# Inverse problems, spring 2015

**The course is lectured in English.**

Inverse problems are about interpreting indirect measurements. The scientific study of inverse problems is an interdisciplinary field combining mathematics, physics, signal processing, and engineering. Examples of inverse problems include

- Three-dimensional X-ray imaging (more information)
- Recovering the inner structure of the Earth based on earthquake measurements
- Sharpening a misfocused photograph (more information )
- Reconstructing electric conductivity from current-to-voltage boundary measurements (see this page and this page)
- Finding cracks inside solid structures
- Prospecting for oil and minerals
- Monitoring underground contaminants
- Finding the shape of asteroids based on light-curve data (see this page)

The common features of all this problems are the need to understand indirect measurements and to overcome extreme sensitivity to noise and modelling inaccuracies.

### What does the course contain?

The goals of the course are

- introduce discrete matrix models of some widely used measurements, such as tomography and convolution
- show how to detect ill-posedness (sensitivity to measurement noise) in matrix models using Singular Value Decomposition
- compute noise-robust reconstructions using
*regularization* - write Matlab algorithms for sharpening photographs and computing tomographic reconstructions
- discussion of nonlinear inverse problems, with Electrical Impedance Tomography as an example

The lectures make up 10 credit units. In addition to lectures the course involves a **project work**. It is done in teams of two and gives 5 credit units to each student.

The course is in total **15 credit units**.

### Lecturer

### Scope

15 sp.

### Type

Advanced studies

### Prerequisites

Recommended courses to take before this course: Linear algebra 1 and 2, Applications of matrix computations.

Some previous experience with Matlab programming is very helpful.

### Lectures

**Period III:** Lectures as follows:

Tuesday 10-12 in room D123

Wednesday 12-14 in room D123

Friday 12-14 in room C123.

Two hours of exercise classes per week.

**Period IV:** Lectures and exercises in the beginning of the period. Later project work, which is reported as a poster in a poster session.

### Exams

There will be an exam after the lecture part of the course.

### Bibliography

**Mueller J L and Siltanen S: ***Linear and Nonlinear Inverse Problems with Practical Applications.* SIAM 2012.

### Registration

Did you forget to register? What to do?

### Exercises

The weekly exercises will appear here.

### Project work

In the project you are supposed to take a subset of the data with only few projections, such as 20 projection directions. Use one of these methods to recover the walnut slice from sparse data:

- Tikhonov regularization based on conjugate gradient method,
- approximate total variation regularization implemented iteratively with the Barzilai-Borwein method,
- some other suitable method.

Optimally, you should have an automatic method for choosing the regularization parameter and an automatic stopping criteria for the iteration. These are both difficult requirements, so have a simple approach as plan B if a more complicated approach does not work.

**First goal consists of two things:** (a) two first sections should be preliminary written in LaTeX (not necessarily in poster format yet) and (b) the Matlab codes at the following webpage should be run and studied:

Two things will be graded in the meeting about the first goal: (a) the draft of project work and (b) your understanding of the Matlab codes at the above webpage relevant to your topic. The grade represents 30% of the final grade of the project work. Please agree on a meeting time (in the period April 1-4) with the lecturer for reviewing and grading the first goal.

**Second and final goal:** poster is presented in the poster session.