Twist equivalence for Nichols algebras over Coxeter groups

Abstract: Bazlov generalized the construction of Fomin-Kirillov algebras to arbitrary finite Coxeter groups. They are quadratic approximations of Nichols algebras associated with the conjugacy class of reflections and a (rack) 2-cocycle q^+ with values in {-1,1}. We prove that q^+ is twist-equivalent to the constant cocycle q^-=-1, generalising a result of Vendramin. As a consequence, the Nichols algebras associated with the two different cocycles have the same Hilbert series and one is quadratic if and only if the other is quadratic. We further apply a recent result of Heckenberger, Meir and Vendramin and Andruskiewitsch, Heckenberger and Vendramin to complete the missing cases in the classification of finite-dimensional Nichols algebras of Yetter-Drinfeld modules over the dihedral groups.