Coefficientwise semicontinuity of Schubert Hilbert numerators

Let GLn/BGL_n/B be the complete flag variety, with Schubert varieties XwX_w indexed by permutations wSnw\in\mathfrak S_n. For uvwu\leq v\leq w in Bruhat order, let Hu,w(q)H_{u,w}(q) and Hv,w(q)H_{v,w}(q) be the Hilbert numerators of the associated graded local rings at eue_u and eve_v, respectively, and write [qt]H(q)[q^t]H(q) for the coefficient of qtq^t in H(q)H(q). Coefficientwise semicontinuity conjecture. If uvwu\leq v\leq w, then

[qt]Hu,w[qt]Hv,w.[q^t]H_{u,w}\geq[q^t]H_{v,w}.

The source describes this as a strengthening of the Cohen–Macaulayness conjecture and notes that it was checked for at least n6n\leq 6 and much of n=7n=7. No general proof or refutation is given.

Sources & referencesView supporting material

Primary source

Alexander Yong, “Castelnuovo-Mumford regularity and Schubert geometry”, arXiv:2202.06362 (2022).

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