An overview of the techniques developed to circumvent computational intractability, a key challenge in many areas of computer science.

Multivariate problems occur in many applications. These problems are defined on spaces of $d$-variate functions and $d$ can be huge--in the hundreds or even in the thousands. Some high-dimensional problems can be solved efficiently to within $\varepsilon$, i.e., the cost increases polynomially in $\varepsilon^{-1}$ and $d$. However, there are many multivariate problems for which even the minimal cost increases exponentially in $d$. This exponential dependence on $d$ is called intractability or the curse of dimensionality. This is the first volume of a three-volume set comprising a comprehensive study of the tractability of multivariate problems. It is devoted to tractability in the case of algorithms using linear information and develops the theory for multivariate problems in various settings: worst case, average case, randomized and probabilistic. A problem is tractable if its minimal cost is not exponential in $\varepsilon^{-1}$ and $d$. There are various notions of tractability, depending on how we measure the lack of exponential dependence. For example, a problem is polynomially tractable if its minimal cost is polynomial in $\varepsilon^{-1}$ and $d$. The study of tractability was initiated about 15 years ago. This is the first and only research monograph on this subject. Many multivariate problems suffer from the curse of dimensionality when they are defined over classical (unweighted) spaces. In this case, all variables and groups of variables play the same role, which causes the minimal cost to be exponential in $d$. But many practically important problems are solved today for huge $d$ in a reasonable time. One of the most intriguing challenges of the theory is to understand why this is possible. Multivariate problems may become weakly tractable, polynomially tractable or even strongly polynomially tractable if they are defined over weighted spaces with properly decaying weights. One of the main purposes of this book is to study weighted spaces and obtain necessary and sufficient conditions on weights for various notions of tractability. The book is of interest for researchers working in computational mathematics, especially in approximation of high-dimensional problems. It may be also suitable for graduate courses and seminars. The text concludes with a list of thirty open problems that can be good candidates for future tractability research.

This is the second volume of a three-volume set comprising a comprehensive study of the tractability of multivariate problems. The second volume deals with algorithms using standard information consisting of function values for the approximation of linear and selected nonlinear functionals. An important example is numerical multivariate integration. The proof techniques used in volumes I and II are quite different. It is especially hard to establish meaningful lower error bounds for the approximation of functionals by using finitely many function values. Here, the concept of decomposable reproducing kernels is helpful, allowing it to find matching lower and upper error bounds for some linear functionals. It is then possible to conclude tractability results from such error bounds. Tractability results, even for linear functionals, are very rich in variety. There are infinite-dimensional Hilbert spaces for which the approximation with an arbitrarily small error of all linear functionals requires only one function value. There are Hilbert spaces for which all nontrivial linear functionals suffer from the curse of dimensionality. This holds for unweighted spaces, where the role of all variables and groups of variables is the same. For weighted spaces one can monitor the role of all variables and groups of variables. Necessary and sufficient conditions on the decay of the weights are given to obtain various notions of tractability. The text contains extensive chapters on discrepancy and integration, decomposable kernels and lower bounds, the Smolyak/sparse grid algorithms, lattice rules and the CBC (component-by-component) algorithms. This is done in various settings. Path integration and quantum computation are also discussed. This volume is of interest to researchers working in computational mathematics, especially in approximation of high-dimensional problems. It is also well suited for graduate courses and seminars. There are 61 open problems listed to stimulate future research in tractability.

This thesis is divided into two main parts. In the first part, we consider the problem of efficiently computing wavefront estimates for use in adaptive optics hardware on ground-based telescopes. Our contribution is a warm-started single-iteration multigrid algorithm that performs as well as conventional vector-matrix-multiplication methods, but at a fraction of the computational cost. We used numerical simulations to compare our algorithm to a variety of other published methods, and validated our findings at the Palomar Observatory. In the second part, we consider feedback control subject to an information constraint. Using a novel algebraic framework, we are able to prove many structural results, including a new convexity result, in a natural and purely algebraic way. We also develop a new condition we call internal quadratic invariance, under which the controller synthesis can be cast as a convex optimization problem. This describes the most general class of tractable decentralized control problems known to date. Both parts of the thesis fit into the broader question of tractability of complex systems. In the first part we look at a practical example which is difficult because of the large number of sensors and actuators. In the second part, we look at decentralized control, which is difficult because of the non-classical information constraint.

Inhaltsangabe:Abstract: This work brings together two areas of science biology and informatics that have only recently been connected in the emerging (and vastly growing) research field of Bioinformatics. In order to achieve a common basis for Parts 2 and 3 of this work, Part 1 intends to introduce the computer scientist to the relevant biological background and terminology (Chapter 2), and to familiarize the biologist with the relevant topics from theoretical computer science (Chapter 3). Chapter 2 first introduces some terminology from the field of genetics, thereby defining SNPs. We then motivate the analysis of SNPs by two applications, i.e. the analysis of evolutionary development and the field of pharmacogenetics. Especially the field of pharmacogenetics is capable of having an enormous impact on medicine and the pharmaceutical industry in the near future by using SNP data to predict the efficacy of medication. Chapter 3 gives a brief introduction to the field of computational complexity. We will see and motivate how algorithms are analyzed in theoretical computer science. This will lead to the definition of complexity classes , introducing the class NP which includes computationally hard problems. Some of the hard problems in the class NP can be solved efficiently using the tool of fixed-parameter tractability, introduced at the end of this chapter. An important application of SNP data is in the analysis of the evolutionary history of species development (phylogenetic analysis part two chapters 4 and 5). As will be made plausible in Chapter 5 using SNP data is in many ways superior to previous approaches of phylogenetic analysis. In order to analyze the development of species using SNP data, an underlying model of evolution must be specified. A popular model is the so-called perfect phylogeny, but the construction of this phylogeny is a computationally hard problem when there are inconsistencies (such as read-errors or an imperfect .t to the model of perfect phylogeny) in the underlying data. Chapter 4 analyzes the problem of forbidden submatrix removal which is closely connected to constructing perfect phylogenies we will see in Chapter 5 that its computational complexity is directly related to that of constructing a perfect phylogeny from data which is partially erroneous. In this chapter, we analyze the algorithmic tractability of forbidden submatrix removal , characterizing cases where this problem is NP-complete (being [...]