X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=mjo%2Feja%2Feja_algebra.py;h=631d695aa75208a65cb457a4ca11280f44823765;hb=76352ef33974a5ec638b8b1fcab5508f915ea976;hp=d38ba87d37d3e6bdb01e23c7a9631bf98ee2c96c;hpb=5dcc3f73aba8c0b71b601c2d8bd65d9229c2ddb0;p=sage.d.git

diff --git a/mjo/eja/eja_algebra.py b/mjo/eja/eja_algebra.py
index d38ba87..631d695 100644
--- a/mjo/eja/eja_algebra.py
+++ b/mjo/eja/eja_algebra.py
@@ -1,9 +1,53 @@
 """
-Euclidean Jordan Algebras. These are formally-real Jordan Algebras;
-specifically those where u^2 + v^2 = 0 implies that u = v = 0. They
-are used in optimization, and have some additional nice methods beyond
-what can be supported in a general Jordan Algebra.
-
+Representations and constructions for Euclidean Jordan algebras.
+
+A Euclidean Jordan algebra is a Jordan algebra that has some
+additional properties:
+
+  1.   It is finite-dimensional.
+  2.   Its scalar field is the real numbers.
+  3a.  An inner product is defined on it, and...
+  3b.  That inner product is compatible with the Jordan product
+       in the sense that `<x*y,z> = <y,x*z>` for all elements
+       `x,y,z` in the algebra.
+
+Every Euclidean Jordan algebra is formally-real: for any two elements
+`x` and `y` in the algebra, `x^{2} + y^{2} = 0` implies that `x = y =
+0`. Conversely, every finite-dimensional formally-real Jordan algebra
+can be made into a Euclidean Jordan algebra with an appropriate choice
+of inner-product.
+
+Formally-real Jordan algebras were originally studied as a framework
+for quantum mechanics. Today, Euclidean Jordan algebras are crucial in
+symmetric cone optimization, since every symmetric cone arises as the
+cone of squares in some Euclidean Jordan algebra.
+
+It is known that every Euclidean Jordan algebra decomposes into an
+orthogonal direct sum (essentially, a Cartesian product) of simple
+algebras, and that moreover, up to Jordan-algebra isomorphism, there
+are only five families of simple algebras. We provide constructions
+for these simple algebras:
+
+  * :class:`BilinearFormEJA`
+  * :class:`RealSymmetricEJA`
+  * :class:`ComplexHermitianEJA`
+  * :class:`QuaternionHermitianEJA`
+
+Missing from this list is the algebra of three-by-three octononion
+Hermitian matrices, as there is (as of yet) no implementation of the
+octonions in SageMath. In addition to these, we provide two other
+example constructions,
+
+  * :class:`HadamardEJA`
+  * :class:`TrivialEJA`
+
+The Jordan spin algebra is a bilinear form algebra where the bilinear
+form is the identity. The Hadamard EJA is simply a Cartesian product
+of one-dimensional spin algebras. And last but not least, the trivial
+EJA is exactly what you think. Cartesian products of these are also
+supported using the usual ``cartesian_product()`` function; as a
+result, we support (up to isomorphism) all Euclidean Jordan algebras
+that don't involve octonions.
 
 SETUP::
 
@@ -13,7 +57,6 @@ EXAMPLES::
 
     sage: random_eja()
     Euclidean Jordan algebra of dimension...
-
 """
 
 from itertools import repeat
@@ -3210,4 +3253,42 @@ class CartesianProductEJA(CombinatorialFreeModule_CartesianProduct,
 
 FiniteDimensionalEJA.CartesianProduct = CartesianProductEJA
 
+class RationalBasisCartesianProductEJA(CartesianProductEJA,
+                                       RationalBasisEJA):
+    r"""
+    A separate class for products of algebras for which we know a
+    rational basis.
+
+    SETUP::
+
+        sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
+        ....:                                  RealSymmetricEJA)
+
+    EXAMPLES:
+
+    This gives us fast characteristic polynomial computations in
+    product algebras, too::
+
+
+        sage: J1 = JordanSpinEJA(2)
+        sage: J2 = RealSymmetricEJA(3)
+        sage: J = cartesian_product([J1,J2])
+        sage: J.characteristic_polynomial_of().degree()
+        5
+        sage: J.rank()
+        5
+
+    """
+    def __init__(self, algebras, **kwargs):
+        CartesianProductEJA.__init__(self, algebras, **kwargs)
+
+        self._rational_algebra = None
+        if self.vector_space().base_field() is not QQ:
+            self._rational_algebra = cartesian_product([
+                r._rational_algebra for r in algebras
+            ])
+
+
+RationalBasisEJA.CartesianProduct = RationalBasisCartesianProductEJA
+
 random_eja = ConcreteEJA.random_instance