  We show that any bijection between two root systems that preserves angles
(but not necessarily lengths) gives rise to inequalities relating tensor
product multiplicities for the corresponding complex semisimple Lie groups (or
Lie algebras). We explain the inequalities in two ways: combinatorially, using
Littelmann's Path Model, and geometrically, using isogenies between algebaric
groups defined over an algebraically closed field of positive characteristic.
