# Magne André Nordaas defended his PhD

Many physical processes are described with partial differential equations (PDE), with important examples such as thermal conduction, Maxwell's laws for electrodynamics and Navier-Stokes equations for viscous fluids.

In many applications, we do not want to find just any solution for differential equation, but an optimal solution in a specific sense: We have one or more parameters in the PDE model that we can tune, and we want to find the parameter values that are most beneficial. A classic example is to determine the shape of an airfoil, so that the airflow around the wing results in the best possible flight characteristics.

A related problem is the reconstruction of physical processes based on incomplete measurements. In such a problem, we seek the solution to the differential equation that best fits the measurement data.

Both examples above can be formulated as optimization problems with PDE constraints. In almost all applications, we can not calculate the exact solution for a PDE or a PDE-constrained optimization problem. Instead, we must compute a numerical approximate solution. For PDE-constrained optimisation, this usually involves solving large sparse saddle point systems.

In my thesis work I have investigated methods to solve such problems in an efficient way. A key technique is operator preconditioning. This involves identifying a mathematical structure in the problem that can be exploited to derive effective solution algorithms. The thesis consists of three papers. The first two paper concerns parameter-robust preconditioning techniques for saddle point systems. In the third paper, a PDE-constrained optimisation method is applied to solve a data assimilation problem for cerebral blood flow.

The thesis is written within the field of Scientific Computing. The work has been conducted at Simula Research Laboratory.

Prior to the defense, at 10:15, Magne André Noraas presented his trial lecture "Optimality conditions for nonlinear equality constrained optimization problems".

## The adjudication committee

- Professor Roland Herzog, Technische Universitet Chemnitz, Germany
- Senior R&D Software Engineer Trygve Karper, Schlumberger Limited, Norway
- Associate Professor Joakim Sundnes, Department of Informatics, University of Oslo

## Chair of the disputation

- Professor Fritz Albregsten, University of Oslo, Institute of Informatics.

## Supervisors

- Professor Kent-Andre Mardal, Department of Mathematics, University of Oslo
- Professor Bjørn Fredrik Nielsen, Norwegian University of Life Sciences

## Read more:

Announcement of the PhD defense at the University of Oslo's web pages (in Norwegian).