Symmetry Properties of High-Order Spectral Densities of Stationary and Periodic-Nonstationary Stochastic Processes

We investigate the symmetry properties of spectral densities of orders $\nu=\overline{2,6}$ of a periodic-nonstationary stochastic process $\xi(t)$, $T\in\mathbf R$. In parallel, we describe the symmetry properties of the high-order spectral densities of stationary stochastic processes in discrete and continuous time.