Relaxation Times in Strictly Disk Systems
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. Therefore 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 result to numerical experiments on galactic structure is discussed.
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