# Hopf-in-Lux

## Conference on Hopf algebras

University of Luxembourg, 13-17 July 2009

## Aims and Scope

The scientific field of the conference lies at the crossroads of Theoretical Physics and Modern Mathematics.

The notion of Symmetry is central in most of physical and mathematical sciences, classically described with the language of group theory. During the last years it turned out that the nature of physical symmetries appearing for example in Quantum Physics (but not only there) goes beyond the mathematical concept of a group. The corresponding new mathematical concept is the structure of a Hopf algebra.

Examples of Hopf algebras are given by quantum groups describing "quantum symmetries". Hopf algebras also helped to reveal the underlying structure of the renormalization scheme in Quantum Field Theory.

In turn these phenomenal applications have pushed for a development and deepening of the theory itself. Hence a great deal of scientists came to this field and new applications have been found.

The core of this conference will be the notion of Hopf algebras and some of its applications. There will be an intensive part devoted to the algebraic foundations of the theory of Hopf algebras and its more recent developments. Since the Hopf algebra approach to renormalization is so important, a big part of the conference will be devoted to Quantum Field Theory in order to understand the renormalization procedure, and another part to the interplays between renormalization and Hopf algebras. Other applications, namely in combinatorics, lattice theory and cohomology of Lie algebras will be covered. Finally there will be some talks on related areas, with the hope to find some new applications of the theory.

## Organizing Committee

### Organizers

• Philippe BONNEAU (Metz)
• Yael FREGIER (Luxembourg) < yael (dot) fregier (at) uni (dot) lu >
• Martin SCHLICHENMAIER (Luxembourg) < martin (dot) schlichenmaier (at) uni (dot) lu >

## Participants

(up to now the following people registered)

## Abstracts

• Susama Agarwala

Title: The Geometry of Renormalization

Abstract: Connes and Marcolli showed that the $\beta$ function of a quantum field can be uniquely written in terms of a flat connection of a renormalization bundle, defined by the Hopf algebra of Feynman diagrams associated to that theory. In this talk, I review the construction of this connection and show how the renormalization bundle can be used to define a $\beta$ function first locally, and then globally over a curved space-time background manifold. Finally, I construct a conformal renormalization bundle that accounts for conformal changes of metric on the background manifold, rather than the scalar changes considered in prior work.

• Martin Bordemann

Title: Graded Hopf algebras, X-infinity structures and deformation quantization

We shall present the graded `cofree' Hopf algebras needed to understand A-infinity and L-infinity structures, viz the tensor (co)algebra and the graded symmetric (co)algebra. We present a pedestrian approach to A- and L-infinity structures as graded coderivations of square zero and show how to transport them by homotopical perturbation theory on the corresponding coalgebras over the (co)homology spaces thereby presenting Gerstenhaber, Nijenhuis-Richardson and Schouten brackets. In this context we shall sketch the Hochschild-Kostant-Rosenberg Theorem for the Hochschild (co)homology of symmetric algebras. Formality will be introduced as an L-infinity morphism between graded Lie-structures. At the end we shall show Kontsevich's result that formality implies the integrability of associative deformations up to order two. As examples we shall take some universal envelopping algebras of Lie algebras.

• Emily Burgunder

Title: Hopf algebra structures and Parking functions

Abstract: We exhibit a family of well-known Hopf algebras which can be endowed with some finer bialgebra structure namely a tridendriform bialgebra structure. One example is the Hopf algebra of Parking functions which are very useful combinatorial objects. In their article Novelli and Thibon conjecture that these Parking functions are free with respect to the tridendriform algebra structure. We prove the conjecture and show that this bialgebra can be reconstructed from its primitives through a structure theorem. Moreover this primitive part is free as a Gerstenhaber-Voronov algebra.

• Kurush Ebrahimi-Fard

LECTURE:

Title: Modern aspects of perturbative renormalization

Abstract: In this series of lectures we describe the process of renormalization in perturbative quantum field theory. In the first lecture we will introduce the basic concepts by using toy models. Based on the lectures at this school on quantum field theory (by A. Frabetti) and Hopf algebras (by A. Makhlouf) we will exemplify these concepts in concrete calculations in quantum field theories relevant to physics. Along these lines Connes-Kreimer's Hopf algebra of renormalization will be introduced. At the end we will outline a Lie theoretic approach to perturbative renormalization using the Dynkin idempotent, one of the fundamental Lie idempotents in the theory of free Lie algebras. These results rely on properties of Hopf algebras encapsulated in the notion of associated descent algebras.

TALK:

Title: On the pre-Lie Magnus and Fer expansions

Abstract: W. Magnus' classical paper on the exponential solution of differential equations for a linear operator received much attention since its appearance in 1954, both in applied mathematics as well as physics. More recently, Magnus' results were explored from a more algebraic-combinatorial perspective using the language of operads and combinatorial Hopf algebras. In this talk we review recent work on the classical Magnus and Fer expansions, unveiling a new structure by using the language of dendriform and pre-Lie algebras as well as Hopf algebras of rooted trees. Let us emphasize that a dendriform algebra may be seen at the same time as an associative as well as a (pre-)Lie algebra. Main examples of dendriform algebras are provided by the shuffle and quasi-shuffle algebras as well as Rota-Baxter operators, e.g. the Riemann integral map or Jackson's q-integral. We introduce the notion of linear equations in dendriform algebras, which include matrix initial value problems as a particular example, and investigate its solution theory. In the end we outline a new approach to Butcher's B-series in the context of pre-Lie algebras. Due to the intimate link between dendriform algebras and Rota-Baxter operators, motivations as well as applications naturally appear. They rang from numerical methods, to control theory to the process of renormalisation in perturbative quantum field theory. (This is joint work with D. Manchon and D. Calaque)

• Alessandra Frabetti

LECTURE

Title: Basic quantum field theory for mathematicians

Abstract: In these lectures I present some basic notions of quantum field theory with the aim of being comprehensible by mathematicians. The first lectures review classical and quantum interacting fields and their perturbative expansion as series over Feynman diagrams.

TALK

Title: Pro algebraic groups and renormalization

Abstract: I present an interpretation of the Connes-Kreimer renormalization Hopf algebra in terms of proalgebraic groups for the perturbative series previously described.

• Anthony Giaquinto

Title: The Principal Element of a Frobenius Lie Algebra

Abstract: We introduce the notion of the principal element of a Frobenius Lie algebra f. The principal element corresponds to a choice of F ∈ f^∗ such that F[−, −] non-degenerate. In many natural instances, the principal element is shown to be semisimple, and when associated to sln , its eigenvalues are integers which constitute a single unbroken string and are independent of F. For certain “small” functionals F, a simple construction is given which readily yields the principal element. When applied to the first maximal parabolic subalgebra of sln , the principal element coin- cides with semisimple element of the principal three-dimensional subalgebra. We also show that Frobenius algebras are stable under deformation, and give specific examples of associated solutions of the classical Yang-Baxter equation. This work is joint with M. Gerstenhaber.

• Abdenacer Maklhouf

Title: Introduction to Hopf algebras and quantum groups

Abstract: The aim of these lectures is to introduce the foundations of Hopf algebras and quantum groups. We set out these fundamental mathematical structures by developing the basic properties and including the structure theorems for finite and infinite dimensional Hopf algebras. We also provide explicit proofs and several relevant examples in order to pick up the techniques needed for working in this field.

• William Schmitt

LECTURE:

Title: Incidence Hopf algebras

Abstract: The theory of incidence Hopf algebras provides a simple, unified framework for constructing Hopf algebras based on families of combinatorial structures, yielding a rich interplay between algebra and combinatorics. The aim of these lectures is to give both an introduction to and an overview of the subject, with emphasis on examples that reveal the combinatorial underpinnings of well-known Hopf algebras that occur in a variety of fields.

TALK:

Title: Noncrossing partition Hopf algebra

Abstract: We introduce and study the incidence Hopf algebra H of the family of noncrossing partition lattices of finite linearly ordered sets. After presenting some basic combinatorial results on noncrossing partitions, we give two formulas for the antipode S of H. The first of these is given as an alternating sum indexed by certain dissections of polygons and is cancellation-free. The second gives the value of S on the canonical set of generators in terms of a second set of generators, related to the first by Lagrange inversion; this formula is not only cancellation-free but is in fact sign-free. We demonstrate the relationship between S and reversion of formal power series and give a sign-free version of the Lagrange inversion formula. Finally, we use an edge labeling of noncrossing partition lattices, defined by Richard Stanley, to construct an isomorphism between H and the Hopf algebra of symmetric functions and an embedding of H in the Hopf algebra of parking functions. These maps have sign-free expressions in terms of the canonical bases of these Hopf algebras. (This joint work with Hillary Einziger.)

## Registration

To register for the Conference please send an email to:
< yael (dot) fregier (at) uni (dot) lu >
before the 01 July 2009.

There is a fee for participating to the conference (30 euros) which should cover lunches.

## Financial support

Having some funding from our ministry, we are able to support partially some participants. Students and young researchers are particularly encouraged to apply. Please contact Yael Fregier:
< yael (dot) fregier (at) uni (dot) lu >

## Practical Information

The workshop will be held in Campus Limpertsberg of the University of Luxembourg.

The Room BS 103 is in the science building (new building).

### Travel

#### By plane

The Luxembourg airport is connected by regular flights to all main destinations.

A regular bus service, operated by Flibco, connects the train station to the Frankfurt-Hahn airport, which is served by low-cost flights to many European destinations.

#### By train

See the websites of the Luxembourg rail company CFL, or SNCF, or Deutsche Bahn.

### Accomodation

A list of hotels in Luxembourg is available here. There is also a youth hostel.

Date: 2009/05/14 12:00:21 PM

HTML generated by org-mode 6.05 in emacs 22