Torus Actions on Matrix Schubert and Kazhdan-Lusztig Varieties, and their Links to Statistical Models

Abstract: We investigate the toric geometry of two families of generalised determinantal varieties arising from permutations: Matrix Schubert varieties ($\overline{X_w}$) and Kazhdan-Lusztig varieties ($\mathcal{N}<em>{v,w}$). Matrix Schubert varieties can be written as $\overline{X_w} = Y_w \times \mathbb C<sup>d$, where $d$ is maximal. We are especially interested in the structure and complexity of these varieties $Y_w$ and $\mathcal{N}</em>{v,w}$ under the so-called usual torus actions. In the case when $Y_w$ is toric, we provide a full characterisation of the simple reflections $s_i$ that render ${Y_{w \cdot s_i}}$ toric, as well as the corresponding changes to the weight cone. For Kazhdan-Lusztig varieties, we consider how moving one of the two permutations $v,w$ along a chain in the Bruhat poset affects their complexity. Additionally, we study the complexity of these varieties, for permutations $v$ and $w$ of a specific structure. Finally, we consider the links between these determinantal varieties and two classes of statistical models; namely conditional independence and quasi-independence models.