arXiv subjects

Frederic Helein

Publications and source records attributed to Frederic Helein.

10 recordsLinked to original sources

arXiv2020

An introduction to supermanifolds and supersymmetry

The aim of these notes (which were partially covered in lectures given at the Peyresq Summer School on 17--22 June, 2002) is to give an introduction to some mathematical aspects of supersymmetry. Some (hopefully) original point of view are added, by using and developing some results in arXiv:math-ph/0603045, where maps of supermanifolds are analyzed in details and, also, in the presentation of the super Minkowski spaces in dimensions 3, 4, 6 and 10.

math-ph↗

arXiv2004

Covariant Hamiltonian formalism for the calculus of variations with several variables: Lepage--Dedecker versus de Donder--Weyl

The main purpose in the present paper is to build a Hamiltonian theory for fields which is consistent with the principles of relativity. For this we consider detailed geometric pictures of Lepage theories in the spirit of Dedecker and try to stress out the interplay between the Lepage-Dedecker (LP) description and the (more usual) de Donder-Weyl (dDW) one. One of the main points is the fact that the Legendre transform in the dDW approach is replaced by a Legendre correspondence in the LP theory (This correspondence behaves differently: ignoring the singularities whenever the Lagrangian is degenerate).

math-ph↗

arXiv2004

The notion of observable in the covariant Hamiltonian formalism for the calculus of variations with several variables

This papers is concerned with multisymplectic formalisms which are the frameworks for Hamiltonian theories for fields theory. Our main purpose is to study the observable $(n-1)$-forms which allows one to construct observable functionals on the set of solutions of the Hamilton equations by integration. We develop here two different points of view: generalizing the law $\{p,q\} = 1$ or the law $dF/dt = \{H,F\}$. This leads to two possible definitions; we explore the relationships and the differences between these two concepts. We show that -- in contrast with the de Donder--Weyl theory -- the two definitions coincides in the Lepage--Dedecker theory.

math-ph↗

arXiv2003

Removability of singularities of harmonic maps into pseudo-Riemannian manifolds

We consider harmonic maps into pseudo-Riemannian manifolds. We show the removability of isolated singularities for continuous maps, i.e. that any continuous map from an open subset of R^m into a pseudo-Riemannian manifold which is two times continuously differentiable and harmonic everywhere outside an isolated point is actually smooth harmonic everywhere.

math.AP↗

arXiv2002

Hamiltonian formalisms for multidimensional calculus of variations and perturbation theory

In a first part we propose an introduction to multisymplectic formalisms, which are generalisations of Hamilton's formulation of Mechanics to the calculus of variations with several variables: we give some physical motivations, related to the quantum field theory, and expound the simplest example, based on a theory due to T. de Donder and H. Weyl. In a second part we explain quickly a work in collaboration with J. Kouneiher (math-ph/0211046) on generalizations of the de Donder--Weyl theory (known as Lepage theories). Lastly we show that in this framework a perturbative classical field theory (analog of the perturbative quantum field theory) can be constructed.

math-ph↗

arXiv2001

On Konopelchenko's representation for surfaces in 4 dimensions

The purpose of this short note is to relate a representation formula due to the Author and P. Romon for Lagrangian surfaces (see math.DG/0009202) to a more general Weierstrass representation type formula found by Konopelchenko for surfaces in 4-dimensional space (see math.DG/9807129). Simplifications are pointed out.

math.DG↗

arXiv2001

Problemes variationnels invariants par transformation conforme en dimension 2

This text proposes geometrical descriptions of all variational problems invariant by conformal transformations in two variables. First a characterisation in terms of C-Finsler manifolds, a suitable generalization of Finsler manifolds, is given. Second Hamiltonian formalisms are explored, with an emphasis on Caratheodory's formalism. Some of these considerations were subsequently developped in papers with Joseph Kouneiher in math-ph/0004020 and math-ph/0010036

math.DG↗

arXiv2000

Hamiltonian stationary Lagrangian surfaces in Hermitian symmetric spaces

This paper is the third of a series on Hamiltonian stationary Lagrangian surfaces. We present here the most general theory, valid for any Hermitian symmetric target space. Using well-chosen moving frame formalism, we show that the equations are equivalent to an integrable system, generalizing the C^2 subcase analyzed in the first article (arXiv:math.DG/0009202). This system shares many features with the harmonic map equation of surfaces into symmetric spaces, allowing us to develop a theory close to Dorfmeister, Pedit and Wu's, including for instance a Weierstrass-type representation. Notice that this article encompasses the article mentioned above, although much fewer details will be given on that particular flat case.

math.DG↗

arXiv2000

Hamiltonian stationary Lagrangian surfaces in C^2

We study Hamiltonian stationary Lagrangian surfaces in C^2, i.e. Lagrangian surfaces in C^2 which are stationary points of the area functional under smooth Hamiltonian variations. Using loop groups, we propose a formulation of the equation as a completely integrable system. We construct a Weierstrass type representation and produce all tori through either the integrable systems machinery or more direct arguments.

math.DG↗

arXiv2000

Hamiltonian formalism with several variables and quantum field theory, PartI

We discuss in this paper the canonical structure of classical field theory in finite dimensions within the {\it{pataplectic}} hamiltonian formulation, where we put forward the role of Legendre correspondance. We define the Poisson $\mathfrak{p}$-brackets and $\mathfrakω$-brackets which are the analogues of the Poisson bracket on forms. We formulate the equations of motion of forms in terms of $\mathfrak{p}$-brackets and $\mathfrakω$-brackets with the $n$-form ${\cal H}ω$. As illustration of our formalism we present two examples: the interacting scalar fields and conformal string theory.

math-ph↗