Variational Approach to the Dynamics of Quantum Fields

In the summer of 1995, Dominique Vautherin came to Kyoto and stayed three months at the Yukawa Institute of Kyoto University where he delivered a series of very informal, pedagogical lectures on the application of variational methods to quantum field theories. [1] This initiated our long lasting enjoyable collaboration on the subject and Yasuhiko Tsue joined the force later. Throughout this collaboration Vautherin was always enthusiastic to explore new physics problems and always came up with new innovative ideas, much of which I suspect have origins in his expertise in nuclear many-body theory. We are very sorry that this fruitful collaboration with Vautherin came to end so soon by his untimely death and that we are no longer able to be inspired by his deep physics insights, charmed by his elegance, and most of all cheered up by his very warm presence. I like to dedicate these lectures to the memory of Dominique Vautherin from whom I learned most of the material presented below.