Stationary and Isotropic Random Functions

We are going to deal with random functions \( X\left( {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}\to {M} } \right) = X\left( {{x_1}, \ldots ,{x_n}} \right) \) of a point \( \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}\to {M} \in {E_n} \) (E n: -dimensional Euclidean space).