From 857eeec29a9b89b0e4f711476771c935757fa8dc Mon Sep 17 00:00:00 2001
From: Michael Orlitzky <michael@orlitzky.com>
Date: Thu, 25 Feb 2021 18:38:29 -0500
Subject: [PATCH] eja: add brute-force associativity test.

---
 mjo/eja/eja_algebra.py | 75 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 75 insertions(+)

diff --git a/mjo/eja/eja_algebra.py b/mjo/eja/eja_algebra.py
index af7d059..3b64ddd 100644
--- a/mjo/eja/eja_algebra.py
+++ b/mjo/eja/eja_algebra.py
@@ -452,6 +452,81 @@ class FiniteDimensionalEJA(CombinatorialFreeModule):
                     for i in range(self.dimension())
                     for j in range(self.dimension()) )
 
+    def _jordan_product_is_associative(self):
+        r"""
+        Return whether or not this algebra's Jordan product is
+        associative; that is, whether or not `x*(y*z) = (x*y)*z`
+        for all `x,y,x`.
+
+        This method should agree with :meth:`is_associative` unless
+        you lied about the value of the ``associative`` parameter
+        when you constructed the algebra.
+
+        SETUP::
+
+            sage: from mjo.eja.eja_algebra import (RealSymmetricEJA,
+            ....:                                  ComplexHermitianEJA,
+            ....:                                  QuaternionHermitianEJA)
+
+        EXAMPLES::
+
+            sage: J = RealSymmetricEJA(4, orthonormalize=False)
+            sage: J._jordan_product_is_associative()
+            False
+            sage: x = sum(J.gens())
+            sage: A = x.subalgebra_generated_by()
+            sage: A._jordan_product_is_associative()
+            True
+
+        ::
+
+            sage: J = ComplexHermitianEJA(2,field=QQ,orthonormalize=False)
+            sage: J._jordan_product_is_associative()
+            False
+            sage: x = sum(J.gens())
+            sage: A = x.subalgebra_generated_by(orthonormalize=False)
+            sage: A._jordan_product_is_associative()
+            True
+
+        ::
+
+            sage: J = QuaternionHermitianEJA(2)
+            sage: J._jordan_product_is_associative()
+            False
+            sage: x = sum(J.gens())
+            sage: A = x.subalgebra_generated_by()
+            sage: A._jordan_product_is_associative()
+            True
+
+        """
+        R = self.base_ring()
+
+        # Used to check whether or not something is zero.
+        epsilon = R.zero()
+        if not R.is_exact():
+            # I don't know of any examples that make this magnitude
+            # necessary because I don't know how to make an
+            # associative algebra when the element subalgebra
+            # construction is unreliable (as it is over RDF; we can't
+            # find the degree of an element because we can't compute
+            # the rank of a matrix). But even multiplication of floats
+            # is non-associative, so *some* epsilon is needed... let's
+            # just take the one from _inner_product_is_associative?
+            epsilon = 1e-15
+
+        for i in range(self.dimension()):
+            for j in range(self.dimension()):
+                for k in range(self.dimension()):
+                    x = self.gens()[i]
+                    y = self.gens()[j]
+                    z = self.gens()[k]
+                    diff = (x*y)*z - x*(y*z)
+
+                    if diff.norm() > epsilon:
+                        return False
+
+        return True
+
     def _inner_product_is_associative(self):
         r"""
         Return whether or not this algebra's inner product `B` is
-- 
2.43.2