Weight-Two Classification for positive tropical Grassmannians

Let I,J([n]k)ncycI,J\in {\binom{[n]}{k}}^{\mathrm{ncyc}} be noncrossing but not weakly separated. For each such II, let tI\mathfrak{t}_I denote the corresponding noncrossing parameter, and let ρ\rho be the positive parameterization into the positive tropical Grassmannian.

Weight-Two Classification. The point

ρ(tI+tJ)\rho(\mathfrak{t}_I+\mathfrak{t}_J)

lies in the direction of a ray of Trop>0X(k,n)\operatorname{Trop}_{>0}X(k,n). Every ray of Trop>0X(k,n)\operatorname{Trop}_{>0}X(k,n) with PK weight 22 arises in this way from a unique noncrossing but not weakly separated pair I,J\\{I,J\\}.

This conjecture gives the proposed classification of the weight-two rays of the positive tropical Grassmannian, extending the preceding classification of weight-one rays. It was stated previously in the cited work, but its resolution is not specified in the supplied context.

Sources & referencesView supporting material

Primary source

Nick Early and Thomas Lam, “Noncrossing Duality and the Geometry of Positive Tropical Linear Spaces”, arXiv:2604.25212 (2026).

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