Hi, I'm recently reading some text on particle physics and there is a section on symmetries and group theory. It gave the definition of SU2 as the group of unitary 2*2 matrices and that SU3 is the group of unitary 3*3 matrices. However, it kind of confuses me when it mentioned representations of higher orders. What's the difference between a 3*3 representation of SU2 and SU3? Also, I don't really understand what it means when it mentioned something like "invariant under SU2 transformations", can anyone give an example of a vector that's invariant under SU2 transformations? Thanks!