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Publication# Toda frames, harmonic maps and extended Dynkin diagrams

Abstract

We consider a natural subclass of harmonic maps from a surface into G/T, namely cyclic primitive maps. Here G is any simple real Lie group (not necessarily compact), T is a Cartan subgroup and both are chosen so that there is a Coxeter automorphism on G(C)/T-C which restricts to give a k-symmetric space structure on G/T. When G is compact, any Coxeter automorphism restricts to the real form. It was shown in [3] that cyclic primitive immersions into compact G/T correspond to solutions of the affine Toda field equations and all those of a genus one surface can be constructed by integrating a pair of commuting vector fields on a finite dimensional vector subspace of a Lie algebra. We generalise these results, removing the assumption that G is compact. The first major obstacle is that a Coxeter automorphism may not restrict to a non-compact real form. We characterise, in terms of extended Dynkin diagrams, those simple real Lie groups G and Cartan subgroups T such that G/T has a k-symmetric space structure induced from a Coxeter automorphism. A Coxeter automorphism preserves the real Lie algebra g if and only if any corresponding Cartan involution defines a permutation of the extended Dynkin diagram for g(C) = g circle times C; we show that every involution of the extended Dynkin diagram for a simple complex Lie algebra g(C) is induced by a Cartan involution of a real form of sf. (C) 2017 Published by Elsevier B.V.

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Related concepts (6)

Lie algebra

In mathematics, a Lie algebra (pronounced liː ) is a vector space together with an operation called the Lie bracket, an alternating bilinear map , that satisfies the Jacobi identity. Otherwise said, a Lie algebra is an algebra over a field where the multiplication operation is now called Lie bracket and has two additional properties: it is alternating and satisfies the Jacobi identity. The Lie bracket of two vectors and is denoted . The Lie bracket does not need to be associative, meaning that the Lie algebra can be non associative.

Harmonic map

In the mathematical field of differential geometry, a smooth map between Riemannian manifolds is called harmonic if its coordinate representatives satisfy a certain nonlinear partial differential equation. This partial differential equation for a mapping also arises as the Euler-Lagrange equation of a functional called the Dirichlet energy. As such, the theory of harmonic maps contains both the theory of unit-speed geodesics in Riemannian geometry and the theory of harmonic functions.

Simple Lie group

In mathematics, a simple Lie group is a connected non-abelian Lie group G which does not have nontrivial connected normal subgroups. The list of simple Lie groups can be used to read off the list of simple Lie algebras and Riemannian symmetric spaces. Together with the commutative Lie group of the real numbers, , and that of the unit-magnitude complex numbers, U(1) (the unit circle), simple Lie groups give the atomic "blocks" that make up all (finite-dimensional) connected Lie groups via the operation of group extension.