ$ .

preprint2015arXiv

Stability of the unique continuation for the wave operator via Tataru inequality and applications

In this paper we study the stability of the unique continuation in the case of the wave equation with variable coefficients independent of time. We prove a logarithmic estimate in a arbitrary domain of ${\mathbb R}^{n+1}$ , where all the parameters are calculated explicitly in terms of the $C^1$ -norm of the coefficients and on the other geometric properties of the problem. We use the Carleman-type estimate proved by Tataru in 1995 and an iteration for locals stability. We apply the result to the case of a wave equation with data on a cylinder an we get a stable estimate for any positive time, also after the first conjugate point for the geodesics of the metric related to the variable coefficients.

preprint2015arXiv

The blow-up of electromagnetic fields in 3-dimensional invisibility cloaking for Maxwell's equations

Transformation optics constructions have allowed the design of cloaking devices that steer electromagnetic, acoustic and quantum waves around a region without penetrating it, so that this region is hidden from external observations. The proposed material parameters are anisotropic, and singular at the interface between the cloaked region and the cloaking device. The presence of these singularities causes various mathematical problems and physical effects on the interface surface. In this paper, we analyze the 3-dimensional cloaking for Maxwell's equations when there are sources or sinks present inside the cloaked region. In particular, we consider nonsingular approximate invisibility cloaks based on the truncation of the singular transformations. We analyze the limit of solutions when the approximate cloaking approaches the ideal cloaking in the sense of distributions. We show that the solutions in the approximate cloaks converge to a distribution that contains Dirac's delta distribution supported on the interface surface. In particular, this implies that the limit of solutions are not measurable functions, making them outside of those classes of functions that have earlier been used in the models of the ideal invisibility cloaks. Also, we give a rigorous meaning for the "extraordinary surface voltage effect" considered in physical literature of invisibility cloaks.

preprint2014arXiv

Inverse acoustic scattering problem in half-space with anisotropic random impedance

We study an inverse acoustic scattering problem in half-space with a probabilistic impedance boundary value condition. The Robin coefficient (surface impedance) is assumed to be a Gaussian random function $λ= λ(x)$ with a pseudodifferential operator describing the covariance. We measure the amplitude of the backscattered field averaged over the frequency band and assume that the data is generated by a single realization of $λ$ . Our main result is to show that under certain conditions the principal symbol of the covariance operator of $λ$ is uniquely determined. Most importantly, no approximations are needed and we can solve the full non-linear inverse problem. We concentrate on anisotropic models for the principal symbol, which leads to the analysis of a novel anisotropic spherical Radon transform and its invertibility.

preprint2014arXiv

Inverse problems in spacetime I: Inverse problems for Einstein equations - Extended preprint version

We consider inverse problems for the coupled Einstein equations and the matter field equations on a 4-dimensional globally hyperbolic Lorentzian manifold $(M,g)$ . We give a positive answer to the question: Do the active measurements, done in a neighborhood $U\subset M$ of a freely falling observed $μ=μ([s_-,s_+])$ , determine the conformal structure of the spacetime in the minimal causal diamond-type set $V_g=J_g^+(μ(s_-))\cap J_g^-(μ(s_+))\subset M$ containing $μ$ ? More precisely, we consider the Einstein equations coupled with the scalar field equations and study the system $Ein(g)=T$ , $T=T(g,ϕ)+F_1$ , and $\square_gϕ-\mathcal V^\prime(ϕ)=F_2$ , where the sources $F=(F_1,F_2)$ correspond to perturbations of the physical fields which we control. The sources $F$ need to be such that the fields $(g,ϕ,F)$ are solutions of this system and satisfy the conservation law $\nabla_jT^{jk}=0$ . Let $(\hat g,\hat ϕ)$ be the background fields corresponding to the vanishing source $F$ . We prove that the observation of the solutions $(g,ϕ)$ in the set $U$ corresponding to sufficiently small sources $F$ supported in $U$ determine $V_{\hat g}$ as a differentiable manifold and the conformal structure of the metric $\hat g$ in the domain $V_{\hat g}$ . The methods developed here have potential to be applied to a large class of inverse problems for non-linear hyperbolic equations encountered e.g. in various practical imaging problems.

preprint2014arXiv

Linearization stability results and active measurements for the Einstein-scalar field equations

We study the Einstein equations coupled with the scalar field equations, $\hbox{Ein}(g)=T$ , $T=T(g,ϕ)+F^1$ , and $\square_gϕ^\ell-m^2ϕ^\ell= F^2$ , where the sources $F=(F^1, F^2)$ correspond to perturbations of the physical fields which we control. Here $ϕ=(ϕ^\ell)_{\ell=1}^L$ and $(M,g)$ is a 4-dimensional globally hyperbolic Lorentzian manifold. The sources $F$ need to be such that the fields $(g,ϕ,F)$ satisfy the conservation law $\hbox{div}_g(T)=0$ . If $(g_ε,ϕ_ε)$ solves the above equations, $\dot g=\partial_εg_ε|_{ε=0}$ , $\dotϕ=ϕ_ε|_{ε=0}$ , and $f=(f^1,f^2)= \partial_εF_ε|_{ε=0}$ solve the linearized Einstein equations and the linearized conservation law $\frac 12 \hat g^{pk}\hat \nabla_p f^1_{kj}+ \sum_{\ell=1}^L f^2_\ell \, \partial_j\hatϕ_\ell=0,$ where $\hat g= g_ε|_{ε=0}$ and $\hat ϕ= ϕ_ε|_{ε=0}$ . Then $(\hat g,\hat ϕ)$ and $f$ have the linearization stability property. Here ask the converse: If $\dot g$ , $\dot ϕ$ , and $f$ solve the linearized Einstein equations and the linearized conservation law, are there $F_ε=(F^1_ε,F^2_ε)$ and $(g_ε,ϕ_ε)$ depending on $ε\in [0,ε_0)$ , $ε_0>0$ , such that $(g_ε,ϕ_ε)$ solves the Einstein-scalar field equations and the conservation law. When $\hat g$ and $\hat ϕ$ vary enough and $L\geq 5$ , we prove a microlocal version of this: When $Y\subset M$ is a 2-surface and $(y,η)\in N^*Y$ , there is $f$ that is a conormal distibutions wrt. the surface $Y$ with a given principal symbol at $(y,η)$ such that $(\hat g,\hat ϕ)$ and $f$ have the linearization stability property.

preprint2014arXiv

The Calderon problem in transversally anisotropic geometries

We consider the anisotropic Calderon problem of recovering a conductivity matrix or a Riemannian metric from electrical boundary measurements in three and higher dimensions. In the earlier work \cite{DKSaU}, it was shown that a metric in a fixed conformal class is uniquely determined by boundary measurements under two conditions: (1) the metric is conformally transversally anisotropic (CTA), and (2) the transversal manifold is simple. In this paper we will consider geometries satisfying (1) but not (2). The first main result states that the boundary measurements uniquely determine a mixed Fourier transform / attenuated geodesic ray transform (or integral against a more general semiclassical limit measure) of an unknown coefficient. In particular, one obtains uniqueness results whenever the geodesic ray transform on the transversal manifold is injective. The second result shows that the boundary measurements in an infinite cylinder uniquely determine the transversal metric. The first result is proved by using complex geometrical optics solutions involving Gaussian beam quasimodes, and the second result follows from a connection between the Calderon problem and Gel'fand's inverse problem for the wave equation and the boundary control method.

preprint2013arXiv

Determination of structures in the space-time from local measurements: a detailed exposition

We consider inverse problems for the Einstein equation with a time-depending metric on a 4-dimensional globally hyperbolic Lorentzian manifold $(M,g)$ . We formulate the concept of active measurements for relativistic models. We do this by coupling the Einstein equation with equations for scalar fields and study the system $Ein(g)=T$ , $T=T(g,ϕ)+F_1$ , and $\square_g ϕ=F_2+S(g,ϕ,F_1,F_2)$ . Here $F=(F_1,F_2)$ correspond to the perturbations of the physical fields which we control and $S$ is a secondary source corresponding to the adaptation of the system to the perturbation so that the conservation law $div_g(T)=0$ will be satisfied. The inverse problem we study is the question, do the observation of the solutions $(g,ϕ)$ in an open subset $U\subset M$ of the space-time corresponding to sources $F$ supported in $U$ determine the properties of the metric in a larger domain $W\subset M$ containing $U$ . To study this problem we define the concept of light observation sets and show that these sets determine the conformal class of the metric. This corresponds to passive observations from a distant area of the space which is filled by light sources (e.g. we see light from stars varying in time). One can apply the obtained result to solve inverse problems encountered in general relativity and in various practical imaging problems.

preprint2013arXiv

Inverse problem for the wave equation with a white noise source

We consider a smooth Riemannian metric tensor $g$ on $\R^n$ and study the stochastic wave equation for the Laplace-Beltrami operator $\p_t^2 u - Δ_g u = F$. Here, $F=F(t,x,ω)$ is a random source that has white noise distribution supported on the boundary of some smooth compact domain $M \subset \R^n$ . We study the following formally posed inverse problem with only one measurement. Suppose that $g$ is known only outside of a compact subset of $M^{int}$ and that a solution $u(t,x,ω_0)$ is produced by a single realization of the source $F(t,x,ω_0)$ . We ask what information regarding $g$ can be recovered by measuring $u(t,x,ω_0)$ on $\R_+ \times \p M$? We prove that such measurement together with the realization of the source determine the scattering relation of the Riemannian manifold $(M, g)$ with probability one. That is, for all geodesics passing through $M$ , the travel times together with the entering and exit points and directions are determined. In particular, if $(M,g)$ is a simple Riemannian manifold and $g$ is conformally Euclidian in $M$ , the measurement determines the metric $g$ in $M$ .

preprint2013arXiv

Inverse problems and invisibility cloaking for FEM models and resistor networks

In this paper we consider inverse problems for resistor networks and for models obtained via the Finite Element Method (FEM) for the conductivity equation. These correspond to discrete versions of the inverse conductivity problem of Calderón. We characterize FEM models corresponding to a given triangulation of the domain that are equivalent to certain resistor networks, and apply the results to study nonuniqueness of the discrete inverse problem. It turns out that the degree of nonuniqueness for the discrete problem is larger than the one for the partial differential equation. We also study invisibility cloaking for FEM models, and show how an arbitrary body can be surrounded with a layer so that the cloaked body has the same boundary measurements as a given background medium.

preprint2013arXiv

Spectral theory and inverse problem on asymptotically hyperbolic orbifolds

We consider an inverse problem associated with $n$ -dimensional asymptotically hyperbolic orbifolds $(n \geq 2)$ having a finite number of cusps and regular ends. By observing solutions of the Helmholtz equation at the cusp, we introduce a generalized $S$ -matrix, and then show that it determines the manifolds with its Riemannian metric and the orbifold structure.

preprint2012arXiv

Inverse problem for the Riemannian wave equation with Dirichlet data and Neumann data on disjoint sets

We consider the inverse problem to determine a smooth compact Riemannian manifold with boundary $(M, g)$ from a restriction $Λ_{\Src, \Rec}$ of the Dirichlet-to-Neumann operator for the wave equation on the manifold. Here $\Src$ and $\Rec$ are open sets in $\p M$ and the restriction $Λ_{\Src, \Rec}$ corresponds to the case where the Dirichlet data is supported on $\R_+\times \Src$ and the Neumann data is measured on $\R_+\times \Rec$. In the novel case where $\bar \Src \cap \bar \Rec = \emptyset$, we show that $Λ_{\Src, \Rec}$ determines the manifold $(M,g)$ uniquely, assuming that the wave equation is exactly controllable from the set of sources $\Src$. Moreover, we show that the exact controllability can be replaced by the Hassell-Tao condition for eigenvalues and eigenfunctions, that is, λ_j \le C \norm{\p_νϕ_j}_{L^2(\Src)}^2, \quad j =1, 2, ..., where $λ_j$ are the Dirichlet eigenvalues and $(ϕ_j)_{j=1}^\infty$ is an orthonormal basis of the corresponding eigenfunctions.

preprint2012arXiv

Reconstruction of a conformally Euclidean metric from local boundary diffraction travel times

We consider a region $M$ in $\mathbb{R}^n$ with boundary $\partial M$ and a metric $g$ on $M$ conformal to the Euclidean metric. We analyze the inverse problem, originally formulated by Dix, of reconstructing $g$ from boundary measurements associated with the single scattering of seismic waves in this region. In our formulation the measurements determine the shape operator of wavefronts outside of $M$ originating at diffraction points within $M$ . We develop an explicit reconstruction procedure which consists of two steps. In the first step we reconstruct the directional curvatures and the metric in what are essentially Riemmanian normal coordinates; in the second step we develop a conversion to Cartesian coordinates. We admit the presence of conjugate points. In dimension $n \geq 3$ both steps involve the solution of a system of ordinary differential equations. In dimension $n=2$ the same is true for the first step, but the second step requires the solution of a Cauchy problem for an elliptic operator which is unstable in general. The first step of the procedure applies for general metrics.

preprint2012arXiv

Recovering the isometry type of a Riemannian manifold from local boundary diffraction travel times

We analyze the inverse problem, originally formulated by Dix in geophysics, of reconstructing the wave speed inside a domain from boundary measurements associated with the single scattering of seismic waves. We consider a domain $\tilde M$ with a varying and possibly anisotropic wave speed which we model as a Riemannian metric $g$ . For our data, we assume that $\tilde M$ contains a dense set of point scatterers and that in a subset $U\subset \tilde M$ , modeling a region containing measurement devices, we can measure the wave fronts of the single scattered waves diffracted from the point scatterers. The inverse problem we study is to recover the metric $g$ in local coordinates anywhere on a set $M \subset \tilde M$ up to an isometry (i.e. we recover the isometry type of $M$ ). To do this we show that the shape operators related to wave fronts produced by the point scatterers within $\tilde M$ satisfy a certain system of differential equations which may be solved along geodesics of the metric. In this way, assuming we know $g$ as well as the shape operator of the wave fronts in the region $U$ , we may recover $g$ in certain coordinate systems (e.g. Riemannian normal coordinates centered at point scatterers). This generalizes the well-known geophysical method of Dix to metrics which may depend on all spatial variables and be anisotropic. In particular, the novelty of this solution lies in the fact that it can be used to reconstruct the metric also in the presence of the caustics.

preprint2011arXiv

An inverse problem for a hyperbolic system on a vector bundle and energy measurements

A uniqueness result in the inverse problem for an inhomogeneous hyperbolic system on a real vector bundle over a smooth compact manifold, based on energy measurements for improperly known sources, is established.

preprint2011arXiv

Conic singularities, generalized scattering matrix, and inverse scattering on asymptotically hyperbolic surfaces

We consider an inverse problem associated with some 2-dimensional non-compact surfaces with conical singularities, cusps and regular ends. Our motivating example is a Riemann surface $\mathcal M = Γ\backslash{\bf H}^2$ associated with a Fuchsian group of the 1st kind $Γ$ containing parabolic elements. $\mathcal M$ is then non-compact, and has a finite number of cusps and elliptic singular points, which is regarded as a hyperbolic orbifold. We introduce a class of Riemannian surfaces with conical singularities on its finite part, having cusps and regular ends at infinity, whose metric is asymptotically hyperbolic. By observing solutions of the Helmholtz equation at the cusp, we define a generalized S-matrix. We then show that this generalized S-matrix determines the Riemannian metric and the structure of conical singularities.

preprint2011arXiv

Determining a first order perturbation of the biharmonic operator by partial boundary measurements

We consider an operator $Δ^2 + A(x)\cdot D+q(x)$ with the Navier boundary conditions on a bounded domain in $R^n$ , $n\ge 3$ . We show that a first order perturbation $A(x)\cdot D+q$ can be determined uniquely by measuring the Dirichlet--to--Neumann map on possibly very small subsets of the boundary of the domain. Notice that the corresponding result does not hold in general for a first order perturbation of the Laplacian.

preprint2011arXiv

Inverse boundary value problems for the perturbed polyharmonic operator

We show that a first order perturbation $A(x)\cdot D+q(x)$ of the polyharmonic operator $(-Δ)^m$ , $m\ge 2$ , can be determined uniquely from the set of the Cauchy data for the perturbed polyharmonic operator on a bounded domain in $R^n$ , $n\ge 3$ . Notice that the corresponding result does not hold in general when $m=1$ .

preprint2011arXiv

Inverse problems with partial data for a magnetic Schrödinger operator in an infinite slab and on a bounded domain

In this paper we study inverse boundary value problems with partial data for the magnetic Schrödinger operator. In the case of an infinite slab in $R^n$ , $n\ge 3$ , we establish that the magnetic field and the electric potential can be determined uniquely, when the Dirichlet and Neumann data are given either on the different boundary hyperplanes of the slab or on the same hyperplane. This is a generalization of the results of [41], obtained for the Schrödinger operator without magnetic potentials. In the case of a bounded domain in $R^n$ , $n\ge 3$ , extending the results of [2], we show the unique determination of the magnetic field and electric potential from the Dirichlet and Neumann data, given on two arbitrary open subsets of the boundary, provided that the magnetic and electric potentials are known in a neighborhood of the boundary. Generalizing the results of [31], we also obtain uniqueness results for the magnetic Schrödinger operator, when the Dirichlet and Neumann data are known on the same part of the boundary, assuming that the inaccessible part of the boundary is a part of a hyperplane.

preprint2011arXiv

Schrodinger's Hat: Electromagnetic, acoustic and quantum amplifiers via transformation optics

The advent of transformation optics and metamaterials has made possible devices producing extreme effects on wave propagation. Here we give theoretical designs for devices, Schrödinger hats, acting as invisible concentrators of waves. These exist for any wave phenomenon modeled by either the Helmholtz or Schrödinger equations, e.g., polarized waves in EM, pressure waves in acoustics and matter waves in QM, and occupy one part of a parameter space continuum of wave-manipulating structures which also contains standard transformation optics based cloaks, resonant cloaks and cloaked sensors. For EM and acoustic Schrödinger hats, the resulting centralized wave is a localized excitation. In QM, the result is a new charged quasiparticle, a \emph{quasmon}, which causes conditional probabilistic illusions. We discuss possible solid state implementations.

preprint2011arXiv

The borderlines of the invisibility and visibility for Calderon's inverse problem

We consider the determination of a conductivity function in a two-dimensional domain from the Cauchy data of the solutions of the conductivity equation on the boundary. We prove uniqueness results for this inverse problem, posed by Calderon, for conductivities that are degenerate, that is, they may not be bounded from above or below. In particular, for scalar conductivities we solve the inverse problem in a class which is larger than $L^\infty$ . Also, we give new counterexamples for the uniqueness of the inverse conductivity problem. We say that a conductivity is visible if the inverse problem is solvable so that the inside of the domain can be uniquely determined, up to a change of coordinates, using the boundary measurements. The present counterexamples for the inverse problem have been related to the invisibility cloaking. This means that there are conductivities for which a part of the domain is shielded from detection via boundary measurements. Such conductivities are called invisibility cloaks. In the present paper we identify the borderline of the visible conductivities and the borderline of invisibility cloaking conductivities. Surprisingly, these borderlines are not the same. We show that between the visible and the cloaking conductivities there are the electric holograms, conductivities which create an illusion of a non-existing body. The electric holograms give counterexamples for the uniqueness of the inverse problem which are less degenerate than the previously known ones.

preprint2011arXiv

The Novikov-Veselov Equation and the Inverse Scattering Method, Part I: Analysis

The Novikov-Veselov (NV) equation is a (2+1)-dimensional nonlinear evolution equation that generalizes the (1+1)-dimensional Korteweg-deVries (KdV) equation. Solution of the NV equation using the inverse scattering method has been discussed in the literature, but only formally (or with smallness assumptions in case of nonzero energy) because of the possibility of exceptional points, or singularities in the scattering data. In this work, absence of exceptional points is proved at zero energy for evolutions with compactly supported, smooth and rotationally symmetric initial data of the conductivity type: $q_0=γ^{-1/2}Δγ^{1/2}$ with a strictly positive function $γ$ . The inverse scattering evolution is shown to be well-defined, real-valued, and preserving conductivity-type. There is no smallness assumption on the initial data.

preprint2011arXiv

Two dimensional invisibility cloaking for Helmholtz equation and non-local boundary conditions

Transformation optics constructions have allowed the design of cloaking devices that steer electromagnetic, acoustic and quantum waves around a region without penetrating it, so that this region is hidden from external observations. The material pa- rameters used to describe these devices are anisotropic, and singular at the interface between the cloaked and uncloaked regions, making physical realization a challenge. These singular material parameters correspond to singular coefficient functions in the partial differential equations modeling these constructions and the presence of these singularities causes various mathematical problems and physical effects on the interface surface. In this paper, we analyze the two dimensional cloaking for Helmholtz equation when there are sources or sinks present inside the cloaked region. In particular, we consider nonsingular approximate invisibility cloaks based on the truncation of the singular transformations. Using such truncation we analyze the limit when the approximate cloaking approaches the ideal cloaking. We show that, surprisingly, a non-local boundary condition appears on the inner cloak interface. This effect in the two dimensional (or cylindrical) invisibility cloaks, which seems to be caused by the infinite phase velocity near the interface between the cloaked and uncloaked regions, is very different from the earlier studied behavior of the solutions in the three dimensional cloaks.

preprint2010arXiv

An inverse problem for the wave equation with one measurement and the pseudorandom noise

We consider the wave equation $(\p_t^2-Δ_g)u(t,x)=f(t,x)$, in $\R^n$ , $u|_{\R_-\times \R^n}=0$ , where the metric $g=(g_{jk}(x))_{j,k=1}^n$ is known outside an open and bounded set $M\subset \R^n$ with smooth boundary $\p M$. We define a deterministic source $f(t,x)$ called the pseudorandom noise as a sum of point sources, $f(t,x)=\sum_{j=1}^\infty a_jδ_{x_j}(x)δ(t)$ , where the points $x_j,\ j\in\Z_+$ , form a dense set on $\p M$. We show that when the weights $a_j$ are chosen appropriately, $u|_{\R\times \p M}$ determines the scattering relation on $\p M$, that is, it determines for all geodesics which pass through $M$ the travel times together with the entering and exit points and directions. The wave $u(t,x)$ contains the singularities produced by all point sources, but when $a_j=λ^{-λ^{j}}$ for some $λ>1$ , we can trace back the point source that produced a given singularity in the data. This gives us the distance in $(\R^n, g)$ between a source point $x_j$ and an arbitrary point $y \in \p M$. In particular, if $(\bar M,g)$ is a simple Riemannian manifold and $g$ is conformally Euclidian in $\bar M$ , these distances are known to determine the metric $g$ in $M$ . In the case when $(\bar M,g)$ is non-simple we present a more detailed analysis of the wave fronts yielding the scattering relation on $\p M$.

preprint2010arXiv

Cloaking a sensor via transformation optics

It is generally believed that transformation optics based cloaking, besides rendering the cloaked region invisible to detection by scattering of incident waves, also shields the region from those same waves. We demonstrate a coupling between the cloaked and uncloaked regions, exposing a difference between cloaking for rays and waves. Interior resonances allow this coupling to be amplified, and careful choice of parameters leads to effective cloaks with degraded shielding. As one application, we describe how to use transformation optics to hide sensors in the cloaked region and yet enable the sensors to efficiently measure waves incident on the exterior of the cloak, an effect similar to the plasmon based approach of Alu' and Engheta.

preprint2010arXiv

Determining electrical and heat transfer parameters using coupled boundary measurements

Let $Ω\subset\R^n$ , $n\ge 3$ , be a smooth bounded domain and consider a coupled system in $Ω$ consisting of a conductivity equation $\nabla \cdot γ(x) \nabla u(t,x)=0$ and an anisotropic heat equation $κ^{-1}(x)\partial_tψ(t,x)=\nabla\cdot (A(x)\nabla ψ(t,x))+(γ\nabla u(t,x))\cdot \nabla u(t,x), \quad t\ge 0$ . It is shown that the coefficients $γ$ , $κ$ and $A=(a_{jk})$ are uniquely determined from the knowledge of the boundary map $u|_{\partialΩ}\mapsto ν\cdot A\nabla ψ|_{\partialΩ}$ , where $ν$ is the unit outer normal to $\partialΩ$ . The coupled system models the following physical phenomenon. Given a fixed voltage distribution, maintained on the boundary $\partialΩ$ , an electric current distribution appears inside $Ω$ . The current in turn acts as a source of heat inside $Ω$ , and the heat flows out of the body through the boundary. The boundary measurements above then correspond to the map taking a voltage distribution on the boundary to the resulting heat flow through the boundary. The presented mathematical results suggest a new hybrid diffuse imaging modality combining electrical prospecting and heat transfer-based probing.

preprint2010arXiv

Inverse problem for wave equation with sources and observations on disjoint sets

We consider an inverse problem for a hyperbolic partial differential equation on a compact Riemannian manifold. Assuming that $Γ_1$ and $Γ_2$ are two disjoint open subsets of the boundary of the manifold we define the restricted Dirichlet-to-Neumann operator $Λ_{Γ_1,Γ_2}$ . This operator corresponds the boundary measurements when we have smooth sources supported on $Γ_1$ and the fields produced by these sources are observed on $Γ_2$ . We show that when $Γ_1$ and $Γ_2$ are disjoint but their closures intersect at least at one point, then the restricted Dirichlet-to-Neumann operator $Λ_{Γ_1,Γ_2}$ determines the Riemannian manifold and the metric on it up to an isometry. In the Euclidian space, the result yields that an anisotropic wave speed inside a compact body is determined, up to a natural coordinate transformations, by measurements on the boundary of the body even when wave sources are kept away from receivers. Moreover, we show that if we have three arbitrary non-empty open subsets $Γ_1,Γ_2$ , and $Γ_3$ of the boundary, then the restricted Dirichlet-to-Neumann operators $Λ_{Γ_j,Γ_k}$ for $1\leq j<k\leq 3$ determine the Riemannian manifold to an isometry. Similar result is proven also for the finite-time boundary measurements when the hyperbolic equation satisfies an exact controllability condition.

preprint2010arXiv

Inverse problems for differential forms on Riemannian manifolds with boundary

Consider a real-analytic orientable connected complete Riemannian manifold $M$ with boundary of dimension $n\ge 2$ and let $k$ be an integer $1\le k\le n$ . In the case when $M$ is compact of dimension $n\ge 3$ , we show that the manifold and the metric on it can be reconstructed, up to an isometry, from the set of the Cauchy data for harmonic $k$ -forms, given on an open subset of the boundary. This extends a result of [13] when $k=0$ . In the two-dimensional case, the same conclusion is obtained when considering the set of the Cauchy data for harmonic $1$ -forms. Under additional assumptions on the curvature of the manifold, we carry out the same program when $M$ is complete non-compact. In the case $n\ge 3$ , this generalizes the results of [12] when $k=0$ . In the two-dimensional case, we are able to reconstruct the manifold from the set of the Cauchy data for harmonic $1$ -forms.

preprint2010arXiv

Reconstruction of Betti numbers of manifolds for anisotropic Maxwell and Dirac systems

We consider an invariant formulation of the system of Maxwell's equations for an anisotropic medium on a compact orientable Riemannian 3-manifold $(M,g)$ with nonempty boundary. The system can be completed to a Dirac type first order system on the manifold. We show that the Betti numbers of the manifold can be recovered from the dynamical response operator for the Dirac system given on a part of the boundary. In the case of the original physical Maxwell system, assuming that the entire boundary is known, all Betti numbers of the manifold can also be determined from the dynamical response operator given on a part of the boundary. Physically, this operator maps the tangential component of the electric field into the tangential component of the magnetic field on the boundary.

preprint2006arXiv

The inverse conductivity problem with an imperfectly known boundary in three dimensions

We consider the inverse conductivity problem in a strictly convex domain whose boundary is not known. Usually the numerical reconstruction from the measured current and voltage data is done assuming the domain has a known fixed geometry. However, in practical applications the geometry of the domain is usually not known. This introduces an error, and effectively changes the problem into an anisotropic one. The main result of this paper is a uniqueness result characterizing the isotropic conductivities on convex domains in terms of measurements done on a different domain, which we call the model domain, up to an affine isometry. As data for the inverse problem, we assume the Robin-to-Neumann map and the contact impedance function on the boundary of the model domain to be given. Also, we present a minimization algorithm based on the use of Cotton--York tensor, that finds the pushforward of the isotropic conductivity to our model domain, and also finds the boundary of the original domain up to an affine isometry. This algorithm works also in dimensions higher than three, but then the Cotton--York tensor has to replaced with the Weyl--tensor.

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math.AP math-ph math.MP math.DG math.PR Machine Learning math.NA math.OC math.FA math.ST Statistics Theory eess.IV math.SP physics.optics math.GT Methodology Numerical Analysis

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arxivconfidence 95%

external id: arxiv:2605.02327:author:3:matti-lassas

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arxivconfidence 95%

external id: arxiv:2605.17968:author:5:matti-lassas

Imported May 20, 2026Synced May 21, 2026

Gunther Uhlmann12 works Lauri Oksanen11 works Yaroslav Kurylev11 works Samuli Siltanen10 works

Matti Lassas

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63 publications

Denoising data using convex relaxations

Function graph transformers universally approximate operators between function spaces

Lens rigidity in 2D: The reconstruction of a Riemann surface from its geodesic lengths

On the lack of external response of a nonlinear medium in the second-harmonic generation process

Inverse problems for discrete heat equations and random walks for a class of graphs

An inverse problem for a semi-linear wave equation: a numerical study

An inverse problem for the Riemannian minimal surface equation

Deep learning architectures for nonlinear operator functions and nonlinear inverse problems

Fitting a manifold of large reach to noisy data

Inverse problems for locally perturbed lattices -- Discrete Hamiltonian and quantum graph

Learning a microlocal prior for limited-angle tomography

Universal Joint Approximation of Manifolds and Densities by Simple Injective Flows

A foliated and reversible Finsler manifold is determined by its broken scattering relation

Random tree Besov priors -- Towards fractal imaging

X-Ray Transform in Asymptotically Conic Spaces

Construction of artificial point sources for a linear wave equation in unknown medium

Deep neural networks for inverse problems with pseudodifferential operators: an application to limited-angle tomography

Inverse scattering on non-compact manifolds with general metric

Uniqueness and stability of an inverse problem for a semi-linear wave equation

Reconstruction and stability in Gel'fand's inverse interior spectral problem

The Light Ray transform on Lorentzian manifolds

Analysis of regularized inversion of data corrupted by white Gaussian noise

Correlation based passive imaging with a white noise source

Inverse problems for semilinear wave equations on Lorentzian manifolds

Inverse scattering for a random potential

Posterior consistency and convergence rates for Bayesian inversion with hypoelliptic operators

The Calderón problem for the conformal Laplacian

A Direct Reconstruction Method for Anisotropic Electrical Impedance Tomography

Determination of the Spacetime from Local Time Measurements

Multi-resolution parameter choice method for total variation regularized tomography

On Absence and Existence of the Anomalous Localized Resonance without the Quasi-static Approximation

On the inverse problem of finding cosmic strings and other topological defects

Positive-energy D-bar method for acoustic tomography: a computational study

Regularization strategy for inverse problem for 1+1 dimensional wave equation

Stability of the unique continuation for the wave operator via Tataru inequality and applications

The blow-up of electromagnetic fields in 3-dimensional invisibility cloaking for Maxwell's equations

Inverse acoustic scattering problem in half-space with anisotropic random impedance

Inverse problems in spacetime I: Inverse problems for Einstein equations - Extended preprint version

Linearization stability results and active measurements for the Einstein-scalar field equations

The Calderon problem in transversally anisotropic geometries

Determination of structures in the space-time from local measurements: a detailed exposition

Inverse problem for the wave equation with a white noise source

Inverse problems and invisibility cloaking for FEM models and resistor networks

Spectral theory and inverse problem on asymptotically hyperbolic orbifolds

Inverse problem for the Riemannian wave equation with Dirichlet data and Neumann data on disjoint sets

Reconstruction of a conformally Euclidean metric from local boundary diffraction travel times

Recovering the isometry type of a Riemannian manifold from local boundary diffraction travel times

An inverse problem for a hyperbolic system on a vector bundle and energy measurements

Conic singularities, generalized scattering matrix, and inverse scattering on asymptotically hyperbolic surfaces

Determining a first order perturbation of the biharmonic operator by partial boundary measurements

Inverse boundary value problems for the perturbed polyharmonic operator

Inverse problems with partial data for a magnetic Schrödinger operator in an infinite slab and on a bounded domain

Schrodinger's Hat: Electromagnetic, acoustic and quantum amplifiers via transformation optics

The borderlines of the invisibility and visibility for Calderon's inverse problem

The Novikov-Veselov Equation and the Inverse Scattering Method, Part I: Analysis

Two dimensional invisibility cloaking for Helmholtz equation and non-local boundary conditions

An inverse problem for the wave equation with one measurement and the pseudorandom noise

Cloaking a sensor via transformation optics

Determining electrical and heat transfer parameters using coupled boundary measurements

Inverse problem for wave equation with sources and observations on disjoint sets

Inverse problems for differential forms on Riemannian manifolds with boundary

Reconstruction of Betti numbers of manifolds for anisotropic Maxwell and Dirac systems

The inverse conductivity problem with an imperfectly known boundary in three dimensions

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