Questions tagged [hopf-algebras]

A Hopf algebra is a vector space $H$ over a field $k$ endowed with an associative product $\times:H\otimes_k H\to H$ and a coassociative coproduct $\Delta:H\to H\otimes_k H$ which is a morphism of algebras. Unit $1:k\to H$, counit $\epsilon:H\to k$ and antipode $S:H\to H$ are also required. Such a structure exists on the group algebra $k G$ of a finite group $G$.

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Does the category of commutative and cocommutative Hopf algebras have enough injectives?

It is well-known that the category of commutative and cocommutative Hopf algebras is abelian (see https://arxiv.org/abs/1502.04001v2 and its references). But does it have enough injectives? What about ...
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Is there a condition such that the $A$ action of a $A \rtimes H$-module is a restriction of the $H$-action?

Let $H$ be a Hopf algebra and $A$ a subalgebra of $H$ such that $A$ is a left coideal of $H$ (that is $\Delta(A) \subset H \otimes A$) and $A$ is preserved by the adjoint action of $H$. Consider now ...
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2 votes
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Primitive elements in the universal enveloping algebra of Lie superalgebra

Let $\mathfrak{g}$ be a Lie superalgebra over $\mathbb{C}$. Denote by $U(\mathfrak{g})$ the universal enveloping algebra of $\mathfrak{g}$. We know that there is a natural super Hopf algebra structure ...
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A finite $H$-module algebra is necessarily inner

Let $H$ be a finite-dimensional Hopf algebra over a field $k$, and $A$ an $H$-module algebra. If $A$ is finite-dimensional over $k$, is the action of $H$ on $A$ necessarily inner? If this is not true ...
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Mayer–Vietoris sequence for coproduct of Hopf algebras

Is there a Mayer–Vietoris-type sequence for the homology of a coproduct of two Hopf algebras over an ideal? The definition of the coproduct can be found in Agore - Categorical Constructions for Hopf ...
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11 votes
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147 views

Natural knot homology

All knot homology theories I've seen share a flaw: their definitions explicitly use some combinatorial choices (such as a diagram presentation). The coin, however, has two sides and the other one is ...
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9 votes
1 answer
321 views

Hopf algebra with a non-invertible antipode

What is an example of a Hopf algebra with a non-invertible antipode?
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7 votes
1 answer
205 views

Group-like elements in quotients of group rings

$\DeclareMathOperator\Gr{Gr}$Let $R$ be a local ring, let $A$ be a finite abelian group, and let $I$ be a Hopf ideal of the ring $R[A]$. The quotient $R[A]\twoheadrightarrow R[A]/I$ induces a map on ...
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3 votes
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How to interpret compositional diagrams for quantum sets algebraically

$\newcommand{\id}{\mathrm{id}}$My reference for this post is Musto, Reutter and Verdon's A compositional approach to quantum functions, arXiv:1711.07945. Questions are in bold below. Allow me to begin ...
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Exterior algebra of free modules over Hopf algebras

Let $H$ be a commutative, cocommutative Hopf algebra over a field $\mathbb{K}$, and $M$ a free Hopf module over $H$. Is the exterior algebra $\Lambda^k_\mathbb{K} M$ with the diagonal $H$-action $$h \...
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Combinatorial proof of a matrix equation

I'm looking for combinatorial proofs (using, e.g., trees) of the following particular matrix equation $(I)$ and also combinatoric operational analogs of its solution via matrix inversion and/or Cramer'...
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2 votes
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Timmerman's "An invitation to quantum groups and duality" corollary 3.2.8

Consider the following propositions of Timmerman's book "An invitation to quantum groups and duality": I do not understand the equivalence $$h(\mathfrak{C}(\delta_V)S(\mathfrak{C}(\delta_W))...
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1 answer
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Characterising algebraic compact quantum groups among Hopf $^*$-algebras

Let $(A, \Delta)$ be a Hopf $^*$-algebra. Assume that $\{u^\alpha\}_{\alpha \in I}$ is a maximal collection of pairwise inequivalent irreducible unitary corepresentation matrices. I want to show that $...
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2 answers
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If $\operatorname{Hom}(\delta_V, \delta_W) = 0$, then $\mathfrak{C}(\delta_V) \cap \mathfrak{C}(\delta_W) = 0.$

Let $(A, \Delta)$ be a Hopf $^*$-algebra and $\delta_V: V \to V \otimes A$ and $\delta_W: W \to W \otimes A$ be two corepresentations of $(A, \Delta).$ Assume that the space of intertwiners $\...
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  • 261
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1 answer
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Orthogonality relations for Haar state and antipode (Timmerman)

Consider the following proposition from Timmerman's "An invitation to quantum groups and duality": I am having trouble seeing why the boxed equations are true (Note that on the left the ...
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  • 261
3 votes
1 answer
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Is there a definition of Heisenberg double for quasi-Hopf algebras?

$\newcommand{\dmod}{\text{-}\mathrm{mod}}$Let $A$ be a finite-dimensional quasi-Hopf algebra over a field $k$ and $A\dmod$ be the category of finite dimensional left $A$-modules. Since $A\dmod$ is a ...
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7 votes
2 answers
240 views

Different Bialgebra/Hopf algebra structures on coalgebras

Given a coalgebra $C$, can there exist more than one algebra structure on $C$ giving it the structure of a bialgebra? I will also ask the same question for Hopf algebras.
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1 vote
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Indecomposable comodules

For a Hopf algebra $A$, we say that a comodule $V$ is indecomposable if it is not equivalent to a direct sum of irreducible comodules. $\bullet$ What is an example of a finite dimensional ...
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2 votes
1 answer
484 views

What is a coalgebra?

A coalgebra is a triple $(A,\Delta,\epsilon)$ consisting of a vector space, a coproduct, and a counit. Now as we all know, just like the unit in an algebra, the counit of a coalgebra is unique, i.e. ...
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2 votes
1 answer
120 views

Bialgebra maps and Hopf algebra maps

Let $H$ and $H'$ be two Hopf algebras, and let $\phi:H \to H'$ be an bialgebra map. Then is $\phi$ automatically a Hopf algebra map?
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2 votes
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Extending elements of the dual of a Hopf subalgebra to the Hopf dual of the whole algebra

I have a question about Hopf duals. To begin we can remind the definition: For an Hopf algebra $A$ over a field $k$, the Hopf dual $A^{\circ}$ is the subspace of the linear dual $\mathrm{Lin}_k(A,k)$ ...
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connected Hopf algebra of infinite Gelfand-Kirillov dimension but of finite dimensional primitive space

I would like to know some examples of connected Hopf algebras which has infinite Gelfand-Kirillov dimension but with primitive space finite dimensional. Any commments are welcome!
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5 votes
2 answers
299 views

Classifying Hopf algebras that admit a single irreducible comodule

Is it possible to classify Hopf algebras $H$, over a field $k$, which admit a unique (up to isomorphism) irreducible comodule, namely the trivial $1$-dim comodule $$ k \to k \otimes H, ~~ k \mapsto k ...
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4 votes
1 answer
454 views

Name for a Hopf algebra whose only grouplike element is the identity?

For a $k$-Hopf algebra $H$ and element $h \in H$ is called grouplike is $\Delta(h) = h \otimes h$ and $\epsilon(h)=1_k$ ($\epsilon$ is the counit). The identity $1_H$ is clearly grouplike, but in ...
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4 votes
1 answer
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Hopf algebra that is unimodular and counimodular but not involutory

I'm looking for a finite-dimensional Hopf algebra (over any field) that is unimodular, has unimodular dual, but is not involutory. Is there such a thing? Here's what I know: By Radford's formula, the ...
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3 votes
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grouplike elements in a completion

Let $G$ be a derived $p$-complete abelian group and $K$ a field. Let $\widehat{K[G]}$ denote the completion of the group ring $K[G]$ at the augmentation ideal. Is there a nice description of the set ...
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2 votes
1 answer
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The shuffle algebra over the rationals is isomorphic to the polynomial algebra in the Lyndon words

On this wikipedia page is stated that over the rational numbers, the shuffle algebra (over a set $X$) is isomorphic to the polynomial algebra in the Lyndon words (on $X$). I was wondering if you can ...
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4 votes
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Bialgebras from mixed Bruhat sheaves

Let $A = \bigoplus_{i=0}^{+\infty} A_i$ be a graded bialgebra in a braided monoidal category $\mathcal V$. Then, according to Kapranov–Schechtman's article "Parabolic induction and perverse ...
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3 votes
3 answers
225 views

Intuition for left Hopf-modules

I'm reading Radford's text on Hopf Algebras right now and I'm a bit confused about the definition of a left $A$-Hopf algebra. The definition given in the book is: Let $A$ be a $\Bbbk$-bialgebra. A ...
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1 answer
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Cosemisimple pointed Hopf algebras

Let $\mathbb{K}$ be an algebraically closed field of characteristic $0$. Every cosemisimple pointed Hopf $\mathbb{K}$-algebra $A$ is easily seen to be cocommutative. Does this imply that $A$ is the ...
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7 votes
2 answers
596 views

Is a Hopf algebra a group object of some category?

The page of ncatlab on group object states that: A group object in $\mathrm{CRing}^{\mathrm{op}}$ is a commutative Hopf algebra. Question: Is a (noncommutative) Hopf algebra a group object of some ...
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4 votes
3 answers
496 views

Name for a Hopf algebra admitting no non-trivial 1-dimensional comodule

A Hopf algebra is called pointed if all its simple left (or right) comodules are one-dimensional. See for example this question for a discussion. Now every Hopf algebra $H$ admits a one-dimensional ...
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11 votes
0 answers
242 views

Is there a homotopy coherent analogue of Dieudonné modules?

Let $K$ be a perfect field with characteristic $>0$ and $\mathcal{H}$ the category of graded connected abelian hopf algebras over $K$. By a theorem of Schoeller there is a canonical equivalence ...
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5 votes
2 answers
274 views

Low dimensional noncommutative non-cocommutative Hopf algebras

Sweedler's Hopf algebra (see here) is the lowest dimesnional ($4$-dimensional) Hopf algebra that is noncommutative and non-cocommutative. What are the next examples? Are there noncommutative, ...
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3 votes
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Do chains send homotopy inverse limits of spaces to homotopy inverse limits of $E_\infty$-coalgebras?

Let $X_\bullet := ... X_2 \to X_1$ be a tower of connected and simple spaces with the following properties: The induced tower $H_\ast(X_\bullet; \mathbb{F}_p)$ of graded $\mathbb{F}_p$-vector spaces ...
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3 votes
0 answers
113 views

Milnor exact sequence for homology of hopf algebras

Let $K$ be a field and $\mathrm{Hopf}^K_{E_\infty}$ the $\infty$-category of homotopy-coherent hopf algebras over $K$ that are coherently commutative and cocommutative. Precisely, $\mathrm{Hopf}^K_{E_\...
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9 votes
1 answer
293 views

The double cover in the classical limit of $U_q(\mathfrak{sl}_2)$

I am trying to learn about Drinfeld–Jimbo quantum groups and I am having trouble with the classical limit of $U_q(\mathfrak{sl}_2)$. When properly expressed the limit makes sense as $q\to 1$ — see for ...
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2 votes
0 answers
61 views

Primitive elements in Hopf algebras over the integers

Let $H$ be a Hopf algebra over $\mathbb Z$, and assume that $H$ is cocommutative, graded, generated in degree $1$, and connected (its degree-$0$ part is $\mathbb Z$). Are there nice, natural ...
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5 votes
1 answer
154 views

Involutive solutions to the Yang-Baxter equation (and triangular Hopf algebras)

I'm interested in solutions to the Yang-Baxter equation $$R_{12}R_{23}R_{12}=R_{23}R_{12}R_{23},$$ that are involutive $R^2_{12}=1$. Or put it another way, I'm interested in representations of the ...
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Superfluous axioms for ribbon Hopf algebra

In his book Foundations of quantum group theory, Majid defines (2.1.10) a ribbon Hopf algebra as a quasi-triangular Hopf algebra $(H, R)$ with a special central element $v \in H$ satisfying (1) $v^2 ...
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8 votes
1 answer
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Trying to understand "a refinement of the Peter–Weyl theorem" by Lusztig

"A refinement of the Peter–Weyl theorem" is the title of Chapter 29 in Lusztig's "Introduction to quantum groups" (Birkhäuser 2010, reprint of the 1994 edition). This chapter is ...
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18 votes
2 answers
1k views

Why does Drinfeld Unitarization work?

In Drinfeld's paper "Quasi-Hopf Algebras" he illuminates a process by which you can replace the $R \in A \otimes A$ associated to a quasi-Hopf QUE-algebra $(A, \Delta, \varepsilon, \Phi)$ ...
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1 vote
1 answer
89 views

About extensions between morphisms on the multiplier algebra

Let $A$ be a non-degenerate algebra and let $\Delta: A \to M(A \otimes A)$ be a non-degenerate morphism. We can extend the algebra morphism $$\iota \otimes \Delta: M(A \otimes A) \to M(A \otimes A \...
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1 vote
1 answer
129 views

Non-degeneracy of comultiplication (multiplier Hopf algebras)

Consider the following fragment from the paper "Multiplier Hopf-algebras" by Van Daele. Can someone explain how the coassociativity in definition 2.2 (ii) and the requirement $(\Delta \...
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0 votes
1 answer
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Antipode on a multiplier Hopf-algebra

Probably an easy question, but here goes: I'm reading the paper Multiplier Hopf algebras by Van Daele. Let $(A, \Delta)$ be a multiplier Hopf algebra. Let $L(A), R(A), M(A)$ be the left, right and ...
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2 votes
0 answers
66 views

Examples of multiplier Hopf algebras

A multiplier Hopf-algebra (introduced by Van Daele) is a pair $(A, \Delta)$ where $A$ is a non-degenerate algebra $A$ together with a non-degenerate algebra morphism $\Delta: A \to M(A \otimes A)$ ...
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1 vote
1 answer
132 views

A comodule algebra map from a Hopf algebra to itself

Let $H$ be a cosemisimple Hopf algebra (or just a bialgebra). Considering $H$ as a left $H$-comodule (i.e. take $\Delta_H$ to be the coaction), can there exist a (non-identity) algebra map $\sigma:H \...
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6 votes
1 answer
127 views

Morphisms between compact quantum groups

Let $(A, \Delta_A)$ and $(B, \Delta_B)$ be two compact quantum groups (in the sense of Woronowicz). I would be tempted to define a morphism $(A, \Delta_A) \to (B, \Delta_B)$ to be a unital $*$-...
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5 votes
1 answer
387 views

Braided monoidal category, example

Let $H$ be a cocommutative hopf algebra. Let $M$ be the category of $H$-bimodules. Does the category $M$ form a braided monoidal category with tensor product $\otimes_{H}$ ?
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3 votes
1 answer
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Show that a certain element is a linear combination of tensors

I posted this question on MSE but got no answer even after putting a bounty on it, so I figured I can try to ask here. Let $(A, \Delta: A \to A \otimes A)$ be bialgebra (unital and counital) such that ...
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