# nLab quantum group

### Context

#### Algebra

higher algebra

universal algebra

# Contents

## Idea

The notion of quantum group refers to various objects which are deformations of (algebras of functions on) groups, but still have very similar properties to (algebras of functions on) groups, and in particular to semisimple Lie groups. Most important are the Hopf algebras deforming the function algebras on semisimple Lie groups or to the enveloping algebras of Kac-Moody Lie algebras.

## Overview

It is a common experience in representation theory that a number of mathematical structures behaves very similarly to algebraic or Lie groups. After the impetus of the theory of quantum integrable systems, mainly the work of Leningrad’s school of mathematical physics around 1980, several mathematicians (including Drinfeld, Manin, Woronowicz, Jimbo, Faddeev–Reshetikhin–Takhtajan) found, in different formalisms, major series of examples which are mostly noncommutative noncocommutative Hopf algebras and which deform enveloping algebras of (semisimple) Lie algebras, or algebras of functions on the corresponding algebraic groups. These deformations $G_q$ depend on a parameter $q$ (sometimes one prefers a formal parameter $h$ with $q = e^{h}$), which may be taken as belonging to the ground field, but also being formal (transcendental over the ground field). A peculiar case is when the parameter $q$ of the deformation is an $l$-th root of unity; the remaining cases are usually called generic $q$.

The representation theory for these ‘quantum’ examples is highly developed; in fact many phenomena in the representation theory of semisimple Lie algebras (e.g. canonical bases) were discovered first as a limiting case of constructions in the quantum case, which become degenerate in the classical case (the principle that quantization removes degeneracy). While representations for generic $q$ parallel classical ones, the theory at roots of unity is peculiar and related to the representation theory of affine Lie algebras; the quantum groups at roots of unity as algebras have big centers.

Nowadays, both the class of examples and the class of formalisms has been extended a lot, hence the term ‘quantum group’ is not a fixed notion but rather a collective term for a rather author-dependent class of group-like objects, most often subclasses or extensions of the concept of Hopf algebras which are sometimes required to belong to families of deformations of their classical counterparts. One of the common features is that if we forget the group-like features, the examples belong to the class of noncommutative spaces (see noncommutative geometry).

Mathematically better defined are notions (sometimes equated by various authors with the class of quantum groups) like quasitriangular Hopf algebras, quantum matrix groups? (quantum linear groups, more general FRT-algebras and Majid’s $A(R)$ where $R$ is a quantum Yang-Baxter equation), quantized enveloping algebras, quantum function algebras, compact matrix pseudogroups, Kac algebras, Yangians etc. The representations of quasitriangular Hopf algebras form braided monoidal categories, which are in main examples related to the mathematics of Iwahori–Hecke algebras, braid groups, knot theory, finite group Chern–Simons theory and Wess–Zumino–Novikov–Witten theory of CFT. One should note that in the classical limit quantum function algebras give not simply (functions on) algebraic (or Lie) groups but also a compatible (= multiplicative) Poisson structure giving rise to Poisson–Lie or Poisson algebraic groups.

There is an extensive geometric theory of homogeneous spaces for quantum groups and fiber bundles whose structure groups are quantum groups.

## Properties

### Tannaka duality

Tannaka duality for categories of modules over monoids/associative algebras

monoid/associative algebracategory of modules
$A$$Mod_A$
$R$-algebra$Mod_R$-2-module
sesquialgebra2-ring = monoidal presentable category with colimit-preserving tensor product
bialgebrastrict 2-ring: monoidal category with fiber functor
Hopf algebrarigid monoidal category with fiber functor
hopfish algebra (correct version)rigid monoidal category (without fiber functor)
weak Hopf algebrafusion category with generalized fiber functor
quasitriangular bialgebrabraided monoidal category with fiber functor
triangular bialgebrasymmetric monoidal category with fiber functor
quasitriangular Hopf algebra (quantum group)rigid braided monoidal category with fiber functor
triangular Hopf algebrarigid symmetric monoidal category with fiber functor
form Drinfeld doubleform Drinfeld center
trialgebraHopf monoidal category

2-Tannaka duality for module categories over monoidal categories

monoidal category2-category of module categories
$A$$Mod_A$
$R$-2-algebra$Mod_R$-3-module
Hopf monoidal categorymonoidal 2-category (with some duality and strictness structure)

3-Tannaka duality for module 2-categories over monoidal 2-categories

monoidal 2-category3-category of module 2-categories
$A$$Mod_A$
$R$-3-algebra$Mod_R$-4-module

## References

• V. G. Drinfel’d, Quantum groups, Proceedings of the International Congress of Mathematicians 986, Vol. 1, 798–820, AMS 1987, djvu:1.3M, pdf:2.5M

• Shahn Majid, Foundations of quantum group theory, Cambridge University Press 1995, 2000.

• Yu. I. Manin, Quantum groups and non-commutative geometry, CRM, Montreal 1988.

• B. Parshall, J.Wang, Quantum linear groups, Mem. Amer. Math. Soc. 89(1991), No. 439, vi+157 pp.

• N. Yu. Reshetikhin, L. A. Takhtajan, L. D. Faddeev, Quantization of Lie groups and Lie algebras, Algebra i analiz 1, 178 (1989) (Russian), English translation in Leningrad Math. J. 1.

• Arun Ram, A survey of quantum groups: background, motivation, and results, in: Geometric analysis and Lie theory in mathematics and physics, A. Carey and M. Murray eds., Australian Math. Soc. Lecture Notes Series 11, Cambridge Univ. Press 1997, pp. 20-104. pdf

• P. Etingof, O. Schiffmann, Lectures on Quantum Groups, Lectures in Math. Phys., International Press (1998).

• P.Etingof, I. Frenkel, Lectures on representation theory and Knizhnik-Zamolodchikov equations

• A. U. Klymik, K. Schmuedgen, Quantum groups and their representations, Springer 1997.

• A. Joseph, Quantum groups and their primitive ideals, Springer 1995.

• Ross Street, Quantum groups : a path to current algebra, Cambridge Univ. Press 2007

• L. I. Korogodski, Ya. S. Soibelman, Algebras of functions on quantum groups I, Math. Surveys and Monographs 56, AMS 1998.

• A. Varchenko, Hypergeometric functions and representation theory of Lie algebras and quantum groups, Advanced Series in Mathematical Physics, Vol. 21, World Scientific (1995)

• George Lusztig, Introduction to quantum groups

• V. Chari, A. Pressley, A guide to quantum groups, Camb. Univ. Press 1994

• C. Kassel, Quantum groups, Graduate Texts in Mathematics 155, Springer 1995 (also errata

• Bangming Deng, Jie Du, Brian Parshall, Jianpan Wang, Finite dimensional algebras and quantum groups, Mathematical Surveys and Monographs 150, Amer. Math. Soc. 2008. xxvi+759 pp. MR2009i:17023)

Revised on April 10, 2013 00:59:02 by Urs Schreiber (82.169.65.155)