This is the first research-level monograph to be written on this vast
and multi-faceted subject on the interface of mathematics and
theoretical physics. The book gives a unified account of what is known,
established theory and a state-of-the-art account of what is still
developing. The mathematical content is not easy, but the results are
nevertheless in demand, not only from physicists, but from engineers
working for example in semi-conductors and composite materials and other
branches of chemical engineering. The book contains many concrete
examples, for such applications.