J-structure
# J-structure

In mathematics, a J-structure is an algebraic structure over a field related to a Jordan algebra. The concept 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. There is a classification of simple structures deriving from the classification of semisimple algebraic groups. Over fields of characteristic not equal to 2, the theory of J-structures is essentially the same as that of Jordan algebras.

## Definition

Let V be a finite-dimensional vector space over a field K and j a rational map from V to itself, expressible in the form n/N with n a polynomial map from V to itself and N a polynomial in K[V]. Let H be the subset of GL(V) × GL(V) containing the pairs (g,h) such that g∘j = j∘h: it is a closed subgroup of the product and the projection onto the first factor, the set of g which occur, is the structure group of j, denoted G'(j).

A J-structure is a triple (V,j,e) where V is a vector space over K, j is a birational map from V to itself and e is a non-zero element of V satisfying the following conditions.

* j is a homogeneous birational involution of degree −1
* j is regular at e and j(e) = e
* if j is regular at x, e + x and e + j(x) then

    $j(e+x)+j(e+j(x))=e$
* the orbit G e of e under the structure group G = G(j) is a Zariski open subset of V.

The norm associated to a J-structure (V,j,e) is the numerator N of j, normalised so that N(e) = 1. The degree of the J-structure is the degree of N as a homogeneous polynomial map.

The quadratic map of the structure is a map P from V to End(V) defined in terms of the differential dj at an invertible x. We put

    $P(x)=-(dj)_{x}^{-1}.$

The quadratic map turns out to be a quadratic polynomial map on V.

The subgroup of the structure group G generated by the invertible quadratic maps is the inner structure group of the J-structure. It is a closed connected normal subgroup.

## J-structures from quadratic forms

Let K have characteristic not equal to 2. Let Q be a quadratic form on the vector space V over K with associated bilinear form Q(x,y) = Q(x+y) − Q(x) − Q(y) and distinguished element e such that Q(e,.) is not trivial. We define a reflection map $x^*$ by

    $x^{*}=Q(x,e)e-x$

and an inversion map j by

    $j(x)=Q(x)^{-1}x^{*}.$

Then (V,j,e) is a J-structure.

### Example

Let Q be the usual sum of squares quadratic function on $K^r$ for fixed integer r, equipped with the standard basis $e^1,...,e^r.$ Then $(K^r,$ Q, $e^r)$ is a J-structure of degree 2. It is denoted $O_2.$

## Link with Jordan algebras

In characteristic not equal to 2, which we assume in this section, the theory of J-structures is essentially the same as that of Jordan algebras.

Let A be a finite-dimensional commutative non-associative algebra over K with identity e. Let L(x) denote multiplication on the left by x. There is a unique birational map i on A such that i(x).x = e if i is regular on x: it is homogeneous of degree −1 and an involution with i(e) = e. It may be defined by i(x) = $L(x)^{−1}.e.$ We call i the inversion on A.

A Jordan algebra is defined by the identity

    $x(x^{2}y)=x^{2}(xy).$

An alternative characterisation is that for all invertible x we have

    $x^{-1}(xy)=x(x^{-1}y).$

If A is a Jordan algebra, then (A,i,e) is a J-structure. If (V,j,e) is a J-structure, then there exists a unique Jordan algebra structure on V with identity e with inversion j.

## Link with quadratic Jordan algebras

In general characteristic, which we assume in this section, J-structures are related to quadratic Jordan algebras. We take a quadratic Jordan algebra to be a finite dimensional vector space V with a quadratic map Q from V to End(V) and a distinguished element e. We let Q also denote the bilinear map Q(x,y) = Q(x+y) − Q(x) − Q(y). The properties of a quadratic Jordan algebra will be

* Q(e) = $id_V,$ Q(x,e)y = Q(x,y)e
* Q(Q(x)y) = Q(x)Q(y)Q(x)
* Q(x)Q(y,z)x = Q(Q(x)y,x)z

We call Q(x)e the square of x. If the squaring is dominant (has Zariski dense image) then the algebra is termed separable.

There is a unique birational involution i such that Q(x)i x = x if Q is regular at x. As before, i is the inversion, definable by i(x) = $Q(x)^{−1}$ x.

If (V,j,e) is a J-structure, with quadratic map Q then (V,Q,e) is a quadratic Jordan algebra. In the opposite direction, if (V,Q,e) is a separable quadratic Jordan algebra with inversion i, then (V,i,e) is a J-structure.

### H-structure

McCrimmon proposed a notion of H-structure by dropping the density axiom and strengthening the third (a form of Hua's identity) to hold in all isotopes. The resulting structure is categorically equivalent to a quadratic Jordan algebra.

## Peirce decomposition

A J-structure has a Peirce decomposition into subspaces determined by idempotent elements. Let a be an idempotent of the J-structure (V,j,e), that is, $a^2$ = a. Let Q be the quadratic map. Define

    $\phi _{a}(t,u)=Q(ta+u(e-a)).$

This is invertible for non-zero t,u in K and so φ defines a morphism from the algebraic torus $GL_1$ × $GL_1$ to the inner structure group $G_1.$ There are subspaces

    $V_{a}=\left\lbrace {x\in V:\phi _{a}(t,u)x=t^{2}x}\right\rbrace$
    $V'_{a}=\left\lbrace {x\in V:\phi _{a}(t,u)x=tux}\right\rbrace$
    $V_{e-a}=\left\lbrace {x\in V:\phi _{a}(t,u)x=u^{2}x}\right\rbrace$

and these form a direct sum decomposition of V. This is the Peirce decomposition for the idempotent a.

## Generalisations

If we drop the condition on the distinguished element e, we obtain "J-structures without identity". These are related to isotopes of Jordan algebras.

