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arXiv · 2206.14001

Computing positroid cells in the Grassmannian of lines, their boundaries and their intersections

Susama Agarwala·Fatemeh Mohammadi·Francesca Zaffalon
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Abstract

Positroids are families of matroids introduced by Postnikov in the study of non-negative Grassmannians. In particular, positroids enumerate a CW decomposition of the totally non-negative Grassmannian. Furthermore, Postnikov has identified several families of combinatorial objects in bijections with positroids. We will provide yet another characterization of positroids for Gr$_{\geq 0}(2,n)$, the Grassmannians of lines, in terms of certain graphs. We use this characterization to compute the dimension and the boundary of positroid cells. This also leads to a combinatorial description of the intersection of positroid cells, that is easily computable. Our techniques rely on determining different ways to enlarge a given collection of subsets of $\{1,\ldots,n\}$ to represent the dependent sets of a positroid, that is the dependencies among the columns of a matrix with non-negative maximal minors. Furthermore, we provide an algorithm to compute all the maximal positroids contained in a set.

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Susama Agarwala, Fatemeh Mohammadi, Francesca Zaffalon. 2022-06-28. Computing positroid cells in the Grassmannian of lines, their boundaries and their intersections. https://arxiv.org/abs/2206.14001

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