Noetherian connected braided Hopf algebras are affine

The well-known Wu--Zhang question asks whether every Noetherian Hopf algebra over a field is affine (finitely generated as an algebra). In this paper, we extend this fundamental problem to the braided setting within the monoidal category ${}_{H}^{H}\mathcal{YD}$ of left Yetter--Drinfel'd modules over a Hopf algebra $H$. By developing term-rewriting methods based on reduction orderings and prime-word factorizations in free prebraided algebras, we establish explicit expansions for YD-triangular comultiplications. We prove that if $(\mathbb{K}\langle X \rangle, ε)$ is an augmented free algebra in ${}_{H}^{H}\mathcal{YD}$ and $I \subseteq \ker ε$ is an ideal invariant under a YD-triangular comultiplication, then every $I$-reducible letter---and consequently every non-empty word in $\langle X \rangle$---can be expressed modulo $I$ as a linear combination of products of $I$-irreducible letters strictly smaller with respect to the reduction order. As a primary consequence, we prove that any right Noetherian connected prebraided Hopf algebra $B$ in ${}_{H}^{H}\mathcal{YD}$ is affine, generated by the image of a finite set $X_{I}$ of $I$-irreducible letters. As an application, we show that the braided component $B = A^{\mathrm{co}\,π}$ of a right Noetherian Radford biproduct $A = B \sharp H$ is affine whenever $A_{(0)} \subseteq \jmath(H)$, reducing the affineness of $A$ entirely to that of $H$.

Authors

Publication Details

Journal
arXiv (Cornell University)
Published
2026-11-10
DOI
https://doi.org/10.48550/arxiv.2611.14304
Primary Topic
Advanced Topics in Algebra
Type
preprint
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preprint

Noetherian connected braided Hopf algebras are affine

Huan Jia, Yinhuo Zhang
arXiv (Cornell University)
Advanced Topics in Algebra
preprint

Noetherian connected braided Hopf algebras are affine

Huan Jia, Yinhuo Zhang
preprint en

Abstract

The well-known Wu--Zhang question asks whether every Noetherian Hopf algebra over a field is affine (finitely generated as an algebra). In this paper, we extend this fundamental problem to the braided setting within the monoidal category ${}_{H}^{H}\mathcal{YD}$ of left Yetter--Drinfel'd modules over a Hopf algebra $H$. By developing term-rewriting methods based on reduction orderings and prime-word factorizations in free prebraided algebras, we establish explicit expansions for YD-triangular comultiplications. We prove that if $(\mathbb{K}\langle X \rangle, ε)$ is an augmented free algebra in ${}_{H}^{H}\mathcal{YD}$ and $I \subseteq \ker ε$ is an ideal invariant under a YD-triangular comultiplication, then every $I$-reducible letter---and consequently every non-empty word in $\langle X \rangle$---can be expressed modulo $I$ as a linear combination of products of $I$-irreducible letters strictly smaller with respect to the reduction order. As a primary consequence, we prove that any right Noetherian connected prebraided Hopf algebra $B$ in ${}_{H}^{H}\mathcal{YD}$ is affine, generated by the image of a finite set $X_{I}$ of $I$-irreducible letters. As an application, we show that the braided component $B = A^{\mathrm{co}\,π}$ of a right Noetherian Radford biproduct $A = B \sharp H$ is affine whenever $A_{(0)} \subseteq \jmath(H)$, reducing the affineness of $A$ entirely to that of $H$.

arXiv (Cornell University)
Advanced Topics in Algebra
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