[1] R. K. Getor: 
The shift operator for non-stationary stochastic processes. Duke Math. J. 23 (1965), 175-187. 
MR 0074717 
[2] K. Karhunen: 
Uber lineare Methoden in der Wahrscheinlichkeitsrechnung. Ann. Acad. Sci. Fenn. Ser. A I Math. 37 (1947), 3-79. 
MR 0023013 
[3] M. Loève: Fonctions aléatoires du second ordre. In: P. Lévy, Processus Stochastiques et Mouvement Brownien. Paris 1948.
[4] J. Michálek: 
Random sequences with normal covariances. Kybernetika 23 (1987), 6, 443-457. 
MR 0922621 
[5] J. Michálek: 
Locally stationary covariances. In: Trans. Tenth Prague Conf. on Inform. Theory, Statist. Dec. Functions, Random Processes, Academia, Prague 1988, pp. 83-103. 
MR 1136266 
[6] J. Michálek: 
Normal covariances. Kybernetika 24 (1988), 1, 17-27. 
MR 0936550 
[7] Y. A. Rozanov: Stationary random processes. Fizmatgiz, Moscow 1963. In Russian.
[8] R. A. Silverman: 
Locally stationary random processes. IRE Trans. Inform. Theory IT-3 (1957), 3, 182-187. 
MR 0090931 
[9] D. V. Widder: 
The Laplace Transform. Princeton University Press, Princeton 1946. 
MR 0005923