In geometry, a Coxeter–Dynkin diagram is a graph with numerically labelled edges representing the spatial relations between a collection of mirrors (or reflecting hyperplanes). It describes a kaleidoscopic construction: each graph node represents a mirror (domain facet) and the label attached to a graph edge encodes the dihedral angle order between two mirrors (on a domain ridge).
In addition, when used to represent a specific uniform polytope, the diagram has rings (circles) around nodes for active mirrors and hollow nodes (holes) to represent alternation.
Each diagram represents a Coxeter group. Dynkin diagrams are used to classify root systems and therefore Lie algebras.[1]
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