a>There are two mathematical–physical descriptions of fluid dynamics. The first of them is a microscopic description, from the Boltzmann equation for the (one-particle) distribution function \( f({\hbox{t}}, {\hbox{x}};{ }\xi ) \): $$ \partial f/\partial {\hbox{t }}\!\! + { }\xi { }.\nabla f{ } = { }\left( {{1}/{\hbox{Kn}}} \right)Q\left( {f;f} \right), $$ where \( f({\hbox{t}},{\hbox{x}};{ }\xi ) \) is precisely the density probability of finding a molecule at the space-position \( {\mathbf{x}} \) (at the time t), with the velocity \( \xi \).