Draisma's topological noetherianity conjecture for the affine infinite Grassmannian

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Let kk be a field. Define the affine infinite Grassmannian Gr⁡~(∞/2,V∞∗):=lim←⁡(n,p)Gr⁡~(p,Vn,p∗) \widetilde{\operatorname{Gr}}(\infty/2,V^*_\infty):=\varprojlim_{(n,p)}\widetilde{\operatorname{Gr}}(p,V^*_{n,p}), and let S∞S_\infty be the infinite symmetric group acting on it by permutation matrices. Let N⁡(∞)\operatorname{N}(\infty) be the direct-limit subgroup formed by the normalizers of the standard maximal tori.

Draisma's conjecture. The affine infinite Grassmannian Gr⁡~(∞/2,V∞∗)\widetilde{\operatorname{Gr}}(\infty/2,V^*_\infty) is topologically S∞S_\infty-noetherian: every descending chain of S∞S_\infty-stable Zariski-closed subsets stabilizes. A weaker form asserts topological N⁡(∞)\operatorname{N}(\infty)-noetherianity, while a stronger form asserts that every ascending chain of S∞S_\infty-stable ideals in its affine coordinate ring stabilizes.

The paper disproves the stronger form by exhibiting an ascending chain of S∞S_\infty-stable ideals that does not stabilize. Thus the topological conjecture and its weaker N⁡(∞)\operatorname{N}(\infty)-form remain unresolved in the supplied text.

References

Primary source

Shrawan Kumar, “Counter Example to a Strong Matroid Minor Conjecture”, arXiv:2412.04288 (2024).

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