Direct involvement with the subject area of the present work dates from my years with NASA at its Electronics Research Center (ERC) in Cambridge, M- sachusetts, in the 1960s. However, my approach to the problems of mathem- ical physics had been shaped earlier in my time as a graduate student in the Mathematics Department at MIT. The passage of time tends merely to further enhance my appreciation of that graduate study program, where I had the b- e't of the intensive courses from Norman Levinson, C.-C. Lin, Jurgen ] Moser, and Eric Reissner. In the case of Reissner, my years as research...