# Jordan Algebras¶

AUTHORS:

• Travis Scrimshaw (2014-04-02): initial version
class sage.algebras.jordan_algebra.JordanAlgebra

A Jordan algebra.

A Jordan algebra is a magmatic algebra (over a commutative ring $$R$$) whose multiplication satisfies the following axioms:

• $$xy = yx$$, and
• $$(xy)(xx) = x(y(xx))$$ (the Jordan identity).

See [Ja1971], [Ch2012], and [McC1978], for example.

These axioms imply that a Jordan algebra is power-associative and the following generalization of Jordan’s identity holds [Al1947]: $$(x^m y) x^n = x^m (y x^n)$$ for all $$m, n \in \ZZ_{>0}$$.

Let $$A$$ be an associative algebra over a ring $$R$$ in which $$2$$ is invertible. We construct a Jordan algebra $$A^+$$ with ground set $$A$$ by defining the multiplication as

$x \circ y = \frac{xy + yx}{2}.$

Often the multiplication is written as $$x \circ y$$ to avoid confusion with the product in the associative algebra $$A$$. We note that if $$A$$ is commutative then this reduces to the usual multiplication in $$A$$.

Jordan algebras constructed in this fashion, or their subalgebras, are called special. All other Jordan algebras are called exceptional.

Jordan algebras can also be constructed from a module $$M$$ over $$R$$ with a symmetric bilinear form $$(\cdot, \cdot) : M \times M \to R$$. We begin with the module $$M^* = R \oplus M$$ and define multiplication in $$M^*$$ by

$(\alpha + x) \circ (\beta + y) = \underbrace{\alpha \beta + (x,y)}_{\in R} + \underbrace{\beta x + \alpha y}_{\in M}$

where $$\alpha, \beta \in R$$ and $$x,y \in M$$.

INPUT:

Can be either an associative algebra $$A$$ or a symmetric bilinear form given as a matrix (possibly followed by, or preceded by, a base ring argument)

EXAMPLES:

We let the base algebra $$A$$ be the free algebra on 3 generators:

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: J = JordanAlgebra(F); J
Jordan algebra of Free Algebra on 3 generators (x, y, z) over Rational Field
sage: a,b,c = map(J, F.gens())
sage: a*b
1/2*x*y + 1/2*y*x
sage: b*a
1/2*x*y + 1/2*y*x


Jordan algebras are typically non-associative:

sage: (a*b)*c
1/4*x*y*z + 1/4*y*x*z + 1/4*z*x*y + 1/4*z*y*x
sage: a*(b*c)
1/4*x*y*z + 1/4*x*z*y + 1/4*y*z*x + 1/4*z*y*x


We check the Jordan identity:

sage: (a*b)*(a*a) == a*(b*(a*a))
True
sage: x = a + c
sage: y = b - 2*a
sage: (x*y)*(x*x) == x*(y*(x*x))
True


Next we construct a Jordan algebra from a symmetric bilinear form:

sage: m = matrix([[-2,3],[3,4]])
sage: J.<a,b,c> = JordanAlgebra(m); J
Jordan algebra over Integer Ring given by the symmetric bilinear form:
[-2  3]
[ 3  4]
sage: a
1 + (0, 0)
sage: b
0 + (1, 0)
sage: x = 3*a - 2*b + c; x
3 + (-2, 1)


We again show that Jordan algebras are usually non-associative:

sage: (x*b)*b
-6 + (7, 0)
sage: x*(b*b)
-6 + (4, -2)


We verify the Jordan identity:

sage: y = -a + 4*b - c
sage: (x*y)*(x*x) == x*(y*(x*x))
True


The base ring, while normally inferred from the matrix, can also be explicitly specified:

sage: J.<a,b,c> = JordanAlgebra(m, QQ); J
Jordan algebra over Rational Field given by the symmetric bilinear form:
[-2  3]
[ 3  4]
sage: J.<a,b,c> = JordanAlgebra(QQ, m); J # either order work
Jordan algebra over Rational Field given by the symmetric bilinear form:
[-2  3]
[ 3  4]


REFERENCES:

class sage.algebras.jordan_algebra.JordanAlgebraSymmetricBilinear(R, form, names=None)

A Jordan algebra given by a symmetric bilinear form $$m$$.

class Element(parent, s, v)

An element of a Jordan algebra defined by a symmetric bilinear form.

bar()

Return the result of the bar involution of self.

The bar involution $$\bar{\cdot}$$ is the $$R$$-linear endomorphism of $$M^*$$ defined by $$\bar{1} = 1$$ and $$\bar{x} = -x$$ for $$x \in M$$.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J.<a,b,c> = JordanAlgebra(m)
sage: x = 4*a - b + 3*c
sage: x.bar()
4 + (1, -3)


We check that it is an algebra morphism:

sage: y = 2*a + 2*b - c
sage: x.bar() * y.bar() == (x*y).bar()
True

monomial_coefficients(copy=True)

Return a dictionary whose keys are indices of basis elements in the support of self and whose values are the corresponding coefficients.

INPUT:

• copy – ignored

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J.<a,b,c> = JordanAlgebra(m)
sage: elt = a + 2*b - c
sage: elt.monomial_coefficients()
{0: 1, 1: 2, 2: -1}

norm()

Return the norm of self.

The norm of an element $$\alpha + x \in M^*$$ is given by $$n(\alpha + x) = \alpha^2 - (x, x)$$.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J.<a,b,c> = JordanAlgebra(m)
sage: x = 4*a - b + 3*c; x
4 + (-1, 3)
sage: x.norm()
13

trace()

Return the trace of self.

The trace of an element $$\alpha + x \in M^*$$ is given by $$t(\alpha + x) = 2 \alpha$$.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J.<a,b,c> = JordanAlgebra(m)
sage: x = 4*a - b + 3*c
sage: x.trace()
8

algebra_generators()

Return a basis of self.

The basis returned begins with the unity of $$R$$ and continues with the standard basis of $$M$$.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J = JordanAlgebra(m)
sage: J.basis()
Family (1 + (0, 0), 0 + (1, 0), 0 + (0, 1))

basis()

Return a basis of self.

The basis returned begins with the unity of $$R$$ and continues with the standard basis of $$M$$.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J = JordanAlgebra(m)
sage: J.basis()
Family (1 + (0, 0), 0 + (1, 0), 0 + (0, 1))

gens()

Return the generators of self.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J = JordanAlgebra(m)
sage: J.basis()
Family (1 + (0, 0), 0 + (1, 0), 0 + (0, 1))

one()

Return the element 1 if it exists.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J = JordanAlgebra(m)
sage: J.one()
1 + (0, 0)

zero()

Return the element 0.

EXAMPLES:

sage: m = matrix([[0,1],[1,1]])
sage: J = JordanAlgebra(m)
sage: J.zero()
0 + (0, 0)

class sage.algebras.jordan_algebra.SpecialJordanAlgebra(A, names=None)

A (special) Jordan algebra $$A^+$$ from an associative algebra $$A$$.

class Element(parent, x)

An element of a special Jordan algebra.

monomial_coefficients(copy=True)

Return a dictionary whose keys are indices of basis elements in the support of self and whose values are the corresponding coefficients.

INPUT:

• copy – (default: True) if self is internally represented by a dictionary d, then make a copy of d; if False, then this can cause undesired behavior by mutating d

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: J = JordanAlgebra(F)
sage: a,b,c = map(J, F.gens())
sage: elt = a + 2*b - c
sage: elt.monomial_coefficients()
{x: 1, y: 2, z: -1}

algebra_generators()

Return the basis of self.

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: J = JordanAlgebra(F)
sage: J.basis()
Lazy family (Term map(i))_{i in Free monoid on 3 generators (x, y, z)}

basis()

Return the basis of self.

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: J = JordanAlgebra(F)
sage: J.basis()
Lazy family (Term map(i))_{i in Free monoid on 3 generators (x, y, z)}

gens()

Return the generators of self.

EXAMPLES:

sage: cat = Algebras(QQ).WithBasis().FiniteDimensional()
sage: C = CombinatorialFreeModule(QQ, ['x','y','z'], category=cat)
sage: J = JordanAlgebra(C)
sage: J.gens()
(B['x'], B['y'], B['z'])

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: J = JordanAlgebra(F)
sage: J.gens()
Traceback (most recent call last):
...
NotImplementedError: infinite set

one()

Return the element $$1$$ if it exists.

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: J = JordanAlgebra(F)
sage: J.one()
1

zero()

Return the element $$0$$.

EXAMPLES:

sage: F.<x,y,z> = FreeAlgebra(QQ)
sage: J = JordanAlgebra(F)
sage: J.zero()
0