Formula_15731

\[\begin{gathered}D\left(t_{1}, t_{2}\right) \equiv\left\langle\left(\xi_{t_{1}}-\left\langle\xi_{t_{1}}\right\rangle\right)\left(\xi_{t_{2}}-\left\langle\xi_{t_{2}}\right\rangle\right)\right\rangle= \\=\int_{\Lambda} \exp \left\{i\left[\lambda,\left(t_{1}-t_{2}\right)\right]\right\} p(\lambda) d \lambda,\end{gathered}\]