Hello everybody

I have a bit of a problem with understanding the conversion from a WSCS process to a WSS process . With the time shift being a uniform random variable on , independent of and being the period of the mean function of

The problem begins with the method to find the mean function of :

First, and it might seem very basic, I don't get the syntax

And second, why by averaging the mean of the WSCS process over its period would we get the mean function of the WSS process ?

If I understand that I could understand the same kind of process used to find the autocorrelation function of from the autocorrelation function of

Please help me !!