Crystallographic root system ($2(\beta ,\alpha )/(\alpha ,\alpha )\in \mathbb{Z}$)

A finite root system in a real inner-product space is crystallographic when every Cartan integer $2(\beta ,\alpha )/(\alpha ,\alpha )$ is an integer. The roots of a complex semisimple Lie algebra satisfy this because each number is a weight of its sl2 subalgebra associated with a root. Reducedness is a separate axiom; the root-space reducedness lemma establishes it for these Lie-algebra roots.