Hardness Results for Structured Linear Systems

We show that if the nearly-linear time solvers for Laplacian matrices and their generalizations can be extended to solve just slightly larger families of linear systems, then they can be used to quickly solve all systems of linear equations over the reals. This result can be viewed either positively or negatively: either we will develop nearly-linear time algorithms for solving all systems of linear equations over the reals, or progress on the families we can solve in nearly-linear time will soon halt

Approximate Gaussian Elimination for Laplacians: Fast, Sparse, and Simple

We show how to perform sparse approximate Gaussian elimination for Laplacian matrices. We present a simple, nearly linear time algorithm that approximates a Laplacian by a matrix with a sparse Cholesky factorization, the version of Gaussian elimination for symmetric matrices. This is the first nearly linear time solver for Laplacian systems that is based purely on random sampling, and does not use any graph theoretic constructions such as low-stretch trees, sparsifiers, or expanders. The crux of our analysis is a novel concentration bound for matrix martingales where the differences are sums of conditionally independent variables

Users and computers: A contextual approach to design of computer artifacts

This article contains a presentation and overview of the papers that I have submitted for the degree doctor scientiarum (dr. scient.). The introduction relates the subject matter of the submitted papers to current discussions in computer science. Section two gives a brief account of the research area in question, how it has developed over the last 25 year~, and its current status. Section three presents and discusses the results structured according to the frame of reference given in section two. For each sub-area the central issues are introduced. Then the results obtained are presented; the practical as well as the theoretical. Finally, a short comparison and evaluation in relation to relevant literature is made. Section four contains a short discussion of the methods used, and section five presents ideas for future research

Bridging the Gap Between Politics and Techniques: On the next practices of participatory design

This paper discusses how we in the participatory design (PD) research community may contribute to the evolution of ICT design1 practices into something that is much more attuned to people using ICT and to their interests. The main idea is that to do so we need to focus more on issues in the gap between politics and techniques, e.g., project funding, types of users and of use settings, the role of companies and of Intellectual Property Rights and the types of projects we work on. The paper presents material illustrating that impor- tant changes are going on in the dimensions outlined by these issues and argues that these changes create important, new opportunities for PD to contribute to the ‘next practices’ of ICT design—as well as serious problems. Thus to exploit these new opportunities we need to improve our understanding of the issues involved and to develop new ways of taking them into account when we design and do research projects

On Creating and Sustaining Alternatives: The case of Danish Telehealth

This paper presents and discusses an initiative aimed at creating direct and long lasting influence on the use and development of telemedicine and telehealth by healthcare professionals, patients and citizens. The initiative draws on ideas, insights, and lessons learned from Participatory Design (PD) as well as from innovation theory and software ecosystems. Last, but not least, the ongoing debate on public finances/economy versus tax evasion by major private companies has been an important element in shaping the vision and creating support for the initiative. This vision is about democratic control, about structures for sustaining such control beyond initial design and imple- mentation and about continued development through Participatory Design projects. We see the “middle element”, the structures for sustaining democratic control beyond initial design and implementation as the most important and novel contribution of the paper