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Mathematics  2015 

Combinatorics of symmetric plabic graphs

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A plabic graph is a planar bicolored graph embedded in a disk, which satisfies some combinatorial conditions. Postnikov's boundary measurement map takes the space of positive edge weights of a plabic graph $G$ to a positroid cell in $\text{Gr}_{\geq 0}(k,n)$. In this note, we investigate plabic graphs which are symmetric about a line of reflection, up to reversing the colors of vertices. These symmetric plabic graphs arise naturally in the study of total positivity for the Lagrangian Grassmannian. We characterize various combinatorial objects associated with symmetric plabic graphs, and describe the subset of $\text{Gr}_{\geq 0}(k,n)$ which can be realized by symmetric weightings of symmetric plabic graphs.


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