In regards to direct product representations, I'm trying to find a proof for:
$R(X_iY_j)=R(X_i)R(Y_j)$
Where $R$ is a symmetry operation of a group and $X_i$ and $Y_j$ are members of different basis sets for the group. See e.g. Chemical Applications of Group Theory by F.A. Cotton, 3rd ed., p.105 or Atkins' Molecular Quantum Mechanics, 5th ed.,p.152.
A lot of books I've seen covering direct product representaions start with this equality but I haven't seen any justification for it yet.
Any help is greatly appreciated!