Cluster-adjacency conjecture for facets of amplituhedron positroid tiles

Let ZπZ_{\pi} be a positroid tile of the amplituhedron An,k,m(Z)\mathcal{A}_{n,k,m}(Z). A facet of ZπZ_{\pi} is a codimension-one boundary positroid tile, and suppose such a facet lies on a hypersurface defined by Q(YZI)=0Q(\langle YZ_I\rangle)=0, where QQ is a polynomial in the Plücker coordinates pIp_I on Grm,n\operatorname{Gr}_{m,n}. Define

Facet(Zπ)={Q(pI)a facet of Zπ lies on Q(YZI)=0}.\operatorname{Facet}(Z_{\pi})=\{Q(p_I)\mid \text{a facet of }Z_{\pi}\text{ lies on }Q(\langle YZ_I\rangle)=0\}.

Cluster-adjacency conjecture. Each element of Facet(Zπ)\operatorname{Facet}(Z_{\pi}) is a cluster variable for Grm,n\operatorname{Gr}_{m,n}, and these cluster variables are pairwise compatible. Moreover, if Q~\widetilde Q is a cluster variable compatible with Facet(Zπ)\operatorname{Facet}(Z_{\pi}), then Q~(YZI)\widetilde Q(\langle YZ_I\rangle) has a fixed sign on the interior ZπZ^{\circ}_{\pi} of the tile. This generalizes the established m=2m=2 cluster-adjacency theorem and remains conjectural for m>2m>2.

Sources & referencesView supporting material

Primary source

Lauren K. Williams, “The positive Grassmannian, the amplituhedron, and cluster algebras”, arXiv:2110.10856 (2022).

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