This volume of the Encyclopaedia contains three contributions in the field of complex analysis. The topics treated are mean periodicity and convolution equations, Yang-Mills fields and the Radon-Penrose transform, and string theory. The latter two have strong links with quantum field theory and the theory of general relativity. In fact, the mathematical results described in the book arose from the need of physicists to find a sound mathematical basis for their theories. The authors present their material in the form of surveys which provide up-to-date accounts of current research. The book will be immensely useful to graduate students and researchers in complex analysis, differential geometry, quantum field theory, string theory and general relativity.