Rigid toric matrix Schubert varieties

For a given permutation $π\in S_N$, Fulton proves that the matrix Schubert variety $\overline{X_π} \cong Y_π \times \mathbb{C}^q$ can be defined via certain rank conditions encoded in the Rothe diagram of $π$. In the case where $Y_π:=\text{TV}(σ_π)$ is toric (with respect to a $(\mathbb{C}^*)^{2N-1}$ action), we show that it can be described as an edge ideal of a bipartite graph $G^π$. We characterize the lower dimensional faces of the associated so-called edge cone $σ_π$ explicitly in terms of subgraphs of $G^π$ and present a combinatorial study for the first order deformations of $Y_π$. We prove that $Y_π$ is rigid if and only if the three-dimensional faces of $σ_π$ are all simplicial. Moreover, we reformulate this result in terms of Rothe diagram of $π$.

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