The unfoldability conjecture for billiard triangles in Weyl chambers

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Let Δ⊂E\Delta\subset E be a Weyl chamber with tip at a special vertex oo. Let x0,xk∈Δx_0,x_k\in\Delta be special vertices, and consider a billiard triangle with geodesic sides ox0‾\overline{ox_0} and oxk‾\overline{ox_k} and broken side x0x1…xk−1xkx_0x_1\ldots x_{k-1}x_k. A billiard triangle is unfoldable when it can be unfolded to a geodesic triangle in the ambient symmetric space or building. It is a generalized Littelmann triangle when its broken side is a weak LS path, namely a broken geodesic satisfying Littelmann's axioms without requiring dist⁡(λi−1,λi)=1\operatorname{dist}(\lambda_{i-1},\lambda_i)=1 in the definition of an aa-chain.

Unfoldability conjecture. A billiard triangle in Δ\Delta with sides ox0‾\overline{ox_0}, oxk‾\overline{ox_k} and x0x1…xk−1xkx_0x_1\ldots x_{k-1}x_k, and with the special vertices 0,x0,xk0,x_0,x_k, is unfoldable if and only if it is a generalized Littelmann triangle.

This conjecture characterizes unfoldable billiard triangles through weak LS paths, extending the known correspondence between Littelmann triangles and tensor-product inclusions for representations of the dual group. The supplied source does not state whether the conjecture has been resolved.

References

Primary source

Michael Kapovich, Bernhard Leeb and John J. Millson, “The generalized triangle inequalities in symmetric spaces and buildings with applications to algebra”, arXiv:math/0210256 (2005).

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