Can anybody help me with two confusions regarding notations in group theory/topology? 1. When we consider symmetry breaking pattern: SU(5) -> [SU(3)XSU(2)XU(1)]/Z_6 what does Z_6 mean? What is the significance of ..../Z_6? is it some kind of mirror symmetry? 2. Is there any difference between two spacetimes: S^1/Z_2 and S^1XZ_2 ? note that the first one denotes compactification in Randall-Sundrum models. I have never seen the second notation but my confusion is am I allowed to write Z_2 term this way or not. When should we write X and when use / notations? My guess is that when we use X sign we mean one or more dimension(s) and its nature(like S^1XR^n etc) and when we use / notation we mean a condition on the immediately previous dimension (in this case miror symmetry). is that correct? Thanks.