A symmetric group action on the irreducible components of the Shi variety associated to $W(\widetilde{A}_n)$

Nathan Chapelier-Laget

Published 2020-10-12Version 1

Let $W_a$ be an affine Weyl group with corresponding finite root system $\Phi$. In the article "Alcoves corresponding to an affine Weyl group" Jian-Yi Shi characterized each element $w \in W_a$ by a $ \Phi^+$-tuple of integers $(k(w,\alpha))_{\alpha \in \Phi^+}$ subject to certain conditions. In the article "Shi variety corresponding to an affine Weyl group" a new interpretation of the coefficients $k(w,\alpha)$ is given. This description led us to define an affine variety $\widehat{X}_{W_a}$, called the Shi variety of $W_a$, whose integral points are in bijection with $W_a$. It turns out that this variety has more than one irreducible component, and the set of these components, denoted $H^0(\widehat{X}_{W_a})$, admits many interesting properties. In particular the group $W_a$ acts on it. In this article we show that the irreducible components of $\widehat{X}_{W(\widetilde{A}_n)}$ are in bijection with the circular permutations of $W(A_n) = S_{n+1}$. We also compute the action of $W(A_n)$ on $H^0(\widehat{X}_{W(\widetilde{A}_n)})$.