Enflo (mechanics, Kungl. Tekniska Högskolan, Stockholm) and Hedberg (mechanical engineering, Blekinge Tekniska Höskola, Karlskrona, Sweden) explain the theory with an equal stress on physical and mathematical foundations, believing that a unified approach is necessary that encompasses both the phenomena studied and the mathematical techniques developed during the past few decades. Their example phenomena are chosen to provide physically interesting illustrations of the mathematical theory. The treatment should be accessible to readers with a graduate level background in physics and mathematical analysis.