Schur's Lemma
Theorem. (Schur’s Lemma) Assume $T:G\to GL(V)$ is an irreducible complex resentation and $A\in \text{End}(T)$. Then $A=\lambda\text{Id}$ for some $\lambda\in \mathbb{C}$.
Theorem. (Schur’s Lemma) Assume $T:G\to GL(V)$ is an irreducible complex resentation and $A\in \text{End}(T)$. Then $A=\lambda\text{Id}$ for some $\lambda\in \mathbb{C}$.