eja: add is_minimal_idempotent() 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 85d45715dae0d787e622293e4f4320d755536911..575c2a4754ae261f284c53de4d423011c9bf5418 100644 (file)
--- a/mjo/eja/eja_element.py
+++ b/mjo/eja/eja_element.py
@@ -1,3 +1,7 @@
+# -*- coding: utf-8 -*-
+
+from itertools import izip
+

 from sage.matrix.constructor import matrix
 from sage.modules.free_module import VectorSpace
 from sage.modules.with_basis.indexed_element import IndexedFreeModuleElement
 from sage.matrix.constructor import matrix
 from sage.modules.free_module import VectorSpace
 from sage.modules.with_basis.indexed_element import IndexedFreeModuleElement


@@ -32,7 +36,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
         Return ``self`` raised to the power ``n``.
 
         Jordan algebras are always power-associative; see for
         Return ``self`` raised to the power ``n``.
 
         Jordan algebras are always power-associative; see for

-        example Faraut and Koranyi, Proposition II.1.2 (ii).
+        example Faraut and Korányi, Proposition II.1.2 (ii).

 
         We have to override this because our superclass uses row
         vectors instead of column vectors! We, on the other hand,
 
         We have to override this because our superclass uses row
         vectors instead of column vectors! We, on the other hand,


@@ -78,7 +82,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
         elif n == 1:
             return self
         else:
         elif n == 1:
             return self
         else:

-            return (self.operator()**(n-1))(self)
+            return (self**(n-1))*self

 
 
     def apply_univariate_polynomial(self, p):
 
 
     def apply_univariate_polynomial(self, p):


@@ -373,7 +377,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             True
 
         Ensure that the determinant is multiplicative on an associative
             True
 
         Ensure that the determinant is multiplicative on an associative

-        subalgebra as in Faraut and Koranyi's Proposition II.2.2::
+        subalgebra as in Faraut and Korányi's Proposition II.2.2::

 
             sage: set_random_seed()
             sage: J = random_eja().random_element().subalgebra_generated_by()
 
             sage: set_random_seed()
             sage: J = random_eja().random_element().subalgebra_generated_by()


@@ -403,7 +407,8 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
 
         SETUP::
 
 
         SETUP::
 

-            sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
+            sage: from mjo.eja.eja_algebra import (ComplexHermitianEJA,
+            ....:                                  JordanSpinEJA,

             ....:                                  random_eja)
 
         EXAMPLES:
             ....:                                  random_eja)
 
         EXAMPLES:


@@ -425,6 +430,13 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             sage: x.inverse() == J.from_vector(x_inverse)
             True
 
             sage: x.inverse() == J.from_vector(x_inverse)
             True
 

+        Trying to invert a non-invertible element throws an error:
+
+            sage: JordanSpinEJA(3).zero().inverse()
+            Traceback (most recent call last):
+            ...
+            ValueError: element is not invertible
+

         TESTS:
 
         The identity element is its own inverse::
         TESTS:
 
         The identity element is its own inverse::


@@ -450,13 +462,32 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             sage: (not x.is_invertible()) or (x.inverse().inverse() == x)
             True
 
             sage: (not x.is_invertible()) or (x.inverse().inverse() == x)
             True
 

-        The zero element is never invertible::
+        Proposition II.2.3 in Faraut and Korányi says that the inverse
+        of an element is the inverse of its left-multiplication operator
+        applied to the algebra's identity, when that inverse exists::

 
             sage: set_random_seed()
 
             sage: set_random_seed()

-            sage: J = random_eja().zero().inverse()
-            Traceback (most recent call last):
-            ...
-            ValueError: element is not invertible
+            sage: J = random_eja()
+            sage: x = J.random_element()
+            sage: (not x.operator().is_invertible()) or (
+            ....:    x.operator().inverse()(J.one()) == x.inverse() )
+            True
+
+        Proposition II.2.4 in Faraut and Korányi gives a formula for
+        the inverse based on the characteristic polynomial and the
+        Cayley-Hamilton theorem for Euclidean Jordan algebras::
+
+            sage: set_random_seed()
+            sage: J = ComplexHermitianEJA(3)
+            sage: x = J.random_element()
+            sage: while not x.is_invertible():
+            ....:     x = J.random_element()
+            sage: r = J.rank()
+            sage: a = x.characteristic_polynomial().coefficients(sparse=False)
+            sage: expected  = (-1)^(r+1)/x.det()
+            sage: expected *= sum( a[i+1]*x^i for i in range(r) )
+            sage: x.inverse() == expected
+            True

 
         """
         if not self.is_invertible():
 
         """
         if not self.is_invertible():


@@ -515,6 +546,96 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
         return not (p(zero) == zero)
 
 
         return not (p(zero) == zero)
 
 

+    def is_minimal_idempotent(self):
+        """
+        Return whether or not this element is a minimal idempotent.
+
+
+        An element of a Euclidean Jordan algebra is a minimal idempotent
+        if it :meth:`is_idempotent` and if its Peirce subalgebra
+        corresponding to the eigenvalue ``1`` has dimension ``1`` (Baes,
+        Proposition 2.7.17).
+
+        SETUP::
+
+            sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
+            ....:                                  RealSymmetricEJA,
+            ....:                                  random_eja)
+
+        WARNING::
+
+        This method is sloooooow.
+
+        EXAMPLES:
+
+        The spectral decomposition of a non-regular element should always
+        contain at least one non-minimal idempotent::
+
+            sage: J = RealSymmetricEJA(3, AA)
+            sage: x = sum(J.gens())
+            sage: x.is_regular()
+            False
+            sage: [ c.is_minimal_idempotent()
+            ....:   for (l,c) in x.spectral_decomposition() ]
+            [False, True]
+
+        On the other hand, the spectral decomposition of a regular
+        element should always be in terms of minimal idempotents::
+
+            sage: J = JordanSpinEJA(4, AA)
+            sage: x = sum( i*J.gens()[i] for i in range(len(J.gens())) )
+            sage: x.is_regular()
+            True
+            sage: [ c.is_minimal_idempotent()
+            ....:   for (l,c) in x.spectral_decomposition() ]
+            [True, True]
+
+        TESTS:
+
+        The identity element is minimal only in an EJA of rank one::
+
+            sage: set_random_seed()
+            sage: J = random_eja()
+            sage: J.rank() == 1 or not J.one().is_minimal_idempotent()
+            True
+
+        A non-idempotent cannot be a minimal idempotent::
+
+            sage: set_random_seed()
+            sage: J = JordanSpinEJA(4)
+            sage: x = J.random_element()
+            sage: (not x.is_idempotent()) and x.is_minimal_idempotent()
+            False
+
+        Proposition 2.7.19 in Baes says that an element is a minimal
+        idempotent if and only if it's idempotent with trace equal to
+        unity::
+
+            sage: set_random_seed()
+            sage: J = JordanSpinEJA(4)
+            sage: x = J.random_element()
+            sage: expected = (x.is_idempotent() and x.trace() == 1)
+            sage: actual = x.is_minimal_idempotent()
+            sage: actual == expected
+            True
+
+        """
+        # TODO: when the Peirce decomposition is implemented for real,
+        # we can use that instead of finding this eigenspace manually.
+        #
+        # Trivial eigenspaces don't appear in the list, so we default to the
+        # trivial one and override it if there's a nontrivial space in the
+        # list.
+        if not self.is_idempotent():
+            return False
+
+        J1 = VectorSpace(self.parent().base_ring(), 0)
+        for (eigval, eigspace) in self.operator().matrix().left_eigenspaces():
+            if eigval == 1:
+                J1 = eigspace
+        return (J1.dimension() == 1)
+
+

     def is_nilpotent(self):
         """
         Return whether or not some power of this element is zero.
     def is_nilpotent(self):
         """
         Return whether or not some power of this element is zero.


@@ -542,10 +663,11 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
 
         TESTS:
 
 
         TESTS:
 

-        The identity element is never nilpotent::
+        The identity element is never nilpotent, except in a trivial EJA::

 
             sage: set_random_seed()
 
             sage: set_random_seed()

-            sage: random_eja().one().is_nilpotent()
+            sage: J = random_eja()
+            sage: J.one().is_nilpotent() and not J.is_trivial()

             False
 
         The additive identity is always nilpotent::
             False
 
         The additive identity is always nilpotent::


@@ -589,11 +711,11 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
         TESTS:
 
         The zero element should never be regular, unless the parent
         TESTS:
 
         The zero element should never be regular, unless the parent

-        algebra has dimension one::
+        algebra has dimension less than or equal to one::

 
             sage: set_random_seed()
             sage: J = random_eja()
 
             sage: set_random_seed()
             sage: J = random_eja()

-            sage: J.dimension() == 1 or not J.zero().is_regular()
+            sage: J.dimension() <= 1 or not J.zero().is_regular()

             True
 
         The unit element isn't regular unless the algebra happens to
             True
 
         The unit element isn't regular unless the algebra happens to


@@ -601,7 +723,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
 
             sage: set_random_seed()
             sage: J = random_eja()
 
             sage: set_random_seed()
             sage: J = random_eja()

-            sage: J.dimension() == 1 or not J.one().is_regular()
+            sage: J.dimension() <= 1 or not J.one().is_regular()

             True
 
         """
             True
 
         """


@@ -645,14 +767,17 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
 
         TESTS:
 
 
         TESTS:
 

-        The zero and unit elements are both of degree one::
+        The zero and unit elements are both of degree one in nontrivial
+        algebras::

 
             sage: set_random_seed()
             sage: J = random_eja()
 
             sage: set_random_seed()
             sage: J = random_eja()

-            sage: J.zero().degree()
-            1
-            sage: J.one().degree()
-            1
+            sage: d = J.zero().degree()
+            sage: (J.is_trivial() and d == 0) or d == 1
+            True
+            sage: d = J.one().degree()
+            sage: (J.is_trivial() and d == 0) or d == 1
+            True

 
         Our implementation agrees with the definition::
 
 
         Our implementation agrees with the definition::
 


@@ -696,15 +821,29 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
 
             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
             ....:                                  RealSymmetricEJA,
 
             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
             ....:                                  RealSymmetricEJA,

+            ....:                                  TrivialEJA,

             ....:                                  random_eja)
 
             ....:                                  random_eja)
 

+        EXAMPLES:
+
+        Keeping in mind that the polynomial ``1`` evaluates the identity
+        element (also the zero element) of the trivial algebra, it is clear
+        that the polynomial ``1`` is the minimal polynomial of the only
+        element in a trivial algebra::
+
+            sage: J = TrivialEJA()
+            sage: J.one().minimal_polynomial()
+            1
+            sage: J.zero().minimal_polynomial()
+            1
+

         TESTS:
 
         The minimal polynomial of the identity and zero elements are
         always the same::
 
             sage: set_random_seed()
         TESTS:
 
         The minimal polynomial of the identity and zero elements are
         always the same::
 
             sage: set_random_seed()

-            sage: J = random_eja()
+            sage: J = random_eja(nontrivial=True)

             sage: J.one().minimal_polynomial()
             t - 1
             sage: J.zero().minimal_polynomial()
             sage: J.one().minimal_polynomial()
             t - 1
             sage: J.zero().minimal_polynomial()


@@ -754,7 +893,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             sage: n_max = RealSymmetricEJA._max_test_case_size()
             sage: n = ZZ.random_element(1, n_max)
             sage: J1 = RealSymmetricEJA(n,QQ)
             sage: n_max = RealSymmetricEJA._max_test_case_size()
             sage: n = ZZ.random_element(1, n_max)
             sage: J1 = RealSymmetricEJA(n,QQ)

-            sage: J2 = RealSymmetricEJA(n,QQ,False)
+            sage: J2 = RealSymmetricEJA(n,QQ,normalize_basis=False)

             sage: X = random_matrix(QQ,n)
             sage: X = X*X.transpose()
             sage: x1 = J1(X)
             sage: X = random_matrix(QQ,n)
             sage: X = X*X.transpose()
             sage: x1 = J1(X)


@@ -830,7 +969,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
         """
         B = self.parent().natural_basis()
         W = self.parent().natural_basis_space()
         """
         B = self.parent().natural_basis()
         W = self.parent().natural_basis_space()

-        return W.linear_combination(zip(B,self.to_vector()))
+        return W.linear_combination(izip(B,self.to_vector()))

 
 
     def norm(self):
 
 
     def norm(self):


@@ -968,10 +1107,10 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             sage: not x.is_invertible() or (
             ....:   x.quadratic_representation(x.inverse())*Qx
             ....:   ==
             sage: not x.is_invertible() or (
             ....:   x.quadratic_representation(x.inverse())*Qx
             ....:   ==

-            ....:   2*x.operator()*Qex - Qx )
+            ....:   2*Lx*Qex - Qx )

             True
 
             True
 

-            sage: 2*x.operator()*Qex - Qx == Lxx
+            sage: 2*Lx*Qex - Qx == Lxx

             True
 
         Property 5:
             True
 
         Property 5:


@@ -1008,12 +1147,82 @@ 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

 
 

-    def subalgebra_generated_by(self):
+        """
+        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.
 
         """
         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
         SETUP::
 
             sage: from mjo.eja.eja_algebra import random_eja


@@ -1038,17 +1247,18 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             sage: A(x^2) == A(x)*A(x)
             True
 
             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... unless the original algebra was trivial, in which
+        case the subalgebra is trivial too::

 
             sage: set_random_seed()
             sage: A = random_eja().zero().subalgebra_generated_by()
 
             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.is_trivial() and A.dimension() == 0) or A.dimension() == 1
+            True

 
         """
 
         """

-        return FiniteDimensionalEuclideanJordanElementSubalgebra(self)
+        return FiniteDimensionalEuclideanJordanElementSubalgebra(self, orthonormalize_basis)

 
 
     def subalgebra_idempotent(self):
 
 
     def subalgebra_idempotent(self):


@@ -1111,14 +1321,23 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
         """
         Return my trace, the sum of my eigenvalues.
 
         """
         Return my trace, the sum of my eigenvalues.
 

+        In a trivial algebra, however you want to look at it, the trace is
+        an empty sum for which we declare the result to be zero.
+

         SETUP::
 
             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
             ....:                                  RealCartesianProductEJA,
         SETUP::
 
             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
             ....:                                  RealCartesianProductEJA,

+            ....:                                  TrivialEJA,

             ....:                                  random_eja)
 
         EXAMPLES::
 
             ....:                                  random_eja)
 
         EXAMPLES::
 

+            sage: J = TrivialEJA()
+            sage: J.zero().trace()
+            0
+
+        ::

             sage: J = JordanSpinEJA(3)
             sage: x = sum(J.gens())
             sage: x.trace()
             sage: J = JordanSpinEJA(3)
             sage: x = sum(J.gens())
             sage: x.trace()


@@ -1142,6 +1361,12 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
         """
         P = self.parent()
         r = P.rank()
         """
         P = self.parent()
         r = P.rank()

+
+        if r == 0:
+            # Special case for the trivial algebra where
+            # the trace is an empty sum.
+            return P.base_ring().zero()
+

         p = P._charpoly_coeff(r-1)
         # The _charpoly_coeff function already adds the factor of
         # -1 to ensure that _charpoly_coeff(r-1) is really what
         p = P._charpoly_coeff(r-1)
         # The _charpoly_coeff function already adds the factor of
         # -1 to ensure that _charpoly_coeff(r-1) is really what


@@ -1160,8 +1385,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
 
         TESTS:
 
 
         TESTS:
 

-        The trace inner product is commutative, bilinear, and satisfies
-        the Jordan axiom:
+        The trace inner product is commutative, bilinear, and associative::

 
             sage: set_random_seed()
             sage: J = random_eja()
 
             sage: set_random_seed()
             sage: J = random_eja()


@@ -1181,7 +1405,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
             ....:              a*x.trace_inner_product(z) )
             sage: actual == expected
             True
             ....:              a*x.trace_inner_product(z) )
             sage: actual == expected
             True

-            sage: # jordan axiom
+            sage: # associative

             sage: (x*y).trace_inner_product(z) == y.trace_inner_product(x*z)
             True
 
             sage: (x*y).trace_inner_product(z) == y.trace_inner_product(x*z)
             True