1. Whether a “few” means less than 10 or several hundred is by no means intuitively clear.
2. H. Grad,Handbuch der Physik, edited by S. Flügge (Springer-Verlag, Berlin, 1958), Vol. XII, Sec. 26;
3. Asymptotic Theory of the Boltzmann Equation
4. A Model for Collision Processes in Gases. I. Small Amplitude Processes in Charged and Neutral One-Component Systems
5. D. R. Willis, Princeton University Department of Aeronautical Engineering Report No. 442 (1958).