On spectral domain of periodically correlated processes

The content of this article primarily falls into three sections. Section 1 deals with a basic structural spectral representation theorem for periodically correlated sequences. Section 2 provides a certain class of square integrable functions that is isomorphic to the time domain of the sequence. This complete class is called the spectral domain of the sequence. Section 3 is related to the unsuccessful previous attempts by others to construct a complete function space as the spectral domain. These are natural extensions of the known results for the stationary case to the periodically correlated sequences.