""" Euclidean Jordan Algebras. These are formally-real Jordan Algebras; specifically those where u^2 + v^2 = 0 implies that u = v = 0. They are used in optimization, and have some additional nice methods beyond what can be supported in a general Jordan Algebra. """ #from sage.algebras.finite_dimensional_algebras.finite_dimensional_algebra import FiniteDimensionalAlgebra from sage.algebras.quatalg.quaternion_algebra import QuaternionAlgebra from sage.categories.finite_dimensional_algebras_with_basis import FiniteDimensionalAlgebrasWithBasis from sage.combinat.free_module import CombinatorialFreeModule from sage.matrix.constructor import matrix from sage.misc.cachefunc import cached_method from sage.misc.prandom import choice from sage.modules.free_module import VectorSpace from sage.rings.integer_ring import ZZ from sage.rings.number_field.number_field import QuadraticField from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing from sage.rings.rational_field import QQ from sage.structure.element import is_Matrix from sage.structure.category_object import normalize_names from mjo.eja.eja_element import FiniteDimensionalEuclideanJordanAlgebraElement from mjo.eja.eja_utils import _mat2vec class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): def __init__(self, field, mult_table, rank, prefix='e', category=None, natural_basis=None): """ SETUP:: sage: from mjo.eja.eja_algebra import random_eja EXAMPLES: By definition, Jordan multiplication commutes:: sage: set_random_seed() sage: J = random_eja() sage: x = J.random_element() sage: y = J.random_element() sage: x*y == y*x True """ self._rank = rank self._natural_basis = natural_basis self._multiplication_table = mult_table if category is None: category = FiniteDimensionalAlgebrasWithBasis(field).Unital() fda = super(FiniteDimensionalEuclideanJordanAlgebra, self) fda.__init__(field, range(len(mult_table)), prefix=prefix, category=category) self.print_options(bracket='') def _repr_(self): """ Return a string representation of ``self``. SETUP:: sage: from mjo.eja.eja_algebra import JordanSpinEJA TESTS: Ensure that it says what we think it says:: sage: JordanSpinEJA(2, field=QQ) Euclidean Jordan algebra of degree 2 over Rational Field sage: JordanSpinEJA(3, field=RDF) Euclidean Jordan algebra of degree 3 over Real Double Field """ # TODO: change this to say "dimension" and fix all the tests. fmt = "Euclidean Jordan algebra of degree {} over {}" return fmt.format(self.dimension(), self.base_ring()) def product_on_basis(self, i, j): ei = self.basis()[i] ej = self.basis()[j] Lei = self._multiplication_table[i] return self.from_vector(Lei*ej.to_vector()) def _a_regular_element(self): """ Guess a regular element. Needed to compute the basis for our characteristic polynomial coefficients. SETUP:: sage: from mjo.eja.eja_algebra import random_eja TESTS: Ensure that this hacky method succeeds for every algebra that we know how to construct:: sage: set_random_seed() sage: J = random_eja() sage: J._a_regular_element().is_regular() True """ gs = self.gens() z = self.sum( (i+1)*gs[i] for i in range(len(gs)) ) if not z.is_regular(): raise ValueError("don't know a regular element") return z @cached_method def _charpoly_basis_space(self): """ Return the vector space spanned by the basis used in our characteristic polynomial coefficients. This is used not only to compute those coefficients, but also any time we need to evaluate the coefficients (like when we compute the trace or determinant). """ z = self._a_regular_element() V = self.vector_space() V1 = V.span_of_basis( (z**k).to_vector() for k in range(self.rank()) ) b = (V1.basis() + V1.complement().basis()) return V.span_of_basis(b) @cached_method def _charpoly_coeff(self, i): """ Return the coefficient polynomial "a_{i}" of this algebra's general characteristic polynomial. Having this be a separate cached method lets us compute and store the trace/determinant (a_{r-1} and a_{0} respectively) separate from the entire characteristic polynomial. """ (A_of_x, x, xr, detA) = self._charpoly_matrix_system() R = A_of_x.base_ring() if i >= self.rank(): # Guaranteed by theory return R.zero() # Danger: the in-place modification is done for performance # reasons (reconstructing a matrix with huge polynomial # entries is slow), but I don't know how cached_method works, # so it's highly possible that we're modifying some global # list variable by reference, here. In other words, you # probably shouldn't call this method twice on the same # algebra, at the same time, in two threads Ai_orig = A_of_x.column(i) A_of_x.set_column(i,xr) numerator = A_of_x.det() A_of_x.set_column(i,Ai_orig) # We're relying on the theory here to ensure that each a_i is # indeed back in R, and the added negative signs are to make # the whole charpoly expression sum to zero. return R(-numerator/detA) @cached_method def _charpoly_matrix_system(self): """ Compute the matrix whose entries A_ij are polynomials in X1,...,XN, the vector ``x`` of variables X1,...,XN, the vector corresponding to `x^r` and the determinent of the matrix A = [A_ij]. In other words, all of the fixed (cachable) data needed to compute the coefficients of the characteristic polynomial. """ r = self.rank() n = self.dimension() # Construct a new algebra over a multivariate polynomial ring... names = tuple('X' + str(i) for i in range(1,n+1)) R = PolynomialRing(self.base_ring(), names) J = FiniteDimensionalEuclideanJordanAlgebra( R, tuple(self._multiplication_table), r) idmat = matrix.identity(J.base_ring(), n) W = self._charpoly_basis_space() W = W.change_ring(R.fraction_field()) # Starting with the standard coordinates x = (X1,X2,...,Xn) # and then converting the entries to W-coordinates allows us # to pass in the standard coordinates to the charpoly and get # back the right answer. Specifically, with x = (X1,X2,...,Xn), # we have # # W.coordinates(x^2) eval'd at (standard z-coords) # = # W-coords of (z^2) # = # W-coords of (standard coords of x^2 eval'd at std-coords of z) # # We want the middle equivalent thing in our matrix, but use # the first equivalent thing instead so that we can pass in # standard coordinates. x = J(W(R.gens())) # Handle the zeroth power separately, because computing # the unit element in J is mathematically suspect. x0 = W.coordinate_vector(self.one().to_vector()) l1 = [ x0.column() ] l1 += [ W.coordinate_vector((x**k).to_vector()).column() for k in range(1,r) ] l2 = [idmat.column(k-1).column() for k in range(r+1, n+1)] A_of_x = matrix.block(R, 1, n, (l1 + l2)) xr = W.coordinate_vector((x**r).to_vector()) return (A_of_x, x, xr, A_of_x.det()) @cached_method def characteristic_polynomial(self): """ Return a characteristic polynomial that works for all elements of this algebra. The resulting polynomial has `n+1` variables, where `n` is the dimension of this algebra. The first `n` variables correspond to the coordinates of an algebra element: when evaluated at the coordinates of an algebra element with respect to a certain basis, the result is a univariate polynomial (in the one remaining variable ``t``), namely the characteristic polynomial of that element. SETUP:: sage: from mjo.eja.eja_algebra import JordanSpinEJA EXAMPLES: The characteristic polynomial in the spin algebra is given in Alizadeh, Example 11.11:: sage: J = JordanSpinEJA(3) sage: p = J.characteristic_polynomial(); p X1^2 - X2^2 - X3^2 + (-2*t)*X1 + t^2 sage: xvec = J.one().to_vector() sage: p(*xvec) t^2 - 2*t + 1 """ r = self.rank() n = self.dimension() # The list of coefficient polynomials a_1, a_2, ..., a_n. a = [ self._charpoly_coeff(i) for i in range(n) ] # We go to a bit of trouble here to reorder the # indeterminates, so that it's easier to evaluate the # characteristic polynomial at x's coordinates and get back # something in terms of t, which is what we want. R = a[0].parent() S = PolynomialRing(self.base_ring(),'t') t = S.gen(0) S = PolynomialRing(S, R.variable_names()) t = S(t) # Note: all entries past the rth should be zero. The # coefficient of the highest power (x^r) is 1, but it doesn't # appear in the solution vector which contains coefficients # for the other powers (to make them sum to x^r). if (r < n): a[r] = 1 # corresponds to x^r else: # When the rank is equal to the dimension, trying to # assign a[r] goes out-of-bounds. a.append(1) # corresponds to x^r return sum( a[k]*(t**k) for k in range(len(a)) ) def inner_product(self, x, y): """ The inner product associated with this Euclidean Jordan algebra. Defaults to the trace inner product, but can be overridden by subclasses if they are sure that the necessary properties are satisfied. SETUP:: sage: from mjo.eja.eja_algebra import random_eja EXAMPLES: The inner product must satisfy its axiom for this algebra to truly be 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: (x*y).inner_product(z) == y.inner_product(x*z) True """ if (not x in self) or (not y in self): raise TypeError("arguments must live in this algebra") return x.trace_inner_product(y) def natural_basis(self): """ Return a more-natural representation of this algebra's basis. Every finite-dimensional Euclidean Jordan Algebra is a direct sum of five simple algebras, four of which comprise Hermitian matrices. This method returns the original "natural" basis for our underlying vector space. (Typically, the natural basis is used to construct the multiplication table in the first place.) Note that this will always return a matrix. The standard basis in `R^n` will be returned as `n`-by-`1` column matrices. SETUP:: sage: from mjo.eja.eja_algebra import (JordanSpinEJA, ....: RealSymmetricEJA) EXAMPLES:: sage: J = RealSymmetricEJA(2) sage: J.basis() Finite family {0: e0, 1: e1, 2: e2} sage: J.natural_basis() ( [1 0] [0 1] [0 0] [0 0], [1 0], [0 1] ) :: sage: J = JordanSpinEJA(2) sage: J.basis() Finite family {0: e0, 1: e1} sage: J.natural_basis() ( [1] [0] [0], [1] ) """ if self._natural_basis is None: return tuple( b.to_vector().column() for b in self.basis() ) else: return self._natural_basis @cached_method def one(self): """ Return the unit element of this algebra. SETUP:: sage: from mjo.eja.eja_algebra import (RealCartesianProductEJA, ....: random_eja) EXAMPLES:: sage: J = RealCartesianProductEJA(5) sage: J.one() e0 + e1 + e2 + e3 + e4 TESTS:: The identity element acts like the identity:: sage: set_random_seed() sage: J = random_eja() sage: x = J.random_element() sage: J.one()*x == x and x*J.one() == x True The matrix of the unit element's operator is the identity:: sage: set_random_seed() sage: J = random_eja() sage: actual = J.one().operator().matrix() sage: expected = matrix.identity(J.base_ring(), J.dimension()) sage: actual == expected True """ # We can brute-force compute the matrices of the operators # that correspond to the basis elements of this algebra. # If some linear combination of those basis elements is the # algebra identity, then the same linear combination of # their matrices has to be the identity matrix. # # Of course, matrices aren't vectors in sage, so we have to # appeal to the "long vectors" isometry. oper_vecs = [ _mat2vec(g.operator().matrix()) for g in self.gens() ] # Now we use basis linear algebra to find the coefficients, # of the matrices-as-vectors-linear-combination, which should # work for the original algebra basis too. A = matrix.column(self.base_ring(), oper_vecs) # We used the isometry on the left-hand side already, but we # still need to do it for the right-hand side. Recall that we # wanted something that summed to the identity matrix. b = _mat2vec( matrix.identity(self.base_ring(), self.dimension()) ) # Now if there's an identity element in the algebra, this should work. coeffs = A.solve_right(b) return self.linear_combination(zip(self.gens(), coeffs)) def rank(self): """ Return the rank of this EJA. ALGORITHM: The author knows of no algorithm to compute the rank of an EJA where only the multiplication table is known. In lieu of one, we require the rank to be specified when the algebra is created, and simply pass along that number here. SETUP:: sage: from mjo.eja.eja_algebra import (JordanSpinEJA, ....: RealSymmetricEJA, ....: ComplexHermitianEJA, ....: QuaternionHermitianEJA, ....: random_eja) EXAMPLES: The rank of the Jordan spin algebra is always two:: sage: JordanSpinEJA(2).rank() 2 sage: JordanSpinEJA(3).rank() 2 sage: JordanSpinEJA(4).rank() 2 The rank of the `n`-by-`n` Hermitian real, complex, or quaternion matrices is `n`:: sage: RealSymmetricEJA(2).rank() 2 sage: ComplexHermitianEJA(2).rank() 2 sage: QuaternionHermitianEJA(2).rank() 2 sage: RealSymmetricEJA(5).rank() 5 sage: ComplexHermitianEJA(5).rank() 5 sage: QuaternionHermitianEJA(5).rank() 5 TESTS: Ensure that every EJA that we know how to construct has a positive integer rank:: sage: set_random_seed() sage: r = random_eja().rank() sage: r in ZZ and r > 0 True """ return self._rank def vector_space(self): """ Return the vector space that underlies this algebra. SETUP:: sage: from mjo.eja.eja_algebra import RealSymmetricEJA EXAMPLES:: sage: J = RealSymmetricEJA(2) sage: J.vector_space() Vector space of dimension 3 over Rational Field """ return self.zero().to_vector().parent().ambient_vector_space() Element = FiniteDimensionalEuclideanJordanAlgebraElement class RealCartesianProductEJA(FiniteDimensionalEuclideanJordanAlgebra): """ Return the Euclidean Jordan Algebra corresponding to the set `R^n` under the Hadamard product. Note: this is nothing more than the Cartesian product of ``n`` copies of the spin algebra. Once Cartesian product algebras are implemented, this can go. SETUP:: sage: from mjo.eja.eja_algebra import RealCartesianProductEJA EXAMPLES: This multiplication table can be verified by hand:: sage: J = RealCartesianProductEJA(3) sage: e0,e1,e2 = J.gens() sage: e0*e0 e0 sage: e0*e1 0 sage: e0*e2 0 sage: e1*e1 e1 sage: e1*e2 0 sage: e2*e2 e2 """ def __init__(self, n, field=QQ): # The superclass constructor takes a list of matrices, the ith # representing right multiplication by the ith basis element # in the vector space. So if e_1 = (1,0,0), then right # (Hadamard) multiplication of x by e_1 picks out the first # component of x; and likewise for the ith basis element e_i. Qs = [ matrix(field, n, n, lambda k,j: 1*(k == j == i)) for i in xrange(n) ] fdeja = super(RealCartesianProductEJA, self) return fdeja.__init__(field, Qs, rank=n) def inner_product(self, x, y): return _usual_ip(x,y) def random_eja(): """ Return a "random" finite-dimensional Euclidean Jordan Algebra. ALGORITHM: For now, we choose a random natural number ``n`` (greater than zero) and then give you back one of the following: * The cartesian product of the rational numbers ``n`` times; this is ``QQ^n`` with the Hadamard product. * The Jordan spin algebra on ``QQ^n``. * The ``n``-by-``n`` rational symmetric matrices with the symmetric product. * The ``n``-by-``n`` complex-rational Hermitian matrices embedded in the space of ``2n``-by-``2n`` real symmetric matrices. * The ``n``-by-``n`` quaternion-rational Hermitian matrices embedded in the space of ``4n``-by-``4n`` real symmetric matrices. Later this might be extended to return Cartesian products of the EJAs above. SETUP:: sage: from mjo.eja.eja_algebra import random_eja TESTS:: sage: random_eja() Euclidean Jordan algebra of degree... """ # The max_n component lets us choose different upper bounds on the # value "n" that gets passed to the constructor. This is needed # because e.g. R^{10} is reasonable to test, while the Hermitian # 10-by-10 quaternion matrices are not. (constructor, max_n) = choice([(RealCartesianProductEJA, 6), (JordanSpinEJA, 6), (RealSymmetricEJA, 5), (ComplexHermitianEJA, 4), (QuaternionHermitianEJA, 3)]) n = ZZ.random_element(1, max_n) return constructor(n, field=QQ) def _real_symmetric_basis(n, field=QQ): """ Return a basis for the space of real symmetric n-by-n matrices. """ # The basis of symmetric matrices, as matrices, in their R^(n-by-n) # coordinates. S = [] for i in xrange(n): for j in xrange(i+1): Eij = matrix(field, n, lambda k,l: k==i and l==j) if i == j: Sij = Eij else: # Beware, orthogonal but not normalized! Sij = Eij + Eij.transpose() S.append(Sij) return tuple(S) def _complex_hermitian_basis(n, field=QQ): """ Returns a basis for the space of complex Hermitian n-by-n matrices. SETUP:: sage: from mjo.eja.eja_algebra import _complex_hermitian_basis TESTS:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: all( M.is_symmetric() for M in _complex_hermitian_basis(n) ) True """ F = QuadraticField(-1, 'I') I = F.gen() # This is like the symmetric case, but we need to be careful: # # * We want conjugate-symmetry, not just symmetry. # * The diagonal will (as a result) be real. # S = [] for i in xrange(n): for j in xrange(i+1): Eij = matrix(field, n, lambda k,l: k==i and l==j) if i == j: Sij = _embed_complex_matrix(Eij) S.append(Sij) else: # Beware, orthogonal but not normalized! The second one # has a minus because it's conjugated. Sij_real = _embed_complex_matrix(Eij + Eij.transpose()) S.append(Sij_real) Sij_imag = _embed_complex_matrix(I*Eij - I*Eij.transpose()) S.append(Sij_imag) return tuple(S) def _quaternion_hermitian_basis(n, field=QQ): """ Returns a basis for the space of quaternion Hermitian n-by-n matrices. SETUP:: sage: from mjo.eja.eja_algebra import _quaternion_hermitian_basis TESTS:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: all( M.is_symmetric() for M in _quaternion_hermitian_basis(n) ) True """ Q = QuaternionAlgebra(QQ,-1,-1) I,J,K = Q.gens() # This is like the symmetric case, but we need to be careful: # # * We want conjugate-symmetry, not just symmetry. # * The diagonal will (as a result) be real. # S = [] for i in xrange(n): for j in xrange(i+1): Eij = matrix(Q, n, lambda k,l: k==i and l==j) if i == j: Sij = _embed_quaternion_matrix(Eij) S.append(Sij) else: # Beware, orthogonal but not normalized! The second, # third, and fourth ones have a minus because they're # conjugated. Sij_real = _embed_quaternion_matrix(Eij + Eij.transpose()) S.append(Sij_real) Sij_I = _embed_quaternion_matrix(I*Eij - I*Eij.transpose()) S.append(Sij_I) Sij_J = _embed_quaternion_matrix(J*Eij - J*Eij.transpose()) S.append(Sij_J) Sij_K = _embed_quaternion_matrix(K*Eij - K*Eij.transpose()) S.append(Sij_K) return tuple(S) def _multiplication_table_from_matrix_basis(basis): """ At least three of the five simple Euclidean Jordan algebras have the symmetric multiplication (A,B) |-> (AB + BA)/2, where the multiplication on the right is matrix multiplication. Given a basis for the underlying matrix space, this function returns a multiplication table (obtained by looping through the basis elements) for an algebra of those matrices. A reordered copy of the basis is also returned to work around the fact that the ``span()`` in this function will change the order of the basis from what we think it is, to... something else. """ # In S^2, for example, we nominally have four coordinates even # though the space is of dimension three only. The vector space V # is supposed to hold the entire long vector, and the subspace W # of V will be spanned by the vectors that arise from symmetric # matrices. Thus for S^2, dim(V) == 4 and dim(W) == 3. field = basis[0].base_ring() dimension = basis[0].nrows() V = VectorSpace(field, dimension**2) W = V.span_of_basis( _mat2vec(s) for s in basis ) Qs = [] for s in basis: # Brute force the multiplication-by-s matrix by looping # through all elements of the basis and doing the computation # to find out what the corresponding row should be. Q_cols = [] for t in basis: this_col = _mat2vec((s*t + t*s)/2) Q_cols.append(W.coordinates(this_col)) Q = matrix.column(field, W.dimension(), Q_cols) Qs.append(Q) return Qs def _embed_complex_matrix(M): """ Embed the n-by-n complex matrix ``M`` into the space of real matrices of size 2n-by-2n via the map the sends each entry `z = a + bi` to the block matrix ``[[a,b],[-b,a]]``. SETUP:: sage: from mjo.eja.eja_algebra import _embed_complex_matrix EXAMPLES:: sage: F = QuadraticField(-1,'i') sage: x1 = F(4 - 2*i) sage: x2 = F(1 + 2*i) sage: x3 = F(-i) sage: x4 = F(6) sage: M = matrix(F,2,[[x1,x2],[x3,x4]]) sage: _embed_complex_matrix(M) [ 4 -2| 1 2] [ 2 4|-2 1] [-----+-----] [ 0 -1| 6 0] [ 1 0| 0 6] TESTS: Embedding is a homomorphism (isomorphism, in fact):: sage: set_random_seed() sage: n = ZZ.random_element(5) sage: F = QuadraticField(-1, 'i') sage: X = random_matrix(F, n) sage: Y = random_matrix(F, n) sage: actual = _embed_complex_matrix(X) * _embed_complex_matrix(Y) sage: expected = _embed_complex_matrix(X*Y) sage: actual == expected True """ n = M.nrows() if M.ncols() != n: raise ValueError("the matrix 'M' must be square") field = M.base_ring() blocks = [] for z in M.list(): a = z.real() b = z.imag() blocks.append(matrix(field, 2, [[a,b],[-b,a]])) # We can drop the imaginaries here. return matrix.block(field.base_ring(), n, blocks) def _unembed_complex_matrix(M): """ The inverse of _embed_complex_matrix(). SETUP:: sage: from mjo.eja.eja_algebra import (_embed_complex_matrix, ....: _unembed_complex_matrix) EXAMPLES:: sage: A = matrix(QQ,[ [ 1, 2, 3, 4], ....: [-2, 1, -4, 3], ....: [ 9, 10, 11, 12], ....: [-10, 9, -12, 11] ]) sage: _unembed_complex_matrix(A) [ 2*i + 1 4*i + 3] [ 10*i + 9 12*i + 11] TESTS: Unembedding is the inverse of embedding:: sage: set_random_seed() sage: F = QuadraticField(-1, 'i') sage: M = random_matrix(F, 3) sage: _unembed_complex_matrix(_embed_complex_matrix(M)) == M True """ n = ZZ(M.nrows()) if M.ncols() != n: raise ValueError("the matrix 'M' must be square") if not n.mod(2).is_zero(): raise ValueError("the matrix 'M' must be a complex embedding") F = QuadraticField(-1, 'i') i = F.gen() # Go top-left to bottom-right (reading order), converting every # 2-by-2 block we see to a single complex element. elements = [] for k in xrange(n/2): for j in xrange(n/2): submat = M[2*k:2*k+2,2*j:2*j+2] if submat[0,0] != submat[1,1]: raise ValueError('bad on-diagonal submatrix') if submat[0,1] != -submat[1,0]: raise ValueError('bad off-diagonal submatrix') z = submat[0,0] + submat[0,1]*i elements.append(z) return matrix(F, n/2, elements) def _embed_quaternion_matrix(M): """ Embed the n-by-n quaternion matrix ``M`` into the space of real matrices of size 4n-by-4n by first sending each quaternion entry `z = a + bi + cj + dk` to the block-complex matrix ``[[a + bi, c+di],[-c + di, a-bi]]`, and then embedding those into a real matrix. SETUP:: sage: from mjo.eja.eja_algebra import _embed_quaternion_matrix EXAMPLES:: sage: Q = QuaternionAlgebra(QQ,-1,-1) sage: i,j,k = Q.gens() sage: x = 1 + 2*i + 3*j + 4*k sage: M = matrix(Q, 1, [[x]]) sage: _embed_quaternion_matrix(M) [ 1 2 3 4] [-2 1 -4 3] [-3 4 1 -2] [-4 -3 2 1] Embedding is a homomorphism (isomorphism, in fact):: sage: set_random_seed() sage: n = ZZ.random_element(5) sage: Q = QuaternionAlgebra(QQ,-1,-1) sage: X = random_matrix(Q, n) sage: Y = random_matrix(Q, n) sage: actual = _embed_quaternion_matrix(X)*_embed_quaternion_matrix(Y) sage: expected = _embed_quaternion_matrix(X*Y) sage: actual == expected True """ quaternions = M.base_ring() n = M.nrows() if M.ncols() != n: raise ValueError("the matrix 'M' must be square") F = QuadraticField(-1, 'i') i = F.gen() blocks = [] for z in M.list(): t = z.coefficient_tuple() a = t[0] b = t[1] c = t[2] d = t[3] cplx_matrix = matrix(F, 2, [[ a + b*i, c + d*i], [-c + d*i, a - b*i]]) blocks.append(_embed_complex_matrix(cplx_matrix)) # We should have real entries by now, so use the realest field # we've got for the return value. return matrix.block(quaternions.base_ring(), n, blocks) def _unembed_quaternion_matrix(M): """ The inverse of _embed_quaternion_matrix(). SETUP:: sage: from mjo.eja.eja_algebra import (_embed_quaternion_matrix, ....: _unembed_quaternion_matrix) EXAMPLES:: sage: M = matrix(QQ, [[ 1, 2, 3, 4], ....: [-2, 1, -4, 3], ....: [-3, 4, 1, -2], ....: [-4, -3, 2, 1]]) sage: _unembed_quaternion_matrix(M) [1 + 2*i + 3*j + 4*k] TESTS: Unembedding is the inverse of embedding:: sage: set_random_seed() sage: Q = QuaternionAlgebra(QQ, -1, -1) sage: M = random_matrix(Q, 3) sage: _unembed_quaternion_matrix(_embed_quaternion_matrix(M)) == M True """ n = ZZ(M.nrows()) if M.ncols() != n: raise ValueError("the matrix 'M' must be square") if not n.mod(4).is_zero(): raise ValueError("the matrix 'M' must be a complex embedding") Q = QuaternionAlgebra(QQ,-1,-1) i,j,k = Q.gens() # Go top-left to bottom-right (reading order), converting every # 4-by-4 block we see to a 2-by-2 complex block, to a 1-by-1 # quaternion block. elements = [] for l in xrange(n/4): for m in xrange(n/4): submat = _unembed_complex_matrix(M[4*l:4*l+4,4*m:4*m+4]) if submat[0,0] != submat[1,1].conjugate(): raise ValueError('bad on-diagonal submatrix') if submat[0,1] != -submat[1,0].conjugate(): raise ValueError('bad off-diagonal submatrix') z = submat[0,0].real() + submat[0,0].imag()*i z += submat[0,1].real()*j + submat[0,1].imag()*k elements.append(z) return matrix(Q, n/4, elements) # The usual inner product on R^n. def _usual_ip(x,y): return x.to_vector().inner_product(y.to_vector()) # The inner product used for the real symmetric simple EJA. # We keep it as a separate function because e.g. the complex # algebra uses the same inner product, except divided by 2. def _matrix_ip(X,Y): X_mat = X.natural_representation() Y_mat = Y.natural_representation() return (X_mat*Y_mat).trace() class RealSymmetricEJA(FiniteDimensionalEuclideanJordanAlgebra): """ The rank-n simple EJA consisting of real symmetric n-by-n matrices, the usual symmetric Jordan product, and the trace inner product. It has dimension `(n^2 + n)/2` over the reals. SETUP:: sage: from mjo.eja.eja_algebra import RealSymmetricEJA EXAMPLES:: sage: J = RealSymmetricEJA(2) sage: e0, e1, e2 = J.gens() sage: e0*e0 e0 sage: e1*e1 e0 + e2 sage: e2*e2 e2 TESTS: The dimension of this algebra is `(n^2 + n) / 2`:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: J = RealSymmetricEJA(n) sage: J.dimension() == (n^2 + n)/2 True The Jordan multiplication is what we think it is:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: J = RealSymmetricEJA(n) sage: x = J.random_element() sage: y = J.random_element() sage: actual = (x*y).natural_representation() sage: X = x.natural_representation() sage: Y = y.natural_representation() sage: expected = (X*Y + Y*X)/2 sage: actual == expected True sage: J(expected) == x*y True """ def __init__(self, n, field=QQ): S = _real_symmetric_basis(n, field=field) Qs = _multiplication_table_from_matrix_basis(S) fdeja = super(RealSymmetricEJA, self) return fdeja.__init__(field, Qs, rank=n, natural_basis=S) def inner_product(self, x, y): return _matrix_ip(x,y) class ComplexHermitianEJA(FiniteDimensionalEuclideanJordanAlgebra): """ The rank-n simple EJA consisting of complex Hermitian n-by-n matrices over the real numbers, the usual symmetric Jordan product, and the real-part-of-trace inner product. It has dimension `n^2` over the reals. SETUP:: sage: from mjo.eja.eja_algebra import ComplexHermitianEJA TESTS: The dimension of this algebra is `n^2`:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: J = ComplexHermitianEJA(n) sage: J.dimension() == n^2 True The Jordan multiplication is what we think it is:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: J = ComplexHermitianEJA(n) sage: x = J.random_element() sage: y = J.random_element() sage: actual = (x*y).natural_representation() sage: X = x.natural_representation() sage: Y = y.natural_representation() sage: expected = (X*Y + Y*X)/2 sage: actual == expected True sage: J(expected) == x*y True """ def __init__(self, n, field=QQ): S = _complex_hermitian_basis(n) Qs = _multiplication_table_from_matrix_basis(S) fdeja = super(ComplexHermitianEJA, self) return fdeja.__init__(field, Qs, rank=n, natural_basis=S) def inner_product(self, x, y): # Since a+bi on the diagonal is represented as # # a + bi = [ a b ] # [ -b a ], # # we'll double-count the "a" entries if we take the trace of # the embedding. return _matrix_ip(x,y)/2 class QuaternionHermitianEJA(FiniteDimensionalEuclideanJordanAlgebra): """ The rank-n simple EJA consisting of self-adjoint n-by-n quaternion matrices, the usual symmetric Jordan product, and the real-part-of-trace inner product. It has dimension `2n^2 - n` over the reals. SETUP:: sage: from mjo.eja.eja_algebra import QuaternionHermitianEJA TESTS: The dimension of this algebra is `n^2`:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: J = QuaternionHermitianEJA(n) sage: J.dimension() == 2*(n^2) - n True The Jordan multiplication is what we think it is:: sage: set_random_seed() sage: n = ZZ.random_element(1,5) sage: J = QuaternionHermitianEJA(n) sage: x = J.random_element() sage: y = J.random_element() sage: actual = (x*y).natural_representation() sage: X = x.natural_representation() sage: Y = y.natural_representation() sage: expected = (X*Y + Y*X)/2 sage: actual == expected True sage: J(expected) == x*y True """ def __init__(self, n, field=QQ): S = _quaternion_hermitian_basis(n) Qs = _multiplication_table_from_matrix_basis(S) fdeja = super(QuaternionHermitianEJA, self) return fdeja.__init__(field, Qs, rank=n, natural_basis=S) def inner_product(self, x, y): # Since a+bi+cj+dk on the diagonal is represented as # # a + bi +cj + dk = [ a b c d] # [ -b a -d c] # [ -c d a -b] # [ -d -c b a], # # we'll quadruple-count the "a" entries if we take the trace of # the embedding. return _matrix_ip(x,y)/4 class JordanSpinEJA(FiniteDimensionalEuclideanJordanAlgebra): """ The rank-2 simple EJA consisting of real vectors ``x=(x0, x_bar)`` with the usual inner product and jordan product ``x*y = (, x0*y_bar + y0*x_bar)``. It has dimension `n` over the reals. SETUP:: sage: from mjo.eja.eja_algebra import JordanSpinEJA EXAMPLES: This multiplication table can be verified by hand:: sage: J = JordanSpinEJA(4) sage: e0,e1,e2,e3 = J.gens() sage: e0*e0 e0 sage: e0*e1 e1 sage: e0*e2 e2 sage: e0*e3 e3 sage: e1*e2 0 sage: e1*e3 0 sage: e2*e3 0 """ def __init__(self, n, field=QQ): Qs = [] id_matrix = matrix.identity(field, n) for i in xrange(n): ei = id_matrix.column(i) Qi = matrix.zero(field, n) Qi.set_row(0, ei) Qi.set_column(0, ei) Qi += matrix.diagonal(n, [ei[0]]*n) # The addition of the diagonal matrix adds an extra ei[0] in the # upper-left corner of the matrix. Qi[0,0] = Qi[0,0] * ~field(2) Qs.append(Qi) # The rank of the spin algebra is two, unless we're in a # one-dimensional ambient space (because the rank is bounded by # the ambient dimension). fdeja = super(JordanSpinEJA, self) return fdeja.__init__(field, Qs, rank=min(n,2)) def inner_product(self, x, y): return _usual_ip(x,y)