# Jordan algebra

In abstract algebra, a **Jordan algebra** is a nonassociative algebra over a field whose multiplication satisfies the following axioms:

The product of two elements *x* and *y* in a Jordan algebra is also denoted *x* ∘ *y*, particularly to avoid confusion with the product of a related associative algebra.

Jordan algebras were first introduced by Pascual Jordan (1933) to formalize the notion of an algebra of observables in quantum mechanics. They were originally called "r-number systems", but were renamed "Jordan algebras" by Abraham Adrian Albert (1946), who began the systematic study of general Jordan algebras.

Given an associative algebra *A* (not of characteristic 2), one can construct a Jordan algebra *A*^{+} using the same underlying addition vector space. Notice first that an associative algebra is a Jordan algebra if and only if it is commutative. If it is not commutative we can define a new multiplication on *A* to make it commutative, and in fact make it a Jordan algebra. The new multiplication *x* ∘ *y* is the **Jordan product**:

This defines a Jordan algebra *A*^{+}, and we call these Jordan algebras, as well as any subalgebras of these Jordan algebras, **special Jordan algebras**. All other Jordan algebras are called **exceptional Jordan algebras**. The Shirshov–Cohn theorem states that any Jordan algebra with two generators is special.^{[2]} Related to this, Macdonald's theorem states that any polynomial in three variables, that has degree one in one of the variables, and that vanishes in every special Jordan algebra, vanishes in every Jordan algebra.^{[3]}

If (*A*, *σ*) is an associative algebra with an involution *σ*, then if *σ*(*x*)=*x* and *σ*(*y*)=*y* it follows that

Thus the set of all elements fixed by the involution (sometimes called the *hermitian* elements) form a subalgebra of *A*^{+}, which is sometimes denoted H(*A*,*σ*).

1. The set of self-adjoint real, complex, or quaternionic matrices with multiplication

2. The set of 3×3 self-adjoint matrices over the octonions, again with multiplication

is a 27 dimensional, exceptional Jordan algebra (it is exceptional because the octonions are not associative). This was the first example of an Albert algebra. Its automorphism group is the exceptional Lie group F_{4}. Since over the complex numbers this is the only simple exceptional Jordan algebra up to isomorphism,^{[4]} it is often referred to as "the" exceptional Jordan algebra. Over the real numbers there are three isomorphism classes of simple exceptional Jordan algebras.^{[4]}

A derivation of a Jordan algebra *A* is an endomorphism *D* of *A* such that *D*(*xy*) = *D*(*x*)*y*+*xD*(*y*). The derivations form a Lie algebra **der**(*A*). The Jordan identity implies that if *x* and *y* are elements of *A*, then the endomorphism sending *z* to *x*(*yz*)−*y*(*xz*) is a derivation. Thus the direct sum of *A* and **der**(*A*) can be made into a Lie algebra, called the **structure algebra** of *A*, **str**(*A*).

A simple example is provided by the Hermitian Jordan algebras H(*A*,*σ*). In this case any element *x* of *A* with *σ*(*x*)=−*x* defines a derivation. In many important examples, the structure algebra of H(*A*,*σ*) is *A*.

Derivation and structure algebras also form part of Tits' construction of the Freudenthal magic square.

A (possibly nonassociative) algebra over the real numbers is said to be **formally real** if it satisfies the property that a sum of *n* squares can only vanish if each one vanishes individually. In 1932, Jordan attempted to axiomatize quantum theory by saying that the algebra of observables of any quantum system should be a formally real algebra that is commutative (*xy* = *yx*) and power-associative (the associative law holds for products involving only *x*, so that powers of any element *x* are unambiguously defined). He proved that any such algebra is a Jordan algebra.

Not every Jordan algebra is formally real, but Jordan, von Neumann & Wigner (1934) classified the finite-dimensional formally real Jordan algebras, also called **Euclidean Jordan algebras**. Every formally real Jordan algebra can be written as a direct sum of so-called **simple** ones, which are not themselves direct sums in a nontrivial way. In finite dimensions, the simple formally real Jordan algebras come in four infinite families, together with one exceptional case:

Of these possibilities, so far it appears that nature makes use only of the *n*×*n* complex matrices as algebras of observables. However, the spin factors play a role in special relativity, and all the formally real Jordan algebras are related to projective geometry.

If *e* is an idempotent in a Jordan algebra *A* (*e*^{2} = *e*) and *R* is the operation of multiplication by *e*, then

so the only eigenvalues of *R* are 0, 1/2, 1. If the Jordan algebra *A* is finite-dimensional over a field of characteristic not 2, this implies that it is a direct sum of subspaces *A* = *A*_{0}(*e*) ⊕ *A*_{1/2}(*e*) ⊕ *A*_{1}(*e*) of the three eigenspaces. This decomposition was first considered by Jordan, von Neumann & Wigner (1934) for totally real Jordan algebras. It was later studied in full generality by Albert (1947) and called the **Peirce decomposition** of *A* relative to the idempotent *e*.^{[5]}

In 1979, Efim Zelmanov classified infinite-dimensional simple (and prime non-degenerate) Jordan algebras. They are either of Hermitian or Clifford type. In particular, the only exceptional simple Jordan algebras are finite-dimensional Albert algebras, which have dimension 27.

The theory of operator algebras has been extended to cover Jordan operator algebras.

The counterparts of C* algebras are JB algebras, which in finite dimensions are called Euclidean Jordan algebras. The norm on the real Jordan algebra must be complete and satisfy the axioms:

These axioms guarantee that the Jordan algebra is formally real, so that, if a sum of squares of terms is zero, those terms must be zero. The complexifications of JB algebras are called Jordan C* algebras or JB* algebras. They have been used extensively in complex geometry to extend Koecher's Jordan algebraic treatment of bounded symmetric domains to infinite dimensions. Not all JB algebras can be realized as Jordan algebras of self-adjoint operators on a Hilbert space, exactly as in finite dimensions. The exceptional Albert algebra is the common obstruction.

The Jordan algebra analogue of von Neumann algebras is played by JBW algebras. These turn out to be JB algebras which, as Banach spaces, are the dual spaces of Banach spaces. Much of the structure theory of von Neumann algebras can be carried over to JBW algebras. In particular the JBW factors—those with center reduced to **R**—are completely understood in terms of von Neumann algebras. Apart from the exceptional Albert algebra, all JWB factors can be realised as Jordan algebras of self-adjoint operators on a Hilbert space closed in the weak operator topology. Of these the spin factors can be constructed very simply from real Hilbert spaces. All other JWB factors are either the self-adjoint part of a von Neumann factor or its fixed point subalgebra under a period 2 *-antiautomorphism of the von Neumann factor.^{[6]}

A Jordan ring is a generalization of Jordan algebras, requiring only that the Jordan ring be over a general ring rather than a field. Alternatively one can define a Jordan ring as a commutative nonassociative ring that respects the Jordan identity.

The concept of J-structure was introduced by Springer (1973) to develop a theory of Jordan algebras using linear algebraic groups and axioms taking the Jordan inversion as basic operation and Hua's identity as a basic relation. In characteristic not equal to 2 the theory of J-structures is essentially the same as that of Jordan algebras.

Quadratic Jordan algebras are a generalization of (linear) Jordan algebras introduced by Kevin McCrimmon (1966). The fundamental identities of the quadratic representation of a linear Jordan algebra are used as axioms to define a quadratic Jordan algebra over a field of arbitrary characteristic. There is a uniform description of finite-dimensional simple quadratic Jordan algebras, independent of characteristic: in characteristic not equal to 2 the theory of quadratic Jordan algebras reduces to that of linear Jordan algebras.