This volume of the Encyclopaedia contains threecontributions in the field of complex analysis. The topicstreated are mean periodicity and convolutionequations, Yang-Mills fields and the Radon-Penrose transform, andstringtheory. The latter two have strong links with quantumfield theory and the theory of general relativity. In fact, the mathematical results described inthe book arose fromthe need of physicists to find a sound mathematical basisfor their theories. The authors present their material inthe formof surveys which provide up-to-date accounts ofcurrent research. The book will be immensely useful to graduate students andresearchers in complex analysis, differential geometry, quantum field theory, string theoryand general relativity.