nLab Goldman bracket




topology (point-set topology, point-free topology)

see also differential topology, algebraic topology, functional analysis and topological homotopy theory


Basic concepts

Universal constructions

Extra stuff, structure, properties


Basic statements


Analysis Theorems

topological homotopy theory



The Goldman bracket of a compact closed surface Σ\Sigma is a Lie algebra structure on the free abelian group generated from the isotopy classes of based loops in Σ\Sigma.

Equivalently, the Goldman bracket on Σ\Sigma is a structure on the 0th homology H 0(LΣ)H_0(L \Sigma) of the free loop space of Σ\Sigma. It is in fact just the lowest degree of the string topology operations on Σ\Sigma. See there for more details.


Let Σ\Sigma be a compact closed and oriented surface (manifold of dimension 2). For γ:S 1Σ\gamma : S^1 \to \Sigma a continuous function from the based circle, write [γ][\gamma] for the corresponding isotopy class.

For [γ 1][\gamma_1] and [γ 2][\gamma_2] two such classes, one can always find differentiable representatives γ 1\gamma_1 and γ 2\gamma_2 that intersect - if they intersect at some point pp - transversally. Write γ 1* pγ 2\gamma_1 \ast_p \gamma_2 for the curve obtained by starting at the intersection point pp, traversing along γ 1\gamma_1 back to that point and then along γ 2\gamma_2.

The Goldman bracket on the free abelian group on classes [γ][\gamma] is defined by

{[γ 1],[γ 2]}:= pγ 1γ 2sgn(p)[γ 1* pγ 2], \left\{ [\gamma_1], [\gamma_2] \right\} := \sum_{p \in \gamma_1 \cap \gamma_2} sgn(p) [\gamma_1 \ast_p \gamma_2] \,,

where sgn(p)sgn(p) is +1 if T pγ 1,T pγ 2T_p \gamma_1, T_p \gamma_2 is an oriented basis of the tangent space T pΣT_p \Sigma, and -1 otherwise.


The original definition is due to

  • W. Goldman, Invariant functions on Lie groups and Hamiltonian flows of surface group representations , Invent. Math. (1986), no. 85, 263302.

The relation to string topology is due to

Last revised on June 19, 2013 at 07:08:20. See the history of this page for a list of all contributions to it.