393 research outputs found
Low Complexity Regularization of Linear Inverse Problems
Inverse problems and regularization theory is a central theme in contemporary
signal processing, where the goal is to reconstruct an unknown signal from
partial indirect, and possibly noisy, measurements of it. A now standard method
for recovering the unknown signal is to solve a convex optimization problem
that enforces some prior knowledge about its structure. This has proved
efficient in many problems routinely encountered in imaging sciences,
statistics and machine learning. This chapter delivers a review of recent
advances in the field where the regularization prior promotes solutions
conforming to some notion of simplicity/low-complexity. These priors encompass
as popular examples sparsity and group sparsity (to capture the compressibility
of natural signals and images), total variation and analysis sparsity (to
promote piecewise regularity), and low-rank (as natural extension of sparsity
to matrix-valued data). Our aim is to provide a unified treatment of all these
regularizations under a single umbrella, namely the theory of partial
smoothness. This framework is very general and accommodates all low-complexity
regularizers just mentioned, as well as many others. Partial smoothness turns
out to be the canonical way to encode low-dimensional models that can be linear
spaces or more general smooth manifolds. This review is intended to serve as a
one stop shop toward the understanding of the theoretical properties of the
so-regularized solutions. It covers a large spectrum including: (i) recovery
guarantees and stability to noise, both in terms of -stability and
model (manifold) identification; (ii) sensitivity analysis to perturbations of
the parameters involved (in particular the observations), with applications to
unbiased risk estimation ; (iii) convergence properties of the forward-backward
proximal splitting scheme, that is particularly well suited to solve the
corresponding large-scale regularized optimization problem
Solving Inverse Conductivity Problems In Doubly Connected Domains By the Homogenization Functions of Two Parameters
In the paper, we make the first attempt to derive a family of two-parameter homogenization functions in the doubly connected domain, which is then applied as the bases of trial solutions for the inverse conductivity problems. The expansion coefficients are obtained by imposing an extra boundary condition on the inner boundary, which results in a linear system for the interpolation of the solution in a weighted Sobolev space. Then, we retrieve the spatial- or temperature-dependent conductivity function by solving a linear system, which is obtained from the collocation method applied to the nonlinear elliptic equation after inserting the solution. Although the required data are quite economical, very accurate solutions of the space-dependent and temperature-dependent conductivity functions, the Robin coefficient function and also the source function are available. It is significant that the nonlinear inverse problems can be solved directly without iterations and solving nonlinear equations. The proposed method can achieve accurate results with high efficiency even for large noise being imposed on the input data
Large Noise in Variational Regularization
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Adaptive Statistical Inference
This workshop in mathematical statistics highlights recent advances in adaptive methods for statistical estimation, testing and confidence sets. Related open mathematical problems are discussed with potential impact on the development of computationally efficient algorithms of data processing under prior uncertainty. Parcticular emphasis is on high dimensional models, inverse problems and discrtete structures
Variable Selection and Estimation in Multivariate Functional Linear Regression via the LASSO
In more and more applications, a quantity of interest may depend on several
covariates, with at least one of them infinite-dimensional (e.g. a curve). To
select the relevant covariates in this context, we propose an adaptation of the
Lasso method. Two estimation methods are defined. The first one consists in the
minimisation of a criterion inspired by classical Lasso inference under group
sparsity (Yuan and Lin, 2006; Lounici et al., 2011) on the whole multivariate
functional space H. The second one minimises the same criterion but on a
finite-dimensional subspace of H which dimension is chosen by a penalized
leasts-squares method base on the work of Barron et al. (1999). Sparsity-oracle
inequalities are proven in case of fixed or random design in our
infinite-dimensional context. To calculate the solutions of both criteria, we
propose a coordinate-wise descent algorithm, inspired by the glmnet algorithm
(Friedman et al., 2007). A numerical study on simulated and experimental
datasets illustrates the behavior of the estimators
VARIABLE SELECTION AND ESTIMATION IN MULTIVARIATE FUNCTIONAL LINEAR REGRESSION VIA THE LASSO
In more and more applications, a quantity of interest may depend on several covariates, with at least one of them infinite-dimensional (e.g. a curve). To select the relevant covariates in this context, we propose an adaptation of the Lasso method. Two estimation methods are defined. The first one consists in the minimisation of a criterion inspired by classical Lasso inference under group sparsity (Yuan and Lin, 2006; Lounici et al., 2011) on the whole multivariate functional space H. The second one minimises the same criterion but on a finite-dimensional subspace of H which dimension is chosen by a penalized leasts-squares method base on the work of Barron et al. (1999). Sparsity- oracle inequalities are proven in case of fixed or random design in our infinite-dimensional context. To calculate the solutions of both criteria, we propose a coordinate-wise descent algorithm, inspired by the glmnet algorithm (Friedman et al., 2007). A numerical study on simulated and experimental datasets illustrates the behavior of the estimators
Variational regularization theory for sparsity promoting wavelet regularization
In many scientific and industrial applications, the quantity of interest is not what is directly observed, but is instead a parameter which has a causal effect on experimental measurements.
To obtain the desired unknown quantity, one must use an inverse transform on the data.
The main challenge in such an inverse problem is that these unknowns may not continuously depend on the observations, and as a result, the effects of noise in data are magnified in the inverted results.
To obtain stable approximations of the desired parameters from noisy observations, regularization methods are used.
This thesis contributes to the mathematical analysis of generalized
Tikhonov regularization, and in particular sparsity promoting Tikhonov regularization, which
are popular examples of regularization methods.
Using variational source conditions as an intermediate step, order optimal upper bounds on the reconstruction error are shown for sparsity promoting wavelet regularization under smoothness assumptions given by Besov spaces. The framework includes practically relevant forward operators, such as the Radon transform, and some nonlinear inverse problems in differential equations with distributed measurements.
In numerical simulations for a parameter identification problem in a differential equation it is demonstrated that these theoretical results correctly predict convergence rates for piecewise smooth unknown coefficients.2022-02-0
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