Take a real vector space $R$ transforming in the adjoint representation of the ${\rm E}_7(7)$ Lie group as $R \rightarrow G R G^{-1}$. One can define invariants using traces of products of $R$ as ${\rm Tr}[R^k]$.
I heard that a basis of invariants is given by $k = 2,6,8,10,12,14,18$. Is this correct? Which theorem states this?
Thanks for your help