A quasisymmetric analog of Grassmannian Schubert varieties

Abstract

We show that the cohomology rings of toric Richardson varieties in the Grassmannian are finite truncations of the ring of quasisymmetric functions. We exhibit an affine paving of each such variety whose cell closures give rise to the basis of fundamental quasisymmetric functions. We similarly interpret the homology of these varieties in terms of the ring of noncommutative symmetric functions and show that the expansion of the homological class of any torus-invariant subvariety into the affine paving basis agrees with the expansion of a corresponding generalized noncommutative ribbon function into the ribbon basis. By taking the direct limit of all toric Richardson varieties, we obtain an ind-variety equipped with a weak H-group structure whose cohomology is the Hopf algebra of quasisymmetric functions. We conjecture that it is isomorphic to a similar H-group constructed by Baker--Richter. As a byproduct, we deduce that the f-vectors of shard polytopes are log-concave, making the first progress on a question of Ferroni--Schröter for matroid base polytopes.

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BibTeX RIS

BibTeX

@misc{gonzales2026quasisymmetric,
  title = {{A quasisymmetric analog of Grassmannian Schubert varieties}},
  author = {Teddy Gonzales and Tuong Le and Chayim Lowen},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.30257},
  eprint = {2609.30257},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.30257},
}

RIS

TY  - UNPB
TI  - A quasisymmetric analog of Grassmannian Schubert varieties
AU  - Teddy Gonzales
AU  - Tuong Le
AU  - Chayim Lowen
PY  - 2026
DA  - 2026/09/24
PB  - arXiv
DO  - 10.48550/arxiv.2609.30257
UR  - https://arxiv.org/abs/2609.30257
AB  - We show that the cohomology rings of toric Richardson varieties in the Grassmannian are finite truncations of the ring of quasisymmetric functions. We exhibit an affine paving of each such variety whose cell closures give rise to the basis of fundamental quasisymmetric functions. We similarly interpret the homology of these varieties in terms of the ring of noncommutative symmetric functions and show that the expansion of the homological class of any torus-invariant subvariety into the affine paving basis agrees with the expansion of a corresponding generalized noncommutative ribbon function into the ribbon basis. By taking the direct limit of all toric Richardson varieties, we obtain an ind-variety equipped with a weak H-group structure whose cohomology is the Hopf algebra of quasisymmetric functions. We conjecture that it is isomorphic to a similar H-group constructed by Baker--Richter. As a byproduct, we deduce that the f-vectors of shard polytopes are log-concave, making the first progress on a question of Ferroni--Schröter for matroid base polytopes.
ER  -

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