eja: add the unique spectral_decomposition() for elements.

[sage.d.git] / mjo / eja / eja_element.py

diff --git a/mjo/eja/eja_element.py b/mjo/eja/eja_element.py
index e7dff7529026cf007056a5ecd66a8f9fba98562a..a4af4eaedbb4ce96c16aa31ab0e98c2fa4c5b6c7 100644 (file)
--- a/mjo/eja/eja_element.py
+++ b/mjo/eja/eja_element.py
@@ -1040,12 +1040,82 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
 
 
 
+    def spectral_decomposition(self):
+        """
+        Return the unique spectral decomposition of this element.
+
+        ALGORITHM:
+
+        Following Faraut and Korányi's Theorem III.1.1, we restrict this
+        element's left-multiplication-by operator to the subalgebra it
+        generates. We then compute the spectral decomposition of that
+        operator, and the spectral projectors we get back must be the
+        left-multiplication-by operators for the idempotents we
+        seek. Thus applying them to the identity element gives us those
+        idempotents.
+
+        Since the eigenvalues are required to be distinct, we take
+        the spectral decomposition of the zero element to be zero
+        times the identity element of the algebra (which is idempotent,
+        obviously).
+
+        SETUP::
+
+            sage: from mjo.eja.eja_algebra import RealSymmetricEJA
+
+        EXAMPLES:
+
+        The spectral decomposition of the identity is ``1`` times itself,
+        and the spectral decomposition of zero is ``0`` times the identity::
+
+            sage: J = RealSymmetricEJA(3,AA)
+            sage: J.one()
+            e0 + e2 + e5
+            sage: J.one().spectral_decomposition()
+            [(1, e0 + e2 + e5)]
+            sage: J.zero().spectral_decomposition()
+            [(0, e0 + e2 + e5)]
+
+        TESTS::
+
+            sage: J = RealSymmetricEJA(4,AA)
+            sage: x = sum(J.gens())
+            sage: sd = x.spectral_decomposition()
+            sage: l0 = sd[0][0]
+            sage: l1 = sd[1][0]
+            sage: c0 = sd[0][1]
+            sage: c1 = sd[1][1]
+            sage: c0.inner_product(c1) == 0
+            True
+            sage: c0.is_idempotent()
+            True
+            sage: c1.is_idempotent()
+            True
+            sage: c0 + c1 == J.one()
+            True
+            sage: l0*c0 + l1*c1 == x
+            True
+
+        """
+        P = self.parent()
+        A = self.subalgebra_generated_by(orthonormalize_basis=True)
+        result = []
+        for (evalue, proj) in A(self).operator().spectral_decomposition():
+            result.append( (evalue, proj(A.one()).superalgebra_element()) )
+        return result
 
     def subalgebra_generated_by(self, orthonormalize_basis=False):
         """
         Return the associative subalgebra of the parent EJA generated
         by this element.
 
+        Since our parent algebra is unital, we want "subalgebra" to mean
+        "unital subalgebra" as well; thus the subalgebra that an element
+        generates will itself be a Euclidean Jordan algebra after
+        restricting the algebra operations appropriately. This is the
+        subalgebra that Faraut and Korányi work with in section II.2, for
+        example.
+
         SETUP::
 
             sage: from mjo.eja.eja_algebra import random_eja
@@ -1070,14 +1140,13 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             sage: A(x^2) == A(x)*A(x)
             True
 
-        The subalgebra generated by the zero element is trivial::
+        By definition, the subalgebra generated by the zero element is the
+        one-dimensional algebra generated by the identity element::
 
             sage: set_random_seed()
             sage: A = random_eja().zero().subalgebra_generated_by()
-            sage: A
-            Euclidean Jordan algebra of dimension 0 over...
-            sage: A.one()
-            0
+            sage: A.dimension()
+            1
 
         """
         return FiniteDimensionalEuclideanJordanElementSubalgebra(self, orthonormalize_basis)