Weak combinatorial invariance conjecture for dominant affine Bruhat intervals

Let W=ZΦ∨⋊WfW=\mathbb{Z}\Phi^\vee\rtimes W_f be an affine Weyl group, let W+W_+ be the set of dominant elements, and let Λ∨\Lambda^\vee be the coweight lattice. Let GG be the group of Bruhat-order automorphisms generated by inversion and Dynkin-diagram symmetries. For x,y∈Wx,y\in W, write y−x∈Λ∨y-x\in\Lambda^\vee when y=x+λy=x+\lambda for some λ∈Λ∨\lambda\in\Lambda^\vee. An interval is full as in the source's affine Bruhat-interval terminology.

Weak combinatorial invariance conjecture. If x,x′,y,y′∈W+x,x',y,y'\in W_+ satisfy that [x,y][x,y] and [x′,y′][x',y'] are full and poset-isomorphic, and there exists g0∈Gg_0\in G with x−g0x′∈Λ∨x-g_0x'\in\Lambda^\vee, then there exists g∈Gg\in G such that

y−gy′=x−gx′∈Λ∨.y-gy'=x-gx'\in\Lambda^\vee.

The conjecture is motivated by computational evidence and is stated for arbitrary affine Weyl groups. The source notes that it is established for thick intervals in type A~2\widetilde{A}_2, but gives no general resolution.

References

Primary source

Gaston Burrull, Nicolas Libedinsky and Rodrigo Villegas, “Shape and class of Bruhat Intervals”, arXiv:2507.14033 (2025).

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