Pre-Lie Structures for Semisimple Lie Algebras
We address the problem of admissibility of pre-Lie structures associated with a given Lie algebra, particularly, semisimple Lie algebras over ${\mathbb C}$. Such structures are collectively referred to as Lie-admissible algebras, which are a class of nonassociative algebras such that the commutator bracket over these algebras satisfies the Jacobi identity. Among the five classes of nonassociative Lie-admissible algebras, left-symmetric algebras (LSAs) and right-symmetric algebras (RSAs), are known to be non-admissible by semisimple Lie algebras of finite dimension $n \geq 3$. Here, we examine the remaining classes starting with those corresponding to the subgroup generated by permutations of order 2: $(1 \; 3)$. These appear in the literature as anti-flexible algebras (AFAs). We discuss properties of AFAs and provide examples of finite-dimensional representations. AFAs geometrically correspond to richer structures than the flat torsion-free affine connections associated with left-symmetric algebras (LSAs) or right-symmetric algebras (RSAs). We compute Lie-admissibility criteria for AFAs and determine a few simple solution classes. Not surprisingly, solvable Lie algebras admit AFAs. Concerning semisimple ones, we report an explicit counterexample demonstrating an AFA admissible by ${\mathfrak sl(2, \, {\mathbb C})}$. We then discuss the remaining two classes of nonassociative Lie-admissible algebras, the $A_3$-associative and $S_3$-associative types. Finally, we prove that $S_3$-associative algebras are universal pre-Lie structures for any Lie algebra over ${\mathbb C}$, including semisimple ones.