Walter Roth – författare

This book presents a wide-ranging approach to operator-valued measures and integrals of both vector-valued and set-valued functions. It covers convergence theorems and an integral representation for linear operators on spaces of continuous vector-valued functions on a locally compact space. These are used to extend Choquet theory, which was originally formulated for linear functionals on spaces of real-valued functions, to operators of this type.

Locally convex  cones were proposed for applications in approximation theory but have matured into a mathematical theory. The aim of this book is to offer a coherent presentation of its basic principles and later developments and to establish a consistent reference source. The build of the theory is modelled after the cone of convex subsets of a locally convex vector space. It does not satisfy the cancellation law and cannot be embedded into a vector space. The topology of the vector space leads to the definition of three cone topologies. Similar settings occur in integration theory, potential theory, and other mathematical areas. Locally convex topological vector spaces are special cases of locally convex cones. Some of the notions for locally convex cones remain close to those for topological vector spaces. The dual cone consists of all continuous linear functionals that can take infinite values. This yields a rich duality theory, including Hahn-Banach type extension and separation theorems.  Other concepts, such as boundedness and connectedness, extend into different territories. This book serves as a primer for students and a reference tool for researchers.

This book presents a unified approach to Korovkin-type approximation theorems. It includes classical material on the approximation of real-valued functions as well as recent and new results on set-valued functions and stochastic processes, and on weighted approximation. The results are not only of qualitative nature, but include quantitative bounds on the order of approximation. The book is addressed to researchers in functional analysis and approximation theory as well as to those that want to apply these methods in other field. It is largely self-contained, but the reader should have a solid background in abstract functional anaysis. The unified approach is based on a new notion of locally convex ordered cones that are not embeddable in vector spaces but allow Hahn-Banach type separation and extension theorems. This concept seems to be of independent interest.

Integration theory deals with extended real-valued, vector-valued, or operator-valued measures and functions. Different approaches are applied in each of these cases using different techniques. The order structure of the (extended) real number system is used for real-valued functions and measures whereas suprema and infima are replaced with topological limits in the vector-valued case. A novel approach employing more general structures, locally convex cones, which are natural generalizations of locally convex vector spaces, is introduced here. This setting allows developing a general theory of integration which simultaneously deals with all of the above-mentioned cases.