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The 1/3–2/3 Conjecture for Coxeter groups

Yibo Gao, Massachusetts Institute of Technology
Wednesday, December 2, 2020 - 3:30pm to 5:00pm
via Zoom

Note: This talk begins with a pre-seminar (aimed at graduate students) at 3:30–4:00. The main talk starts at 4:10.

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Meeting ID: 950 2088 4635

The 1/3–2/3 Conjecture, originally formulated in 1968, is one of the best-known open problems in the theory of posets, stating that the balance constant of any non-total order is at least 1/3. By reinterpreting balance constants of posets in terms of convex subsets of the symmetric group, we extend the study of balance constants to convex subsets \$C\$ of any Coxeter group. Remarkably, we conjecture that the lower bound of 1/3 still applies in any finite Coxeter group, with new and interesting equality cases appearing. We generalize several of the main results towards the 1/3–2/3 Conjecture to this new setting: we prove our conjecture when \$C\$ is a weak order interval below a fully commutative element in any acyclic Coxeter group (a generalization of the case of width-two posets), we give a uniform lower bound for balance constants in all finite Weyl groups using a new generalization of order polytopes to this context, and we introduce generalized semiorders for which we resolve the conjecture. We hope this new perspective may shed light on the proper level of generality in which to consider the 1/3–2/3 Conjecture, and therefore on which methods are likely to be successful in resolving it. This is joint work with Christian Gaetz.