Diagrammatics for Kazhdan-Lusztig R-polynomials

Abstract: Let $(W,S)$ be an arbitrary Coxeter system. We introduce a family of polynomials, ${ \tilde{\mathcal{R}}{u,\underline{v}}(t)}$, indexed by pairs $(u,\underline{v})$ formed by an element $u\in W$ and a (non-necessarily reduced) word $\underline{v}$ in the alphabet $S$. The polynomial $\tilde{\mathcal{R}}{u,\underline{v}}(t)$ is obtained by considering a certain subset of Libedinsky's light leaves associated to the pair $(u,\underline{v})$. Given a reduced expression $\underline{v}$ of an element $v\in W$; we show that $\tilde{\mathcal{R}}{u,\underline{v}}(t)$ coincides with the Kazhdan-- Lusztig $\tilde{R}$-polynomial $\tilde{R}{u,v}(t)$. Using the diagrammatic approach, we obtain some closed formulas for $\tilde{R}$- polynomials.