Past exam of the mathematics course of the University of Cambridge / 2008 / iii / Paper 3 / 2 / i

Work with a complex semisimple Lie algebra and its root system $R$ in the real span $E$ of the roots of a root system. The Killing form induces a positive definite inner product on this real root space, after the usual identification with the dual of the real span of the coroots. For each root $\alpha$, its Weyl reflection isThe Weyl group is $\boxed{W=⟨s_{\alpha }:\alpha \in R⟩}$. The structural facts being used are that these reflections preserve the finite root system, that the roots span $E$, and that the reflections in the simple roots already generate $W$. Thus $W$ is finite: it acts faithfully as permutations of $R$. Equivalently, it is the normalizer of the Cartan subalgebra modulo its centralizer in the associated adjoint group.