Journal of Lie Theory

Let $A$ be a symmetrizable generalized Cartan matrix with corresponding Kac-Moody algebra $\frak{g}$ over $\mathbb Q$. Let $V=V^{\lambda}$ be an integrable highest weight $\frak{g}$-module with dominant regular integral weight $\lambda$ and representation $\rho: \frak{g} \to$ End($V$), and let $V_\mathbb Z=V^{\lambda}_\mathbb{Z}$ be a $\mathbb Z$-form of $V$. Let $G_V(\mathbb Q)$ be the associated minimal Kac-Moody group generated by the automorphisms $\exp(t\rho(e_{i}))$ and $\exp(t\rho(f_{i}))$ of $V$, where $e_i$ and $f_i$ are the Chevalley-Serre generators and $t\in\mathbb Q$. Let $G(\mathbb Z)$ be the group generated by $\exp(t\rho(e_{i}))$ and $\exp(t\rho(f_{i}))$ for $t\in\mathbb Z$. Let $\Gamma(\mathbb Z)$ be the Chevalley subgroup of $G_V(\mathbb Q)$, that is, the subgroup that stabilizes the lattice $V_{\mathbb Z}$ in $V$. For a subgroup $M$ of $G_V(\mathbb Q)$, we say that $M$ is integral if $M\cap G(\mathbb Z) = M\cap \Gamma(\mathbb Z)$ and that $M$ is strongly integral if there exists $v\in V_\mathbb Z$ such that $g\cdot v\in V_{\mathbb{Z}}$ implies $g\in G({\mathbb{Z}})$ for all $g\in M$. We prove strong integrality of inversion subgroups $U_{(w)}$ of $G_V(\mathbb Q)$ for $w$ in the Weyl group, where $U_{(w)}$ is the group generated by positive real root groups that are flipped to negative root groups by $w^{-1}$. We use this to prove strong integrality of subgroups of the unipotent subgroup $U$ of $G_V(\mathbb Q)$ that are generated by commuting real root groups. When $A$ has rank 2, this gives strong integrality of subgroups $U_1$ and $U_2$ where $U=U_{1}{\Large{*}}\ U_{2}$ and each $U_{i}$ is generated by `half' the positive real roots.