Fellow stackexchangers, I did my best to put a topic that describes the question that I am going to ask. I am reading Probability, Random Variables, and Stochastic Processes by Papoulis and I am having problem with the proof of one of the theorems (bottom left corner of attached image). As I'm showing with blue bounding box in the image, the author reaches the conclusion that blue bounding box is equivalent to (9-71). I don't understand how can we get rid of c in the blue bounding box because our signal is not strict sense stationary so how can we get rid of c then. Below the blue bounding box, the author is saying see 9-70 but not sure how this can lead to the aforementioned conclusions.
