“…The eigenvalue problem has been solved for several other Gaussian processes and other Hilbert spaces including stationary processes with regular highfrequency behaviour of the spectral density, fractional Brownian sheets, Lévy's fractional Brownian motion, integrated Gaussian processes and Gaussian martingales. See [15], [14, p. 79], [11], [2], [9], [3], [12], [13].…”