Relaxation Times in Strictly Disk Systems (Papers appear in the Proceedings of IAU Colloquium No. 10 Gravitational N-Body Problem (ed. by Myron Lecar), R. Reidel Publ. Co. , Dordrecht-Holland.)
It is shown that the time of relaxation by particle encounters of self-gravitating systems in the plane interacting by 1/r 2 forces is of the same order of magnitude as the mean orbit time. There-fore such a system does not have a Vlasov limit for large numbers of particles, unless appeal is made to some non-zero thickness of the disk. The relevance of this results to numerical experiments on galactic structure is discussed.