Reflection subgroups of finite and affine Weyl groups
Reflection subgroups of finite and affine Weyl groups
We give complete classifications of the reflection subgroups of finite and affine Weyl groups from the point of view of their root systems. A short case-free proof is given of the well-known classification of the isomorphism classes of reflection subgroups using completed Dynkin diagrams, for which there seems to be …