Upper-root description of the Cambrian cluster–noncrossing bijection

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Let WW be a finite Coxeter group, let cc be a Coxeter element, and let a cc-cluster CC have upper roots among its roots. Associate to CC the intersection of the hyperplanes orthogonal to those upper roots. Upper-root bijection conjecture. The resulting map from cc-clusters to cc-noncrossing subspaces is a bijection and agrees with the established map

NC⁡c∘cl⁡c−1.\operatorname{NC}_c\circ\operatorname{cl}_c^{-1}.

This conjecture is presented as an immediate consequence of the span-intersection conjecture together with the preceding theorem identifying NC⁡c∘cl⁡c−1\operatorname{NC}_c\circ\operatorname{cl}_c^{-1} using the lower roots; the source reports computational verification of the needed orthogonality in rank at most 77.

References

Primary source

Nathan Reading and David E Speyer, “Cambrian fans”, arXiv:math/0606201 (2008).

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