On the Mean Number of Crossings of a Level by a Stationary Gaussian Process

Let $\xi(t)$ be a stationary Gaussian process and $N_\xi (u)$ denote the number of solutions of $\xi(t)=u, 0\ne t\ne1$. We prove the well-known formula for $\mathbf M_\xi(u)$ under conditions that are very close to the necessary ones.