A factorization theorem for affine Kazhdan-Lusztig basis elements

Abstract

The lowest two-sided cell of the extended affine Weyl group $W_e$ is the set $\{w \in W_e: w = x \cdot w_0 \cdot z, \text{for some} x,z \in W_e\}$, denoted $W_{(\nu)}$. We prove that for any $w \in W_{(\nu)}$, the canonical basis element $\C_w$ can be expressed as $\frac{1}{[n]!} \chi_\lambda({\y}) \C_{v_1 w_0} \C_{w_0 v_2}$, where $\chi_\lambda({\y})$ is the character of the irreducible representation of highest weight $\lambda$ in the Bernstein generators, and $v_1$ and $v_2^{-1}$ are what we call primitive elements. Primitive elements are naturally in bijection with elements of the finite Weyl group $W_f \subseteq W_e$, thus this theorem gives an expression for any $\C_w$, $w \in W_{(\nu)}$ in terms of only finitely many canonical basis elements. After completing this paper, we realized that this result was first proved by Xi in \cite{X}. The proof given here is significantly different and somewhat longer than Xi's, however our proof has the advantage of being mostly self-contained, while Xi's makes use of results of Lusztig from \cite{L Jantzen} and Cells in affine Weyl groups I-IV and the positivity of Kazhdan-Lusztig coefficients.