in the floor" (for example, Hilbert's 4-th problem).
In some cases, more than one proof to get the
offered; I believe in the maxim that it is more instructi
ferent proofs to the same result than the same proof t
results. Much attention is paid to examples: the be
derstand a general concept is to study, in detail, the f
I am grateful to the colleagues and to the student
cussed billiards with and learned from; they are too n
mentioned here by name. It is a pleasure to acknowled
of the National Science Foundation.