Line bundles on $G$-Bott-Samelson-Demazure-Hansen varieties

TL;DR

This paper characterizes line bundles on Bott-Samelson-Demazure-Hansen varieties, providing conditions for their positivity properties and identifying when these varieties are Fano or weak-Fano.

Contribution

It offers necessary and sufficient conditions for BSDH-varieties to be Fano or weak-Fano, and describes the structure of line bundles and the Picard group on these varieties.

Findings

Characterization of Fano and weak-Fano BSDH-varieties.

Line bundles on $Z_w$ are globally generated iff nef.

Picard group of $ ilde{Z}_w$ is free abelian with an explicit basis.

Abstract

Let $G$ be a semi-simple simply connected algebraic group over an algebraically closed field $k$ of arbitrary characteristic. Let $B$ be a Borel subgroup of $G$ containing a maximal torus $T$ of $G.$ Let $W$ be the Weyl group of $G$ with respect to $T$. For an arbitrary sequence $w=(s_{i_{1}},s_{i_{2}},\ldots, s_{i_{r}})$ of simple reflections in $W,$ let $Z_{w}$ be the Bott-Samelson-Demazure-Hansen variety (BSDH-variety for short) corresponding to $w.$ Let $\widetilde{Z_{w}}:=G\times^{B}Z_{w}$ denote the fibre bundle over $G/B$ with the fibre over $B/B$ is $Z_{w}.$ In this article, we give necessary and sufficient conditions for the varieties $Z_{w}$ and $\widetilde{Z_{w}}$ to be Fano (weak-Fano). We show that a line bundle on $Z_{w}$ is globally generated if and only if it is nef. We show that Picard group $\text{Pic}(\widetilde{Z_{w}})$ is free abelian and we construct a $\mathcal{O}(1)$-basis. We characterize the nef, globally generated, ample and very ample line bundles on $\widetilde{Z_{w}}$ in terms of the $\mathcal{O}(1)$-basis.