Contents
Preface vii
1 Characteristics of Inverse Problems 1
1.1 Preliminaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 The least squares estimator . . . . . . . . . . . . . . . . . . . . . 6
1.3 The statistical properties of xLS and ill-posedness . . . . . . . . . 8
1.4 An illustrative example . . . . . . . . . . . . . . . . . . . . . . . 11
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2 Regularization by Spectral Filtering 15
2.1 Spectral filtering methods . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Regularization parameter selection methods . . . . . . . . . . . . 19
2.3 Periodic and data-driven boundary conditions . . . . . . . . . . . 25
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
3 Two-Dimensional Test Cases 33
3.1 Two-dimensional image deblurring . . . . . . . . . . . . . . . . . 33
3.2 Computed tomography . . . . . . . . . . . . . . . . . . . . . . . 42
3.3 The preconditioned conjugate gradient iteration . . . . . . . . . . 45
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4 Bayes’ Law, Markov Random Field Priors, and MAP Estimation 53
4.1 Bayes’ law and regularization . . . . . . . . . . . . . . . . . . . . 53
4.2 Choosing p(x|δ): Gaussian Markov random fields . . . . . . . . . 54
4.3 Choosing p(x|δ): Laplace Markov random fields . . . . . . . . . . 65
4.4 The infinite-dimensional limit . . . . . . . . . . . . . . . . . . . . 68
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
5 Markov Chain Monte Carlo Methods for Linear Inverse Problems 77
5.1 Sampling from high-dimensional Gaussian random vectors . . . . 77
5.2 Hierarchical modeling of λ and δ and sampling from p(x, λ, δ|b) . 81
5.3 Alternative MCMC methods for sampling from p(x, λ, δ|b) . . . . 89
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6 Markov Chain Monte Carlo Methods for Nonlinear Inverse Problems 105
6.1 A general setup for nonlinear inverse problems . . . . . . . . . . . 105
6.2 Levenburg–Marquardt nonlinear least squares optimization . . . . 106
v
vi Contents
6.3 Randomize-then-optimize as a proposal for Metropolis–Hastings . 109
6.4 Nonlinear test cases . . . . . . . . . . . . . . . . . . . . . . . . . 112
6.5 Hierarchical modeling of λ and δ and sampling from p(x, λ, δ|b) . 118
Exercises . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
Bibliography 125
Index 131