##### Actions

(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)

A (non-associative) algebra (cf. Non-associative rings and algebras) whose commutator algebra becomes a Lie algebra. It was first introduced by A.A. Albert in 1948 and originated from one of the defining identities for standard algebras [a1]. For an algebra over a field , the commutator algebra of is the anti-commutative algebra with multiplication defined on the vector space . If is a Lie algebra, i.e. satisfies the Jacobi identity , then is called Lie-admissible (LA). Much of the structure theory of Lie-admissible algebras has been carried out initially under additional conditions such as the flexible identity or power-associativity (i.e. every element generates an associative subalgebra), or both. An algebra is flexible Lie-admissible (FLA) if and only if it satisfies the identity

 (a1)

if and only if the mapping is a Lie module homomorphism of to for under the adjoint action. For this reason, representations of Lie algebras play a main role in the structure theory of FLA algebras [a2]. Lie and associative algebras are examples of FLA algebras.

Beginning with Albert's problem of classifying all power-associative FLA algebras with semi-simple [a1], a common theme of the structure theory in various mathematical, physical and geometrical settings has been to focus on the case of a prescribed Lie algebra structure on . Albert's problem was first solved in 1962 for finite-dimensional algebras over an algebraically closed field of characteristic 0, and such algebras turned out to be Lie algebras [a3]. This result was extended to the case of when is a classical Lie algebra or a generalized Witt algebra [a2], [a4] (cf. Witt algebra). In 1981, these algebras were classified without the assumption of power-associativity [a5], [a6]: When is simple over the base field as above, the multiplication in is given by

 (a2)

for a fixed scalar , where for not of type (), and for of type (), and is defined on by

where denotes the matrix product of and and is the identity matrix. Thus, the algebra with of type () can not be power-associative. If is semi-simple, is a direct sum of simple algebras given by (a2). The classification was extended to the case where the solvable radical (cf. Radical of rings and algebras) of is a direct summand of or Abelian [a2]. In 1984, the algebras in Albert's problem were determined in the absence of flexibility [a7]: If is semi-simple with decomposition (), where the are simple ideals of , then the multiplication in has the form for , , where the are linear functionals on the and satisfy certain conditions prescribed in terms of graphs having 2, 3 or 4 vertices.

R.M. Santilli in 1978 obtained LA algebras (brackets) from a modified form of Hamilton's equations with external terms which represent a general non-self-adjoint Newtonian system in classical mechanics [a8]. Such a form leads to a time evolution

 (a3)

From a different point of view, S. Okubo [a12] in 1978 used FLA algebras to generalize the framework of the consistent canonical quantization procedure based on the associative law. A quantization is called consistent if the Hamiltonian equation of motion can reproduce the original Lagrange equation. Such a quantization can be done in based only on the canonical commutation relation and the identity (a1). If consists of operators in a physical system, then using (a1) it can be shown that the Heisenberg equation for some is essentially the most general time-development equation in , where is the commutator in . If the Hamiltonian is power-associative in , then the time-development operator is well defined for the Schrödinger formulation in with state vector satisfying . If, in addition, is weakly associative in , i.e. for all positive integers and , then the solution to the Heisenberg equation in has the form , as in the usual quantum mechanics. An example of such an algebra is the real pseudo-octonion algebra , which has the multiplication defined on the -space of Hermitian matrices of trace 0, where [a2]. is an FLA division algebra, with isomorphic to , and has some relevance to particle physics. It also plays an important role in the structure theory of real division algebras [a2].

An algebra over a field is called Mal'tsev-admissible (MA) if its commutator algebra becomes a Mal'tsev algebra, i.e., satisfies the Mal'tsev identity

It arises as a natural generalization of LA algebras as well as Mal'tsev algebras, and its structure theory is parallel to that of LA algebras [a2]. Alternative algebras (cf. Alternative rings and algebras) are examples of flexible Mal'tsev-admissible (FMA) algebras, and octonion algebras (also called Cayley–Dickson algebras, cf. Cayley–Dickson algebra) are FMA but not LA. For an octonion algebra over a field of characteristic with standard involution , one obtains an algebra with multiplication defined on . The algebra , called a para-octonion algebra, is a simple FMA algebra without identity and so not alternative [a2]. An algebra (not necessarily with identity) is called a composition algebra if there exists a non-degenerate quadratic form on such that for all . Any finite-dimensional flexible composition algebra () is an MA algebra of dimension 1, 2, 4, or 8, and for dimension 8 octonion, pseudo-octonion and para-octonion algebras are the only such algebras [a13]. For an MA algebra , let , where is the adjoint mapping given by . Then is a Lie subalgebra of the derivation algebra of (cf. also Derivation in a ring), and if is FMA, then is also a subalgebra of and the mapping is a Lie module homomorphism of to for [a2]. Let be finite dimensional over a field of characteristic 0. If is semi-simple, then so is . Because of this, virtually all results about FLA algebras can be extended to FMA algebras [a2]. If is FMA with central simple, non-Lie over , then is a Mal'tsev algebra isomorphic to a -dimensional simple Mal'tsev algebra obtained from an octonion algebra (cf. Mal'tsev algebra). If is semi-simple and is algebraically closed, then is the direct sum of simple algebras given by (a2) and simple Mal'tsev algebras. Some of the work on MA algebras was motivated by algebraic formalisms in physics aimed at generalizing both the Lie-admissible and the octonionic approach in quantum mechanics.

LA and MA algebras also arise from differential geometry on Lie groups and reductive homogeneous spaces. For a (connected) Lie group with Lie algebra , the determination of all (left) invariant affine connections (cf. Affine connection) on reduces to the problem of classifying all algebras with a multiplication defined on ; the relation is given by for . In this case, is called the connection algebra of . Those connections which are torsion free correspond to the LA algebras with (i.e., ) [a14]. If, in addition, the curvature tensor of is zero (i.e., is flat), then satisfies the left-symmetric identity

The classification of left-invariant affine structures on reduces to that of left-symmetric algebras with [a15]. Other geometrical properties of on , such as geodesic, holonomy, pseudo-Riemannian structure, and infinitesimal generator, can be described in terms of . For example, if every vector field in is an infinitesimal generator for a one-parameter group of affine diffeomorphisms on for , then the connection algebra of is FLA [a14].

Let be a reductive homogeneous space with a fixed decomposition (direct sum), where is the Lie algebra of a closed Lie subgroup of and is a subspace of such that (or, equivalently, ). There is a one-one correspondence between the set of -invariant affine connections on and the set of algebras with , i.e., , the automorphism group of . The projection of onto for converts into an anti-commutative algebra , called a reductive algebra. More generally, an algebra is called reductive-admissible if is isomorphic to for some reductive decomposition of a Lie algebra . Those connections on which are torsion free correspond to the reductive-admissible algebras such that and or . Any MA algebra is reductive-admissible with , where is a Lie algebra with multiplication for and . Geometrical properties of on such as those above are described in terms of the connection algebra . For a detailed account of these, see [a15][a17].