Pre-Lie algebras have been considered under some other names, among which one can cite left-symmetric algebras, right-symmetric algebras or Vinberg algebras.
Definition
A pre-Lie algebra is a vector space with a bilinear map , satisfying the relation
This identity can be seen as the invariance of the associator under the exchange of the two variables and .
Every associative algebra is hence also a pre-Lie algebra, as the associator vanishes identically. Although weaker than associativity, the defining relation of a pre-Lie algebra still implies that the commutator is a Lie bracket. In particular, the Jacobi identity for the commutator follows from cycling the terms in the defining relation for pre-Lie algebras, above.
Let be an open neighborhood of , parameterised by variables . Given vector fields , we define .
The difference between and , is
which is symmetric in and . Thus defines a pre-Lie algebra structure.
Given a manifold and homeomorphisms from to overlapping open neighborhoods of , they each define a pre-Lie algebra structure on vector fields defined on the overlap. Whilst need not agree with , their commutators do agree: , the Lie bracket of and .
Rooted trees
Let be the vector space spanned by all rooted trees.
One can introduce a bilinear product on as follows. Let and be two rooted trees.
where is the rooted tree obtained by adding to the disjoint union of and an edge going from the vertex of to the root vertex of .