X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=mjo%2Feja%2Feja_element.py;h=7c4c79ddcd7315e654620a0be8f8bccf5ab9ac11;hb=99ca9f8c24194ad6be7b8e325575e58b53429c2b;hp=fb1983859c849c40ae5294f8e45e8d2f14a13b57;hpb=3b7cb5424101ae1cde5f868d672dd613763fc439;p=sage.d.git diff --git a/mjo/eja/eja_element.py b/mjo/eja/eja_element.py index fb19838..7c4c79d 100644 --- a/mjo/eja/eja_element.py +++ b/mjo/eja/eja_element.py @@ -1,3 +1,5 @@ +# -*- coding: utf-8 -*- + from sage.matrix.constructor import matrix from sage.modules.free_module import VectorSpace from sage.modules.with_basis.indexed_element import IndexedFreeModuleElement @@ -5,7 +7,7 @@ from sage.modules.with_basis.indexed_element import IndexedFreeModuleElement # TODO: make this unnecessary somehow. from sage.misc.lazy_import import lazy_import lazy_import('mjo.eja.eja_algebra', 'FiniteDimensionalEuclideanJordanAlgebra') -lazy_import('mjo.eja.eja_subalgebra', +lazy_import('mjo.eja.eja_element_subalgebra', 'FiniteDimensionalEuclideanJordanElementSubalgebra') from mjo.eja.eja_operator import FiniteDimensionalEuclideanJordanAlgebraOperator from mjo.eja.eja_utils import _mat2vec @@ -25,69 +27,6 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): dir(self.__class__) ) - def __init__(self, A, elt): - """ - - SETUP:: - - sage: from mjo.eja.eja_algebra import (RealSymmetricEJA, - ....: random_eja) - - EXAMPLES: - - The identity in `S^n` is converted to the identity in the EJA:: - - sage: J = RealSymmetricEJA(3) - sage: I = matrix.identity(QQ,3) - sage: J(I) == J.one() - True - - This skew-symmetric matrix can't be represented in the EJA:: - - sage: J = RealSymmetricEJA(3) - sage: A = matrix(QQ,3, lambda i,j: i-j) - sage: J(A) - Traceback (most recent call last): - ... - ArithmeticError: vector is not in free module - - TESTS: - - Ensure that we can convert any element of the parent's - underlying vector space back into an algebra element whose - vector representation is what we started with:: - - sage: set_random_seed() - sage: J = random_eja() - sage: v = J.vector_space().random_element() - sage: J(v).to_vector() == v - True - - """ - # Goal: if we're given a matrix, and if it lives in our - # parent algebra's "natural ambient space," convert it - # into an algebra element. - # - # The catch is, we make a recursive call after converting - # the given matrix into a vector that lives in the algebra. - # This we need to try the parent class initializer first, - # to avoid recursing forever if we're given something that - # already fits into the algebra, but also happens to live - # in the parent's "natural ambient space" (this happens with - # vectors in R^n). - ifme = super(FiniteDimensionalEuclideanJordanAlgebraElement, self) - try: - ifme.__init__(A, elt) - except ValueError: - natural_basis = A.natural_basis() - if elt in natural_basis[0].matrix_space(): - # Thanks for nothing! Matrix spaces aren't vector - # spaces in Sage, so we have to figure out its - # natural-basis coordinates ourselves. - V = VectorSpace(elt.base_ring(), elt.nrows()**2) - W = V.span( _mat2vec(s) for s in natural_basis ) - coords = W.coordinate_vector(_mat2vec(elt)) - ifme.__init__(A, coords) def __pow__(self, n): @@ -95,7 +34,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): 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, @@ -141,7 +80,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): elif n == 1: return self else: - return (self.operator()**(n-1))(self) + return (self**(n-1))*self def apply_univariate_polynomial(self, p): @@ -157,7 +96,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): SETUP:: - sage: from mjo.eja.eja_algebra import (RealCartesianProductEJA, + sage: from mjo.eja.eja_algebra import (HadamardEJA, ....: random_eja) EXAMPLES:: @@ -165,7 +104,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: R = PolynomialRing(QQ, 't') sage: t = R.gen(0) sage: p = t^4 - t^3 + 5*t - 2 - sage: J = RealCartesianProductEJA(5) + sage: J = HadamardEJA(5) sage: J.one().apply_univariate_polynomial(p) == 3*J.one() True @@ -198,7 +137,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): SETUP:: - sage: from mjo.eja.eja_algebra import RealCartesianProductEJA + sage: from mjo.eja.eja_algebra import HadamardEJA EXAMPLES: @@ -206,14 +145,14 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): the identity element is `(t-1)` from which it follows that the characteristic polynomial should be `(t-1)^3`:: - sage: J = RealCartesianProductEJA(3) + sage: J = HadamardEJA(3) sage: J.one().characteristic_polynomial() t^3 - 3*t^2 + 3*t - 1 Likewise, the characteristic of the zero element in the rank-three algebra `R^{n}` should be `t^{3}`:: - sage: J = RealCartesianProductEJA(3) + sage: J = HadamardEJA(3) sage: J.zero().characteristic_polynomial() t^3 @@ -223,11 +162,26 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): to zero on that element:: sage: set_random_seed() - sage: x = RealCartesianProductEJA(3).random_element() + sage: x = HadamardEJA(3).random_element() sage: p = x.characteristic_polynomial() sage: x.apply_univariate_polynomial(p) 0 + The characteristic polynomials of the zero and unit elements + should be what we think they are in a subalgebra, too:: + + sage: J = HadamardEJA(3) + sage: p1 = J.one().characteristic_polynomial() + sage: q1 = J.zero().characteristic_polynomial() + sage: e0,e1,e2 = J.gens() + sage: A = (e0 + 2*e1 + 3*e2).subalgebra_generated_by() # dim 3 + sage: p2 = A.one().characteristic_polynomial() + sage: q2 = A.zero().characteristic_polynomial() + sage: p1 == p2 + True + sage: q1 == q2 + True + """ p = self.parent().characteristic_polynomial() return p(*self.to_vector()) @@ -291,9 +245,8 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: set_random_seed() sage: J = random_eja() - sage: x = J.random_element() - sage: y = J.random_element() - sage: x.inner_product(y) in RR + sage: x,y = J.random_elements(2) + sage: x.inner_product(y) in RLF True """ @@ -328,9 +281,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): Test Lemma 1 from Chapter III of Koecher:: sage: set_random_seed() - sage: J = random_eja() - sage: u = J.random_element() - sage: v = J.random_element() + sage: u,v = random_eja().random_elements(2) sage: lhs = u.operator_commutes_with(u*v) sage: rhs = v.operator_commutes_with(u^2) sage: lhs == rhs @@ -340,9 +291,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): Chapter III, or from Baes (2.3):: sage: set_random_seed() - sage: J = random_eja() - sage: x = J.random_element() - sage: y = J.random_element() + sage: x,y = random_eja().random_elements(2) sage: Lx = x.operator() sage: Ly = y.operator() sage: Lxx = (x*x).operator() @@ -354,10 +303,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): Baes (2.4):: sage: set_random_seed() - sage: J = random_eja() - sage: x = J.random_element() - sage: y = J.random_element() - sage: z = J.random_element() + sage: x,y,z = random_eja().random_elements(3) sage: Lx = x.operator() sage: Ly = y.operator() sage: Lz = z.operator() @@ -371,10 +317,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): Baes (2.5):: sage: set_random_seed() - sage: J = random_eja() - sage: u = J.random_element() - sage: y = J.random_element() - sage: z = J.random_element() + sage: u,y,z = random_eja().random_elements(3) sage: Lu = u.operator() sage: Ly = y.operator() sage: Lz = z.operator() @@ -431,6 +374,15 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: x.is_invertible() == (x.det() != 0) True + Ensure that the determinant is multiplicative on an associative + 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: x,y = J.random_elements(2) + sage: (x*y).det() == x.det()*y.det() + True + """ P = self.parent() r = P.rank() @@ -453,7 +405,8 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): SETUP:: - sage: from mjo.eja.eja_algebra import (JordanSpinEJA, + sage: from mjo.eja.eja_algebra import (ComplexHermitianEJA, + ....: JordanSpinEJA, ....: random_eja) EXAMPLES: @@ -462,8 +415,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): Example 11.11:: sage: set_random_seed() - sage: n = ZZ.random_element(1,10) - sage: J = JordanSpinEJA(n) + sage: J = JordanSpinEJA.random_instance() sage: x = J.random_element() sage: while not x.is_invertible(): ....: x = J.random_element() @@ -473,9 +425,16 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: coeff = ~(x0^2 - x_bar.inner_product(x_bar)) sage: inv_vec = x_vec.parent()([x0] + (-x_bar).list()) sage: x_inverse = coeff*inv_vec - sage: x.inverse() == J(x_inverse) + 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:: @@ -501,13 +460,32 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): 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: 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(): @@ -545,19 +523,136 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: J.one().is_invertible() True - The zero element is never invertible:: + The zero element is never invertible in a non-trivial algebra:: sage: set_random_seed() sage: J = random_eja() - sage: J.zero().is_invertible() + sage: (not J.is_trivial()) and J.zero().is_invertible() False """ - zero = self.parent().zero() + if self.is_zero(): + if self.parent().is_trivial(): + return True + else: + return False + + # In fact, we only need to know if the constant term is non-zero, + # so we can pass in the field's zero element instead. + zero = self.base_ring().zero() p = self.minimal_polynomial() return not (p(zero) == zero) + def is_primitive_idempotent(self): + """ + Return whether or not this element is a primitive (or minimal) + idempotent. + + A primitive idempotent is a non-zero idempotent that is not + the sum of two other non-zero idempotents. Remark 2.7.15 in + Baes shows that this is what he refers to as 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, + ....: TrivialEJA, + ....: 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_primitive_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_primitive_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_primitive_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_primitive_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_primitive_idempotent() + sage: actual == expected + True + + Primitive idempotents must be non-zero:: + + sage: set_random_seed() + sage: J = random_eja() + sage: J.zero().is_idempotent() + True + sage: J.zero().is_primitive_idempotent() + False + + As a consequence of the fact that primitive idempotents must + be non-zero, there are no primitive idempotents in a trivial + Euclidean Jordan algebra:: + + sage: J = TrivialEJA() + sage: J.one().is_idempotent() + True + sage: J.one().is_primitive_idempotent() + False + + """ + if not self.is_idempotent(): + return False + + if self.is_zero(): + return False + + (_,_,J1) = self.parent().peirce_decomposition(self) + return (J1.dimension() == 1) + + def is_nilpotent(self): """ Return whether or not some power of this element is zero. @@ -585,10 +680,11 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): TESTS: - The identity element is never nilpotent:: + The identity element is never nilpotent, except in a trivial EJA:: 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:: @@ -632,11 +728,11 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): 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: 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 @@ -644,7 +740,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): 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 """ @@ -681,22 +777,24 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): aren't multiples of the identity are regular:: sage: set_random_seed() - sage: n = ZZ.random_element(1,10) - sage: J = JordanSpinEJA(n) + sage: J = JordanSpinEJA.random_instance() sage: x = J.random_element() sage: x == x.coefficient(0)*J.one() or x.degree() == 2 True 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: 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:: @@ -706,6 +804,11 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): True """ + if self.is_zero() and not self.parent().is_trivial(): + # The minimal polynomial of zero in a nontrivial algebra + # is "t"; in a trivial algebra it's "1" by convention + # (it's an empty product). + return 1 return self.subalgebra_generated_by().dimension() @@ -734,15 +837,30 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): SETUP:: sage: from mjo.eja.eja_algebra import (JordanSpinEJA, + ....: RealSymmetricEJA, + ....: TrivialEJA, ....: 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() - sage: J = random_eja() + sage: J = random_eja(nontrivial=True) sage: J.one().minimal_polynomial() t - 1 sage: J.zero().minimal_polynomial() @@ -759,10 +877,12 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): The minimal polynomial and the characteristic polynomial coincide and are known (see Alizadeh, Example 11.11) for all elements of the spin factor algebra that aren't scalar multiples of the - identity:: + identity. We require the dimension of the algebra to be at least + two here so that said elements actually exist:: sage: set_random_seed() - sage: n = ZZ.random_element(2,10) + sage: n_max = max(2, JordanSpinEJA._max_test_case_size()) + sage: n = ZZ.random_element(2, n_max) sage: J = JordanSpinEJA(n) sage: y = J.random_element() sage: while y == y.coefficient(0)*J.one(): @@ -783,9 +903,36 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: x.apply_univariate_polynomial(p) 0 + The minimal polynomial is invariant under a change of basis, + and in particular, a re-scaling of the basis:: + + sage: set_random_seed() + 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,normalize_basis=False) + sage: X = random_matrix(QQ,n) + sage: X = X*X.transpose() + sage: x1 = J1(X) + sage: x2 = J2(X) + sage: x1.minimal_polynomial() == x2.minimal_polynomial() + True + """ + if self.is_zero(): + # We would generate a zero-dimensional subalgebra + # where the minimal polynomial would be constant. + # That might be correct, but only if *this* algebra + # is trivial too. + if not self.parent().is_trivial(): + # Pretty sure we know what the minimal polynomial of + # the zero operator is going to be. This ensures + # consistency of e.g. the polynomial variable returned + # in the "normal" case without us having to think about it. + return self.operator().minimal_polynomial() + A = self.subalgebra_generated_by() - return A.element_class(A,self).operator().minimal_polynomial() + return A(self).operator().minimal_polynomial() @@ -838,10 +985,37 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): """ B = self.parent().natural_basis() - W = B[0].matrix_space() + W = self.parent().natural_basis_space() return W.linear_combination(zip(B,self.to_vector())) + def norm(self): + """ + The norm of this element with respect to :meth:`inner_product`. + + SETUP:: + + sage: from mjo.eja.eja_algebra import (JordanSpinEJA, + ....: HadamardEJA) + + EXAMPLES:: + + sage: J = HadamardEJA(2) + sage: x = sum(J.gens()) + sage: x.norm() + sqrt(2) + + :: + + sage: J = JordanSpinEJA(4) + sage: x = sum(J.gens()) + sage: x.norm() + 2 + + """ + return self.inner_product(self).sqrt() + + def operator(self): """ Return the left-multiplication-by-this-element @@ -855,8 +1029,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: set_random_seed() sage: J = random_eja() - sage: x = J.random_element() - sage: y = J.random_element() + sage: x,y = J.random_elements(2) sage: x.operator()(y) == x*y True sage: y.operator()(x) == x*y @@ -864,10 +1037,12 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): """ P = self.parent() + left_mult_by_self = lambda y: self*y + L = P.module_morphism(function=left_mult_by_self, codomain=P) return FiniteDimensionalEuclideanJordanAlgebraOperator( P, P, - self.to_matrix() ) + L.matrix() ) def quadratic_representation(self, other=None): @@ -885,10 +1060,9 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): Alizadeh's Example 11.12:: sage: set_random_seed() - sage: n = ZZ.random_element(1,10) - sage: J = JordanSpinEJA(n) - sage: x = J.random_element() + sage: x = JordanSpinEJA.random_instance().random_element() sage: x_vec = x.to_vector() + sage: n = x_vec.degree() sage: x0 = x_vec[0] sage: x_bar = x_vec[1:] sage: A = matrix(QQ, 1, [x_vec.inner_product(x_vec)]) @@ -905,8 +1079,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: set_random_seed() sage: J = random_eja() - sage: x = J.random_element() - sage: y = J.random_element() + sage: x,y = J.random_elements(2) sage: Lx = x.operator() sage: Lxx = (x*x).operator() sage: Qx = x.quadratic_representation() @@ -923,7 +1096,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): Property 2 (multiply on the right for :trac:`28272`): - sage: alpha = QQ.random_element() + sage: alpha = J.base_ring().random_element() sage: (alpha*x).quadratic_representation() == Qx*(alpha^2) True @@ -951,10 +1124,10 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: not x.is_invertible() or ( ....: x.quadratic_representation(x.inverse())*Qx ....: == - ....: 2*x.operator()*Qex - Qx ) + ....: 2*Lx*Qex - Qx ) True - sage: 2*x.operator()*Qex - Qx == Lxx + sage: 2*Lx*Qex - Qx == Lxx True Property 5: @@ -991,21 +1164,95 @@ 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:: - def subalgebra_generated_by(self): + 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 - TESTS:: + TESTS: + + This subalgebra, being composed of only powers, is associative:: sage: set_random_seed() - sage: x = random_eja().random_element() - sage: x.subalgebra_generated_by().is_associative() + sage: x0 = random_eja().random_element() + sage: A = x0.subalgebra_generated_by() + sage: x,y,z = A.random_elements(3) + sage: (x*y)*z == x*(y*z) True Squaring in the subalgebra should work the same as in @@ -1017,8 +1264,18 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: A(x^2) == A(x)*A(x) True + 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: (A.is_trivial() and A.dimension() == 0) or A.dimension() == 1 + True + """ - return FiniteDimensionalEuclideanJordanElementSubalgebra(self) + return FiniteDimensionalEuclideanJordanElementSubalgebra(self, orthonormalize_basis) def subalgebra_idempotent(self): @@ -1030,10 +1287,13 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: from mjo.eja.eja_algebra import random_eja - TESTS:: + TESTS: + + Ensure that we can find an idempotent in a non-trivial algebra + where there are non-nilpotent elements:: sage: set_random_seed() - sage: J = random_eja() + sage: J = random_eja(nontrivial=True) sage: x = J.random_element() sage: while x.is_nilpotent(): ....: x = J.random_element() @@ -1042,17 +1302,20 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): True """ + if self.parent().is_trivial(): + return self + if self.is_nilpotent(): raise ValueError("this only works with non-nilpotent elements!") J = self.subalgebra_generated_by() - u = J.from_vector(self.to_vector()) + u = J(self) # The image of the matrix of left-u^m-multiplication # will be minimal for some natural number s... s = 0 minimal_dim = J.dimension() - for i in xrange(1, minimal_dim): + for i in range(1, minimal_dim): this_dim = (u**i).operator().matrix().image().dimension() if this_dim < minimal_dim: minimal_dim = this_dim @@ -1071,7 +1334,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): # Our FiniteDimensionalAlgebraElement superclass uses rows. u_next = u**(s+1) A = u_next.operator().matrix() - c = J(A.solve_right(u_next.to_vector())) + c = J.from_vector(A.solve_right(u_next.to_vector())) # Now c is the idempotent we want, but it still lives in the subalgebra. return c.superalgebra_element() @@ -1081,14 +1344,23 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): """ 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, + ....: HadamardEJA, + ....: TrivialEJA, ....: random_eja) EXAMPLES:: + sage: J = TrivialEJA() + sage: J.zero().trace() + 0 + + :: sage: J = JordanSpinEJA(3) sage: x = sum(J.gens()) sage: x.trace() @@ -1096,7 +1368,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): :: - sage: J = RealCartesianProductEJA(5) + sage: J = HadamardEJA(5) sage: J.one().trace() 5 @@ -1106,12 +1378,18 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): sage: set_random_seed() sage: J = random_eja() - sage: J.random_element().trace() in J.base_ring() + sage: J.random_element().trace() in RLF True """ 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 @@ -1130,22 +1408,16 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): TESTS: - The trace inner product is commutative:: + The trace inner product is commutative, bilinear, and associative:: sage: set_random_seed() sage: J = random_eja() - sage: x = J.random_element(); y = J.random_element() + sage: x,y,z = J.random_elements(3) + sage: # commutative sage: x.trace_inner_product(y) == y.trace_inner_product(x) True - - The trace inner product is bilinear:: - - sage: set_random_seed() - sage: J = random_eja() - sage: x = J.random_element() - sage: y = J.random_element() - sage: z = J.random_element() - sage: a = QQ.random_element(); + sage: # bilinear + sage: a = J.base_ring().random_element(); sage: actual = (a*(x+z)).trace_inner_product(y) sage: expected = ( a*x.trace_inner_product(y) + ....: a*z.trace_inner_product(y) ) @@ -1156,15 +1428,7 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): ....: a*x.trace_inner_product(z) ) sage: actual == expected True - - The trace inner product satisfies the compatibility - condition in the definition of a Euclidean Jordan algebra:: - - sage: set_random_seed() - sage: J = random_eja() - sage: x = J.random_element() - sage: y = J.random_element() - sage: z = J.random_element() + sage: # associative sage: (x*y).trace_inner_product(z) == y.trace_inner_product(x*z) True @@ -1173,3 +1437,30 @@ class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement): raise TypeError("'other' must live in the same algebra") return (self*other).trace() + + + def trace_norm(self): + """ + The norm of this element with respect to :meth:`trace_inner_product`. + + SETUP:: + + sage: from mjo.eja.eja_algebra import (JordanSpinEJA, + ....: HadamardEJA) + + EXAMPLES:: + + sage: J = HadamardEJA(2) + sage: x = sum(J.gens()) + sage: x.trace_norm() + sqrt(2) + + :: + + sage: J = JordanSpinEJA(4) + sage: x = sum(J.gens()) + sage: x.trace_norm() + 2*sqrt(2) + + """ + return self.trace_inner_product(self).sqrt()