Hopf algebras

We survey Hopf algebras and their generalizations. In particular, we compare and contrast three well-studied generalizations (quasi-Hopf algebras, weak Hopf algebras, and Hopf algebroids), and two newer ones (Hopf monads and hopfish algebras). Each of these notions was originally introduced for a specific purpose within a particular context; our discussion favors applicability to the theory of dynamical quantum groups. Throughout the note, we provide several definitions and examples in order to make this exposition accessible to readers with differing backgrounds.

Since the first optical instruments were invented, the idea that the visible image of an object under observation depends on tools of observation became commonly assumed in physics. A way of formalizing this idea in mathematics is the construction that assigns to an arbitrary object A in a category K its envelope Env Ω Φ A in a given class of morphisms (a class of representations) Ω with respect to a given class of morphisms (a class of observation tools) Φ. It turns out that if we take a sufficiently wide category of topological algebras as K, then each choice of the classes Ω and Φ defines a "projection of functional analysis into geometry," and the standard "geometric disciplines," like complex geometry, differential geometry, and topology, become special cases of this construction. This gives a formal scheme of "categorical construction of geometries" with many interesting applications, in particular, "geometric generalizations of the Pontryagin duality" (to the classes of noncommutative groups). In this paper we describe this scheme in topology and in differential geometry.

We introduce a graphical syntax for signal flow diagrams based on the language of symmetric monoidal categories. Using universal categorical constructions, we provide a stream semantics and a sound complete axiomatisation.
A certain class of diagrams captures the orthodox notion of signal flow graph used in control theory; we show that any diagram of our syntax can be realised, via rewriting in the equational theory, as a signal flow graph.

Scientists in diverse fields use diagrammatic formalisms to reason about various kinds of networks, or compound systems. Examples include electrical circuits, signal flow graphs, Penrose and Feynman diagrams, Bayesian networks, Petri nets, Kahn process networks, proof nets, UML specifications, amongst many others.
Graphical languages provide a convenient abstraction of some underlying mathematical formalism, which gives meaning to diagrams. For instance, signal flow graphs, foundational structures in control theory, are traditionally translated into systems of linear equations. This is typical: diagrammatic languages are used as an interface for more traditional mathematics, but rarely studied per se.
Recent trends in computer science analyse diagrams as first-class objects using formal methods from programming language semantics. In these approaches, diagrams are generated as the arrows of a PROP --- a special kind of monoidal category --- by a two-dimensional syntax and equations. The domain of interpretation of diagrams is also formalised as a PROP, and the (compositional) semantics is expressed as a functor preserving the PROP structure.
The first main contribution of this thesis is the characterisation of SV, the PROP of linear subspaces over a field k. This is an important domain of interpretation for diagrams appearing in diverse research areas, like the signal flow graphs mentioned above. We present by generators and equations the PROP IH of string diagrams whose free model is SV. The name IH stands for interacting Hopf algebras: indeed, the equations ofIH arise by distributive laws between Hopf algebras, which we obtain using S. Lack's technique for composing PROPs.
The significance of the result is two-fold. On the one hand, it offers a canonical string diagrammatic syntax for linear algebra: linear maps, kernels, subspaces and the standard linear algebraic transformations are all faithfully represented in the graphical language. On the other hand, the equations of IH describe familiar algebraic structures --- Hopf algebras and Frobenius algebras --- which are at the heart of graphical formalisms as seemingly diverse as quantum circuits, signal flow graphs, simple electrical circuits and Petri nets. Our characterisation enlightens the provenance of these axioms and reveals their linear algebraic nature.
Our second main contribution is an application of IH to the semantics of signal processing circuits. We develop a formal theory of signal flow graphs, featuring a string diagrammatic syntax for circuits, a structural operational semantics and a denotational semantics. We prove soundness and completeness of the equations ofIH for denotational equivalence. Also, we study the full abstraction question: it turns out that the purely operational picture is too concrete --- two graphs that are denotationally equal may exhibit different operational behaviour. We classify the ways in which this can occur and show that any graph can be realised --- rewritten, using the equations of IH, into an executable form where the operational behavior and the denotation coincide. This realisability theorem --- which is the culmination of our developments --- suggests a reflection about the role of causality in the semantics of signal flow graphs and, more generally, of computing devices.

Since the first optical instruments were invented, an idea that the visible image of an object under observation depends on tools of observation became commonly assumed in physics. A way of formalizing this idea in mathematics is the construction that assigns to an arbitrary object A in a category K its envelope Env Ω Φ A in a given class of morphisms (a class of representations) Ω with respect to a given class of morphisms (a class of observation tools) Φ. It turns out that if we take a sufficiently wide category of topological algebras as K, then each choice of the classes Ω and Φ defines a "projection of functional analysis into geometry," and the standard "geometric disciplines," like complex geometry, differential geometry, and topology, become special cases of this construction. This gives a formal scheme of "categorical construction of geometries" with many interesting applications, in particular, "geometric generalizations of the Pontryagin duality" (to the classes of noncommutative groups). In this paper we describe this scheme in topology and in differential geometry.

We suggest a generalization of Pontryagin duality from the category of commutative, complex Lie groups to the category of (not necessarily commutative) Stein groups with algebraic connected component of identity. In contrast to the other similar generalizations, in our approach the enveloping category consists of Hopf algebras (in a proper symmetrical monoidal category).

The Structure Theorem for Hopf modules states that if a bialgebra H is a Hopf algebra (i.e. it is endowed with a so-called antipode) then every Hopf module M is of the form M coH ⊗ H, where M coH denotes the space of coinvariant elements in M. Actually, it has been shown that this result characterizes Hopf algebras: H is a Hopf algebra if and only if every Hopf module M can be decomposed in such a way. The main aim of this paper is to extend this characterization to the framework of quasi-bialgebras by introducing the notion of preantipode and by proving a Structure Theorem for quasi-Hopf bimodules. We will also establish the uniqueness of the preantipode and the closure of the family of quasi-bialgebras with preantipode under gauge transformation. Then, we will prove that every Hopf and quasi-Hopf algebra (i.e. a quasi-bialgebra with quasi-antipode) admits a preantipode and we will show how some previous results, as the Structure Theorem for Hopf modules, the Hausser-Nill theorem and the Bulacu-Caenepeel theorem for quasi-Hopf algebras, can be deduced from our Structure Theorem. Furthermore, we will investigate the relationship between the preantipode and the quasi-antipode and we will study a number of cases in which the two notions are equivalent: ordinary bialgebras endowed with trivial reassociator, commutative quasi-bialgebras, finite-dimensional quasi-bialgebras.

We give a construction of cyclic cocycles representing the equivariant characteristic classes of equivariant bundles. Our formulas generalize Connes' Godbillon-Vey cyclic cocycle. An essential tool of our construction is Connes-Moscovici's theory of cyclic cohomology of Hopf algebras.

We compute the finite-dimensional Nichols algebras over the sum of two simple Yetter-Drinfeld modules V and W over non-abelian quotients of a certain central extension of the dihedral group of eight elements or SL(2,3), and such that the Weyl groupoid of the pair (V,W) is finite. These central extensions appear in the classification of non-elementary finite-dimensional Nichols algebras with finite Weyl groupoid of rank two. We deduce new information on the structure of primitive elements of finite-dimensional Nichols algebras over groups.

The purpose of this paper is to develop a theory of bimonads and Hopf monads on arbitrary categories thus providing the possibility to transfer the essentials of the theory of Hopf algebras in vector spaces to more general settings. There are several extensions of this theory to {\em monoidal} categories which in a certain sense follow the classical trace. Here we do not pose any conditions on our base category but we do refer to the monoidal structure of the category of endofunctors on any category $\A$ and by this we retain some of the combinatorial complexity which makes the theory so interesting. As a basic tool we use distributive laws between monads and comonads (entwinings) on $\A$: we define a {\em bimonad} on $\A$ as an endofunctor $B$ which is a monad and a comonad with an entwining $\lambda:BB\to BB$ satisfying certain conditions. This $\lambda$ is also employed to define the category $\A^B_B$ of (mixed) $B$-bimodules. In the classical situation, an entwining $\lambda$ is derived from the twist map for vector spaces. Here this need not be the case but there may exist special distributive laws $\tau:BB\to BB$ satisfying the Yang-Baxter equation ({\em local prebraidings}) which induce an entwining $\lambda$ and lead to an extension of the theory of {\em braided Hopf algebras}. An antipode is defined as a natural transformation $S:B\to B$ with special properties and for categories $\A$ with limits or colimits and bimonads $B$ preserving them, the existence of an antipode is equivalent to $B$ inducing an equivalence between $\A$ and the category $\A^B_B$ of $B$-bimodules. This is a general form of the {\em Fundamental Theorem} of Hopf algebras.

In the recent years’ Hopf algebras have been introduced to describe certain combinatorial properties of quantum field theories.I have a short review of Hopf algebras and Quantum groups in this lecture. I will give a basic introduction to these algebras and objects and review some occurrences in particle
physics and explain our conclude and ideas in this matter with some examples.

The purpose of this note is two-fold: (1) to study when quasi-Euclidean rings, regular rings and regular separative rings have the property (∗) that each right (left) singular element is a product of idempotents, and (2) to consider the question: “when is a singular nonnegative square matrix a product of nonnegative idempotent matrices?” The importance of the class of quasi- Euclidean rings in connection with the property (∗) is given by the first three authors and T.Y. Lam [Journal of Algebra, 406:154–170, 2014], where it is shown that every singular matrix over a right and left quasi-Euclidean domain is a product of idempotents, generalizing the results of J. A Erdos [Glasgow Mathematical Journal, 8: 118–122, 1967] for matrices over fields and that of T. J. Laffey [Linear and Multilinear Algebra, 14:309–314, 1983] for matrices over commutative Euclidean domains. We have shown in this paper that quasi-Euclidean rings appear among many interesting classes of rings and ...

Since the first optical instruments have been invented, an idea that the visible image of an object under observation depends on tools of observation became commonly assumed in physics. A way to formalize it in mathematics is the construction that assigns to an arbitrary object A in a category K its envelope Env^Ω_Φ A in a given class of morphisms (a class of representations) Ω with respect to a given class of morphisms (a class of observation tools) Φ. It turns out that if we take a sufficiently wide category of topological algebras as K, then each choice of the classes Ω and Φ defines a "projection of functional analysis into geometry", and the standard "geometric disciplines", like complex geometry, differential geometry and topology, become special cases of this construction. This gives a formal scheme of "categorical construction of geometries" with many interesting applications, in particular, "geometric generalizations of the Pontryagin dual...

Network theory uses the string diagrammatic language of monoidal categories to study graphical structures formally, eschewing specialised translations into intermediate formalisms. Recently, there has been a concerted research focus on developing a network theoretic approach to signal flow graphs, which are classical structures in control theory, signal processing and a cornerstone in the study of feedback. In this approach, signal flow graphs are given a relational denotational semantics in terms of formal power series.

Thus far, the operational behaviour of such signal flow graphs has only been discussed at an intuitive level. In this paper we equip them with a structural operational semantics. As is typically the case, the purely operational picture is too concrete -- two graphs that are denotationally equal may exhibit different operational behaviour. We classify the ways in which this can occur and show that any graph can be realised -- rewritten, using the graphical theory, into an executable form where the operational behavior and the denotation coincides.

We introduce and study the definition, main properties and applications of iterated twisted tensor products of algebras, motivated by the problem of defining a suitable representative for the product of spaces in noncommutative geometry. We find conditions for constructing an iterated product of three factors, and prove that they are enough for building an iterated product of any number of factors. As an example of the geometrical aspects of our construction, we show how to construct differential forms and involutions on iterated products starting from the corresponding structures on the factors, and give some examples of algebras that can be described within our theory. We prove a certain result (called ``invariance under twisting'') for a twisted tensor product of two algebras, stating that the twisted tensor product does not change when we apply certain kind of deformation. Under certain conditions, this invariance can be iterated, containing as particular cases a number ...

We present a short overview of Hopf algebra theory. Starting with definitions of coalgebra.
bialgebra, Hopf algebra and these structures and pass to Hopf algebra symmetry and We introduce the notation, which is called Sweedler notation.
In this article we introduce the afine group scheme and circular group of a Hopf algebra H.Then pass to finite Hopf algebras and Radfords formula for S^4 and the Nichols-Zoeller theorem with proofs. Then pass to character theory for finite Hopf algebras, and finally, we introduce the class equation for Hopf algebras. and we finish this article with Hopf algebras of prime order and theorem of Zhu in 1994.

The main goal of this paper is to investigate the structure of Hopf algebras with the property that either its Jacobson radical is a Hopf ideal or its coradical is a subalgebra. Let us consider a Hopf algebra A A such that its Jacobson radical J J is a nilpotent Hopf ideal and H := A / J H:=A/J is a semisimple algebra. We prove that the canonical projection of A A on H H has a section which is an H H –colinear algebra map. Furthermore, if H H is cosemisimple too, then we can choose this section to be an ( H , H ) (H,H) –bicolinear algebra morphism. This fact allows us to describe A A as a ‘generalized bosonization’ of a certain algebra R R in the category of Yetter–Drinfeld modules over H H . As an application we give a categorical proof of Radford’s result about Hopf algebras with projections. We also consider the dual situation. Let A A be a bialgebra such that its coradical is a Hopf sub-bialgebra with antipode. Then there is a retraction of the canonical injection of H H into A ...

In this paper we continue the investigation started in [A.M.St.-Small], dealing with bialgebras $A$ with an $H$-bilinear coalgebra projection over an arbitrary subbialgebra $H$ with antipode. These bialgebras can be described as deformed bosonizations $R#_{\xi} H$ of a pre-bialgebra $R$ by $H$ with a cocycle $\xi$. Here we describe the behavior of $\xi$ in the case when $R$ is f.d. and thin i.e. it is connected with one dimensional space of primitive elements. This is used to analyze the arithmetic properties of $A$. Meaningful results are obtained when $H$ is cosemisimple. By means of Ore extension construction, we provide some examples of atypical situations (e.g. the multiplication of $R$ is not $H$-colinear or $\xi$ is non-trivial).

We classify all groups G and all pairs (V,W) of absolutely simple Yetter-Drinfeld modules over G such that the support of the direct sum of V and W generates G, the square of the braiding between V and W is not the identity, and the Nichols algebra of the direct sum of V and W admits a finite root system. As a byproduct, we determine the dimensions of such Nichols algebras, and several new families of finite-dimensional Nichols algebras are obtained. Our main tool is the Weyl groupoid of pairs of absolutely simple Yetter-Drinfeld modules over groups.

Abstract. Within the quantum function algebra Fq[GLn], we study the subset Fq[GLn] — introduced in [Ga1] — of all elements of Fq[GLn] which are Z ˆ q, q −1 ˜ –valued when paired with Uq(gl n), the unrestricted Z ˆ q, q −1 ˜ –integral form of Uq(gl n) introduced by De Concini, Kac and Procesi. In particular we obtain a presentation of it by generators and relations, and a PBW-like theorem. Moreover, we give a direct proof that Fq[GLn] is a Hopf subalgebra of Fq[GLn], and that Fq[GLn] ∼= UZ(gln). We describe explicitly its specializations at roots of 1, say ε, and q=1 the associated quantum Frobenius (epi)morphism from Fε[GLn] to F1[GLn] ∼ = UZ(gl n), also introduced in [Ga1]. The same analysis is done for Fq[SLn] and (as key step) for Fq[Mn].

We construct several pairings in Hopf-cyclic cohomology of (co)module (co)algebras with arbitrary coefficients. The key ideas instrumental in constructing these pairings are the derived functor interpretation of Hopf-cyclic and equivariant cyclic (co)homology, and the Yoneda interpretation of Ext-groups. As a special case of one of these pairings, we recover the Connes-Moscovici characteristic map in Hopf-cyclic cohomology. We also prove that

We introduce and study the definition, main properties and applications of iterated twisted tensor products of algebras, motivated by the problem of defining a suitable representative for the product of spaces in noncommutative geometry. We find conditions for constructing an iterated product of three factors and prove that they are enough for building an iterated product of any number of factors. As an example of the geometrical aspects of our construction, we show how to construct differential forms and involutions on iterated products starting from the corresponding structures on the factors and give some examples of algebras that can be described within our theory. We prove a certain result (called "invariance under twisting") for a twisted tensor product of two algebras, stating that the twisted tensor product does not change when we apply certain kind of deformation. Under certain conditions, this invariance can be iterated, containing as particular cases a number of...