X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=mjo%2Feja%2Feja_algebra.py;h=a7e56187550111a928823d536dc3ff88347b7424;hb=0848d2345f4813e9f176efef6864d65b1915c495;hp=a9bb6c7de4fc829f881978de63d1e78597a51d73;hpb=208bc5d64bd206684e59f757d932768552e9f7ba;p=sage.d.git diff --git a/mjo/eja/eja_algebra.py b/mjo/eja/eja_algebra.py index a9bb6c7..a7e5618 100644 --- a/mjo/eja/eja_algebra.py +++ b/mjo/eja/eja_algebra.py @@ -9,14 +9,16 @@ from sage.algebras.quatalg.quaternion_algebra import QuaternionAlgebra from sage.categories.magmatic_algebras import MagmaticAlgebras from sage.combinat.free_module import CombinatorialFreeModule from sage.matrix.constructor import matrix +from sage.matrix.matrix_space import MatrixSpace from sage.misc.cachefunc import cached_method from sage.misc.prandom import choice from sage.misc.table import table -from sage.modules.free_module import VectorSpace +from sage.modules.free_module import FreeModule, VectorSpace from sage.rings.integer_ring import ZZ -from sage.rings.number_field.number_field import QuadraticField +from sage.rings.number_field.number_field import NumberField, QuadraticField from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing from sage.rings.rational_field import QQ +from sage.rings.real_lazy import CLF, RLF from sage.structure.element import is_Matrix from mjo.eja.eja_element import FiniteDimensionalEuclideanJordanAlgebraElement @@ -134,13 +136,17 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): return self.zero() natural_basis = self.natural_basis() - if elt not in natural_basis[0].matrix_space(): + basis_space = natural_basis[0].matrix_space() + if elt not in basis_space: raise ValueError("not a naturally-represented algebra element") - # 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()*elt.ncols()) + # Thanks for nothing! Matrix spaces aren't vector spaces in + # Sage, so we have to figure out its natural-basis coordinates + # ourselves. We use the basis space's ring instead of the + # element's ring because the basis space might be an algebraic + # closure whereas the base ring of the 3-by-3 identity matrix + # could be QQ instead of QQbar. + V = VectorSpace(basis_space.base_ring(), elt.nrows()*elt.ncols()) W = V.span_of_basis( _mat2vec(s) for s in natural_basis ) coords = W.coordinate_vector(_mat2vec(elt)) return self.from_vector(coords) @@ -207,8 +213,13 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): 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()) ) + # Don't use the parent vector space directly here in case this + # happens to be a subalgebra. In that case, we would be e.g. + # two-dimensional but span_of_basis() would expect three + # coordinates. + V = VectorSpace(self.base_ring(), self.vector_space().dimension()) + basis = [ (z**k).to_vector() for k in range(self.rank()) ] + V1 = V.span_of_basis( basis ) b = (V1.basis() + V1.complement().basis()) return V.span_of_basis(b) @@ -259,16 +270,40 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): r = self.rank() n = self.dimension() - # Construct a new algebra over a multivariate polynomial ring... + # Turn my vector space into a module so that "vectors" can + # have multivatiate polynomial entries. names = tuple('X' + str(i) for i in range(1,n+1)) R = PolynomialRing(self.base_ring(), names) - # Hack around the fact that our multiplication table is in terms of - # algebra elements but the constructor wants it in terms of vectors. - vmt = [ tuple(map(lambda x: x.to_vector(), ls)) - for ls in self._multiplication_table ] - J = FiniteDimensionalEuclideanJordanAlgebra(R, tuple(vmt), r) - idmat = matrix.identity(J.base_ring(), n) + # Using change_ring() on the parent's vector space doesn't work + # here because, in a subalgebra, that vector space has a basis + # and change_ring() tries to bring the basis along with it. And + # that doesn't work unless the new ring is a PID, which it usually + # won't be. + V = FreeModule(R,n) + + # Now let x = (X1,X2,...,Xn) be the vector whose entries are + # indeterminates... + x = V(names) + + # And figure out the "left multiplication by x" matrix in + # that setting. + lmbx_cols = [] + monomial_matrices = [ self.monomial(i).operator().matrix() + for i in range(n) ] # don't recompute these! + for k in range(n): + ek = self.monomial(k).to_vector() + lmbx_cols.append( + sum( x[i]*(monomial_matrices[i]*ek) + for i in range(n) ) ) + Lx = matrix.column(R, lmbx_cols) + + # Now we can compute powers of x "symbolically" + x_powers = [self.one().to_vector(), x] + for d in range(2, r+1): + x_powers.append( Lx*(x_powers[-1]) ) + + idmat = matrix.identity(R, n) W = self._charpoly_basis_space() W = W.change_ring(R.fraction_field()) @@ -288,18 +323,10 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): # 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.from_vector(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()) + x_powers = [ W.coordinate_vector(xp) for xp in x_powers ] + l2 = [idmat.column(k-1) for k in range(r+1, n+1)] + A_of_x = matrix.column(R, n, (x_powers[:r] + l2)) + return (A_of_x, x, x_powers[r], A_of_x.det()) @cached_method @@ -394,6 +421,29 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): return x.trace_inner_product(y) + def is_trivial(self): + """ + Return whether or not this algebra is trivial. + + A trivial algebra contains only the zero element. + + SETUP:: + + sage: from mjo.eja.eja_algebra import ComplexHermitianEJA + + EXAMPLES:: + + sage: J = ComplexHermitianEJA(3) + sage: J.is_trivial() + False + sage: A = J.zero().subalgebra_generated_by() + sage: A.is_trivial() + True + + """ + return self.dimension() == 0 + + def multiplication_table(self): """ Return a visual representation of this algebra's multiplication @@ -453,8 +503,8 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): Finite family {0: e0, 1: e1, 2: e2} sage: J.natural_basis() ( - [1 0] [0 1] [0 0] - [0 0], [1 0], [0 1] + [1 0] [ 0 1/2*sqrt2] [0 0] + [0 0], [1/2*sqrt2 0], [0 1] ) :: @@ -470,11 +520,23 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): """ if self._natural_basis is None: - return tuple( b.to_vector().column() for b in self.basis() ) + M = self.natural_basis_space() + return tuple( M(b.to_vector()) for b in self.basis() ) else: return self._natural_basis + def natural_basis_space(self): + """ + Return the matrix space in which this algebra's natural basis + elements live. + """ + if self._natural_basis is None or len(self._natural_basis) == 0: + return MatrixSpace(self.base_ring(), self.dimension(), 1) + else: + return self._natural_basis[0].matrix_space() + + @cached_method def one(self): """ @@ -536,6 +598,15 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): return self.linear_combination(zip(self.gens(), coeffs)) + def random_element(self): + # Temporary workaround for https://trac.sagemath.org/ticket/28327 + if self.is_trivial(): + return self.zero() + else: + s = super(FiniteDimensionalEuclideanJordanAlgebra, self) + return s.random_element() + + def rank(self): """ Return the rank of this EJA. @@ -608,7 +679,7 @@ class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule): sage: J = RealSymmetricEJA(2) sage: J.vector_space() - Vector space of dimension 3 over Rational Field + Vector space of dimension 3 over... """ return self.zero().to_vector().parent().ambient_vector_space() @@ -649,14 +720,32 @@ class RealCartesianProductEJA(FiniteDimensionalEuclideanJordanAlgebra): sage: e2*e2 e2 + TESTS: + + We can change the generator prefix:: + + sage: RealCartesianProductEJA(3, prefix='r').gens() + (r0, r1, r2) + + Our inner product satisfies the Jordan axiom:: + + sage: set_random_seed() + sage: n = ZZ.random_element(1,5) + sage: J = RealCartesianProductEJA(n) + 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 + """ - def __init__(self, n, field=QQ): + def __init__(self, n, field=QQ, **kwargs): V = VectorSpace(field, n) mult_table = [ [ V.gen(i)*(i == j) for j in range(n) ] for i in range(n) ] fdeja = super(RealCartesianProductEJA, self) - return fdeja.__init__(field, mult_table, rank=n) + return fdeja.__init__(field, mult_table, rank=n, **kwargs) def inner_product(self, x, y): return _usual_ip(x,y) @@ -713,9 +802,22 @@ def random_eja(): -def _real_symmetric_basis(n, field=QQ): +def _real_symmetric_basis(n, field, normalize): """ Return a basis for the space of real symmetric n-by-n matrices. + + SETUP:: + + sage: from mjo.eja.eja_algebra import _real_symmetric_basis + + TESTS:: + + sage: set_random_seed() + sage: n = ZZ.random_element(1,5) + sage: B = _real_symmetric_basis(n, QQbar, False) + sage: all( M.is_symmetric() for M in B) + True + """ # The basis of symmetric matrices, as matrices, in their R^(n-by-n) # coordinates. @@ -726,16 +828,23 @@ def _real_symmetric_basis(n, field=QQ): if i == j: Sij = Eij else: - # Beware, orthogonal but not normalized! Sij = Eij + Eij.transpose() + if normalize: + Sij = Sij / _real_symmetric_matrix_ip(Sij,Sij).sqrt() S.append(Sij) return tuple(S) -def _complex_hermitian_basis(n, field=QQ): +def _complex_hermitian_basis(n, field, normalize): """ Returns a basis for the space of complex Hermitian n-by-n matrices. + Why do we embed these? Basically, because all of numerical linear + algebra assumes that you're working with vectors consisting of `n` + entries from a field and scalars from the same field. There's no way + to tell SageMath that (for example) the vectors contain complex + numbers, while the scalar field is real. + SETUP:: sage: from mjo.eja.eja_algebra import _complex_hermitian_basis @@ -744,11 +853,15 @@ def _complex_hermitian_basis(n, field=QQ): sage: set_random_seed() sage: n = ZZ.random_element(1,5) - sage: all( M.is_symmetric() for M in _complex_hermitian_basis(n) ) + sage: field = QuadraticField(2, 'sqrt2') + sage: B = _complex_hermitian_basis(n, field, False) + sage: all( M.is_symmetric() for M in B) True """ - F = QuadraticField(-1, 'I') + R = PolynomialRing(field, 'z') + z = R.gen() + F = NumberField(z**2 + 1, 'I', embedding=CLF(-1).sqrt()) I = F.gen() # This is like the symmetric case, but we need to be careful: @@ -759,7 +872,7 @@ def _complex_hermitian_basis(n, field=QQ): S = [] for i in xrange(n): for j in xrange(i+1): - Eij = matrix(field, n, lambda k,l: k==i and l==j) + Eij = matrix(F, n, lambda k,l: k==i and l==j) if i == j: Sij = _embed_complex_matrix(Eij) S.append(Sij) @@ -770,13 +883,27 @@ def _complex_hermitian_basis(n, field=QQ): S.append(Sij_real) Sij_imag = _embed_complex_matrix(I*Eij - I*Eij.transpose()) S.append(Sij_imag) + + # Since we embedded these, we can drop back to the "field" that we + # started with instead of the complex extension "F". + S = [ s.change_ring(field) for s in S ] + if normalize: + S = [ s / _complex_hermitian_matrix_ip(s,s).sqrt() for s in S ] + return tuple(S) -def _quaternion_hermitian_basis(n, field=QQ): + +def _quaternion_hermitian_basis(n, field, normalize): """ Returns a basis for the space of quaternion Hermitian n-by-n matrices. + Why do we embed these? Basically, because all of numerical linear + algebra assumes that you're working with vectors consisting of `n` + entries from a field and scalars from the same field. There's no way + to tell SageMath that (for example) the vectors contain complex + numbers, while the scalar field is real. + SETUP:: sage: from mjo.eja.eja_algebra import _quaternion_hermitian_basis @@ -785,7 +912,8 @@ def _quaternion_hermitian_basis(n, field=QQ): sage: set_random_seed() sage: n = ZZ.random_element(1,5) - sage: all( M.is_symmetric() for M in _quaternion_hermitian_basis(n) ) + sage: B = _quaternion_hermitian_basis(n, QQ, False) + sage: all( M.is_symmetric() for M in B ) True """ @@ -861,7 +989,7 @@ def _embed_complex_matrix(M): EXAMPLES:: - sage: F = QuadraticField(-1,'i') + sage: F = QuadraticField(-1, 'i') sage: x1 = F(4 - 2*i) sage: x2 = F(1 + 2*i) sage: x3 = F(-i) @@ -895,8 +1023,8 @@ def _embed_complex_matrix(M): field = M.base_ring() blocks = [] for z in M.list(): - a = z.real() - b = z.imag() + a = z.vector()[0] # real part, I guess + b = z.vector()[1] # imag part, I guess blocks.append(matrix(field, 2, [[a,b],[-b,a]])) # We can drop the imaginaries here. @@ -939,7 +1067,10 @@ def _unembed_complex_matrix(M): if not n.mod(2).is_zero(): raise ValueError("the matrix 'M' must be a complex embedding") - F = QuadraticField(-1, 'i') + field = M.base_ring() # This should already have sqrt2 + R = PolynomialRing(field, 'z') + z = R.gen() + F = NumberField(z**2 + 1,'i', embedding=CLF(-1).sqrt()) i = F.gen() # Go top-left to bottom-right (reading order), converting every @@ -1087,6 +1218,15 @@ def _matrix_ip(X,Y): Y_mat = Y.natural_representation() return (X_mat*Y_mat).trace() +def _real_symmetric_matrix_ip(X,Y): + return (X*Y).trace() + +def _complex_hermitian_matrix_ip(X,Y): + # This takes EMBEDDED matrices. + Xu = _unembed_complex_matrix(X) + Yu = _unembed_complex_matrix(Y) + # The trace need not be real; consider Xu = (i*I) and Yu = I. + return ((Xu*Yu).trace()).vector()[0] # real part, I guess class RealSymmetricEJA(FiniteDimensionalEuclideanJordanAlgebra): """ @@ -1105,7 +1245,7 @@ class RealSymmetricEJA(FiniteDimensionalEuclideanJordanAlgebra): sage: e0*e0 e0 sage: e1*e1 - e0 + e2 + 1/2*e0 + 1/2*e2 sage: e2*e2 e2 @@ -1135,19 +1275,65 @@ class RealSymmetricEJA(FiniteDimensionalEuclideanJordanAlgebra): sage: J(expected) == x*y True + We can change the generator prefix:: + + sage: RealSymmetricEJA(3, prefix='q').gens() + (q0, q1, q2, q3, q4, q5) + + Our inner product satisfies the Jordan axiom:: + + 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: z = J.random_element() + sage: (x*y).inner_product(z) == y.inner_product(x*z) + True + + Our basis is normalized with respect to the natural inner product:: + + sage: set_random_seed() + sage: n = ZZ.random_element(1,5) + sage: J = RealSymmetricEJA(n) + sage: all( b.norm() == 1 for b in J.gens() ) + True + + Left-multiplication operators are symmetric because they satisfy + the Jordan axiom:: + + sage: set_random_seed() + sage: n = ZZ.random_element(1,5) + sage: x = RealSymmetricEJA(n).random_element() + sage: x.operator().matrix().is_symmetric() + True + """ - def __init__(self, n, field=QQ): - S = _real_symmetric_basis(n, field=field) + def __init__(self, n, field=QQ, normalize_basis=True, **kwargs): + if n > 1 and normalize_basis: + # We'll need sqrt(2) to normalize the basis, and this + # winds up in the multiplication table, so the whole + # algebra needs to be over the field extension. + R = PolynomialRing(field, 'z') + z = R.gen() + p = z**2 - 2 + if p.is_irreducible(): + field = NumberField(p, 'sqrt2', embedding=RLF(2).sqrt()) + + S = _real_symmetric_basis(n, field, normalize_basis) Qs = _multiplication_table_from_matrix_basis(S) fdeja = super(RealSymmetricEJA, self) return fdeja.__init__(field, Qs, rank=n, - natural_basis=S) + natural_basis=S, + **kwargs) def inner_product(self, x, y): - return _matrix_ip(x,y) + X = x.natural_representation() + Y = y.natural_representation() + return _real_symmetric_matrix_ip(X,Y) class ComplexHermitianEJA(FiniteDimensionalEuclideanJordanAlgebra): @@ -1187,27 +1373,66 @@ class ComplexHermitianEJA(FiniteDimensionalEuclideanJordanAlgebra): sage: J(expected) == x*y True + We can change the generator prefix:: + + sage: ComplexHermitianEJA(2, prefix='z').gens() + (z0, z1, z2, z3) + + Our inner product satisfies the Jordan axiom:: + + 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: z = J.random_element() + sage: (x*y).inner_product(z) == y.inner_product(x*z) + True + + Our basis is normalized with respect to the natural inner product:: + + sage: set_random_seed() + sage: n = ZZ.random_element(1,4) + sage: J = ComplexHermitianEJA(n) + sage: all( b.norm() == 1 for b in J.gens() ) + True + + Left-multiplication operators are symmetric because they satisfy + the Jordan axiom:: + + sage: set_random_seed() + sage: n = ZZ.random_element(1,5) + sage: x = ComplexHermitianEJA(n).random_element() + sage: x.operator().matrix().is_symmetric() + True + """ - def __init__(self, n, field=QQ): - S = _complex_hermitian_basis(n) + def __init__(self, n, field=QQ, normalize_basis=True, **kwargs): + if n > 1 and normalize_basis: + # We'll need sqrt(2) to normalize the basis, and this + # winds up in the multiplication table, so the whole + # algebra needs to be over the field extension. + R = PolynomialRing(field, 'z') + z = R.gen() + p = z**2 - 2 + if p.is_irreducible(): + field = NumberField(p, 'sqrt2', embedding=RLF(2).sqrt()) + + S = _complex_hermitian_basis(n, field, normalize_basis) Qs = _multiplication_table_from_matrix_basis(S) fdeja = super(ComplexHermitianEJA, self) return fdeja.__init__(field, Qs, rank=n, - natural_basis=S) + natural_basis=S, + **kwargs) 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 + X = x.natural_representation() + Y = y.natural_representation() + return _complex_hermitian_matrix_ip(X,Y) class QuaternionHermitianEJA(FiniteDimensionalEuclideanJordanAlgebra): @@ -1247,16 +1472,33 @@ class QuaternionHermitianEJA(FiniteDimensionalEuclideanJordanAlgebra): sage: J(expected) == x*y True + We can change the generator prefix:: + + sage: QuaternionHermitianEJA(2, prefix='a').gens() + (a0, a1, a2, a3, a4, a5) + + Our inner product satisfies the Jordan axiom:: + + 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: z = J.random_element() + sage: (x*y).inner_product(z) == y.inner_product(x*z) + True + """ - def __init__(self, n, field=QQ): - S = _quaternion_hermitian_basis(n) + def __init__(self, n, field=QQ, normalize_basis=True, **kwargs): + S = _quaternion_hermitian_basis(n, field, normalize_basis) Qs = _multiplication_table_from_matrix_basis(S) fdeja = super(QuaternionHermitianEJA, self) return fdeja.__init__(field, Qs, rank=n, - natural_basis=S) + natural_basis=S, + **kwargs) def inner_product(self, x, y): # Since a+bi+cj+dk on the diagonal is represented as @@ -1303,8 +1545,24 @@ class JordanSpinEJA(FiniteDimensionalEuclideanJordanAlgebra): sage: e2*e3 0 + We can change the generator prefix:: + + sage: JordanSpinEJA(2, prefix='B').gens() + (B0, B1) + + Our inner product satisfies the Jordan axiom:: + + sage: set_random_seed() + sage: n = ZZ.random_element(1,5) + sage: J = JordanSpinEJA(n) + 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 + """ - def __init__(self, n, field=QQ): + def __init__(self, n, field=QQ, **kwargs): V = VectorSpace(field, n) mult_table = [[V.zero() for j in range(n)] for i in range(n)] for i in range(n): @@ -1325,7 +1583,7 @@ class JordanSpinEJA(FiniteDimensionalEuclideanJordanAlgebra): # one-dimensional ambient space (because the rank is bounded by # the ambient dimension). fdeja = super(JordanSpinEJA, self) - return fdeja.__init__(field, mult_table, rank=min(n,2)) + return fdeja.__init__(field, mult_table, rank=min(n,2), **kwargs) def inner_product(self, x, y): return _usual_ip(x,y)