From: Michael Orlitzky <michael@orlitzky.com>
Date: Fri, 26 Jul 2019 14:05:45 +0000 (-0400)
Subject: eja: use the quadratic representation for the element inverse.
X-Git-Url: https://gitweb.michael.orlitzky.com/?a=commitdiff_plain;h=9144c528176a467bec604e334ba05379cc995ae3;p=sage.d.git

eja: use the quadratic representation for the element inverse.
---

diff --git a/mjo/eja/euclidean_jordan_algebra.py b/mjo/eja/euclidean_jordan_algebra.py
index da3f600..4b46380 100644
--- a/mjo/eja/euclidean_jordan_algebra.py
+++ b/mjo/eja/euclidean_jordan_algebra.py
@@ -658,8 +658,10 @@ class FiniteDimensionalEuclideanJordanAlgebra(FiniteDimensionalAlgebra):
             """
             Return the Jordan-multiplicative inverse of this element.
 
-            We can't use the superclass method because it relies on the
-            algebra being associative.
+            ALGORITHM:
+
+            We appeal to the quadratic representation as in Koecher's
+            Theorem 12 in Chapter III, Section 5.
 
             EXAMPLES:
 
@@ -698,34 +700,28 @@ class FiniteDimensionalEuclideanJordanAlgebra(FiniteDimensionalAlgebra):
                 sage: (not x.is_invertible()) or (x.inverse()*x == J.one())
                 True
 
-            """
-            if not self.is_invertible():
-                raise ValueError("element not invertible")
+            The inverse of the inverse is what we started with::
 
-            if self.parent().is_associative():
-                elt = FiniteDimensionalAlgebraElement(self.parent(), self)
-                # elt is in the right coordinates, but has the wrong class.
-                return self.parent()(elt.inverse().vector())
+                sage: set_random_seed()
+                sage: J = random_eja()
+                sage: x = J.random_element()
+                sage: (not x.is_invertible()) or (x.inverse().inverse() == x)
+                True
 
-            # We do this a little different than the usual recursive
-            # call to a finite-dimensional algebra element, because we
-            # wind up with an inverse that lives in the subalgebra and
-            # we need information about the parent to convert it back.
-            V = self.span_of_powers()
-            assoc_subalg = self.subalgebra_generated_by()
-            # Mis-design warning: the basis used for span_of_powers()
-            # and subalgebra_generated_by() must be the same, and in
-            # the same order!
-            elt = assoc_subalg(V.coordinates(self.vector()))
+            The zero element is never invertible::
+
+                sage: set_random_seed()
+                sage: J = random_eja().zero().inverse()
+                Traceback (most recent call last):
+                ...
+                ValueError: element is not invertible
 
-            # This will be in the subalgebra's coordinates...
-            fda_elt = FiniteDimensionalAlgebraElement(assoc_subalg, elt)
-            subalg_inverse = fda_elt.inverse()
+            """
+            if not self.is_invertible():
+                raise ValueError("element is not invertible")
 
-            # So we have to convert back...
-            basis = [ self.parent(v) for v in V.basis() ]
-            pairs = zip(subalg_inverse.vector(), basis)
-            return self.parent().linear_combination(pairs)
+            P = self.parent()
+            return P(self.quadratic_representation().inverse()*self.vector())
 
 
         def is_invertible(self):