Characterization of Cyclically Fully commutative elements in finite and affine Coxeter Groups

Abstract

An element of a Coxeter group W is fully commutative if any two of its reduced decompositions are related by a series of transpositions of adjacent commuting generators. An element of a Coxeter group W is cyclically fully commutative if any of its cyclic shifts remains fully commutative. These elements were studied in Boothby et al.. In particular the authors enumerated cyclically fully commutative elements in all Coxeter groups having a finite number of them. In this work we characterize and enumerate cyclically fully commutative elements according to their Coxeter length in all finite or affine Coxeter groups by using a new operation on heaps, the cylindric transformation. In finite types, this refines the work of Boothby et al., by adding a new parameter. In affine type, all the results are new. In particular, we prove that there is a finite number of cyclically fully commutative logarithmic elements in all affine Coxeter groups. We study afterwards the cyclically fully commutative involutions and prove that their number is finite in all Coxeter groups.