2 Euclidean Jordan Algebras. These are formally-real Jordan Algebras;
3 specifically those where u^2 + v^2 = 0 implies that u = v = 0. They
4 are used in optimization, and have some additional nice methods beyond
5 what can be supported in a general Jordan Algebra.
8 from itertools
import repeat
10 from sage
.algebras
.quatalg
.quaternion_algebra
import QuaternionAlgebra
11 from sage
.categories
.magmatic_algebras
import MagmaticAlgebras
12 from sage
.combinat
.free_module
import CombinatorialFreeModule
13 from sage
.matrix
.constructor
import matrix
14 from sage
.matrix
.matrix_space
import MatrixSpace
15 from sage
.misc
.cachefunc
import cached_method
16 from sage
.misc
.lazy_import
import lazy_import
17 from sage
.misc
.prandom
import choice
18 from sage
.misc
.table
import table
19 from sage
.modules
.free_module
import FreeModule
, VectorSpace
20 from sage
.rings
.all
import (ZZ
, QQ
, AA
, QQbar
, RR
, RLF
, CLF
,
23 from mjo
.eja
.eja_element
import FiniteDimensionalEuclideanJordanAlgebraElement
24 lazy_import('mjo.eja.eja_subalgebra',
25 'FiniteDimensionalEuclideanJordanSubalgebra')
26 from mjo
.eja
.eja_utils
import _mat2vec
28 class FiniteDimensionalEuclideanJordanAlgebra(CombinatorialFreeModule
):
30 def _coerce_map_from_base_ring(self
):
32 Disable the map from the base ring into the algebra.
34 Performing a nonsense conversion like this automatically
35 is counterpedagogical. The fallback is to try the usual
36 element constructor, which should also fail.
40 sage: from mjo.eja.eja_algebra import random_eja
44 sage: set_random_seed()
45 sage: J = random_eja()
47 Traceback (most recent call last):
49 ValueError: not a naturally-represented algebra element
64 sage: from mjo.eja.eja_algebra import (
65 ....: FiniteDimensionalEuclideanJordanAlgebra,
71 By definition, Jordan multiplication commutes::
73 sage: set_random_seed()
74 sage: J = random_eja()
75 sage: x,y = J.random_elements(2)
81 The ``field`` we're given must be real with ``check=True``::
83 sage: JordanSpinEJA(2,QQbar)
84 Traceback (most recent call last):
86 ValueError: field is not real
88 The multiplication table must be square with ``check=True``::
90 sage: FiniteDimensionalEuclideanJordanAlgebra(QQ,((),()))
91 Traceback (most recent call last):
93 ValueError: multiplication table is not square
97 if not field
.is_subring(RR
):
98 # Note: this does return true for the real algebraic
99 # field, and any quadratic field where we've specified
101 raise ValueError('field is not real')
103 self
._natural
_basis
= natural_basis
106 category
= MagmaticAlgebras(field
).FiniteDimensional()
107 category
= category
.WithBasis().Unital()
109 # The multiplication table had better be square
112 if not all( len(l
) == n
for l
in mult_table
):
113 raise ValueError("multiplication table is not square")
115 fda
= super(FiniteDimensionalEuclideanJordanAlgebra
, self
)
120 self
.print_options(bracket
='')
122 # The multiplication table we're given is necessarily in terms
123 # of vectors, because we don't have an algebra yet for
124 # anything to be an element of. However, it's faster in the
125 # long run to have the multiplication table be in terms of
126 # algebra elements. We do this after calling the superclass
127 # constructor so that from_vector() knows what to do.
128 self
._multiplication
_table
= [
129 list(map(lambda x
: self
.from_vector(x
), ls
))
134 if not self
._is
_commutative
():
135 raise ValueError("algebra is not commutative")
136 if not self
._is
_jordanian
():
137 raise ValueError("Jordan identity does not hold")
138 if not self
._inner
_product
_is
_associative
():
139 raise ValueError("inner product is not associative")
141 def _element_constructor_(self
, elt
):
143 Construct an element of this algebra from its natural
146 This gets called only after the parent element _call_ method
147 fails to find a coercion for the argument.
151 sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
153 ....: RealSymmetricEJA)
157 The identity in `S^n` is converted to the identity in the EJA::
159 sage: J = RealSymmetricEJA(3)
160 sage: I = matrix.identity(QQ,3)
161 sage: J(I) == J.one()
164 This skew-symmetric matrix can't be represented in the EJA::
166 sage: J = RealSymmetricEJA(3)
167 sage: A = matrix(QQ,3, lambda i,j: i-j)
169 Traceback (most recent call last):
171 ArithmeticError: vector is not in free module
175 Ensure that we can convert any element of the two non-matrix
176 simple algebras (whose natural representations are their usual
177 vector representations) back and forth faithfully::
179 sage: set_random_seed()
180 sage: J = HadamardEJA.random_instance()
181 sage: x = J.random_element()
182 sage: J(x.to_vector().column()) == x
184 sage: J = JordanSpinEJA.random_instance()
185 sage: x = J.random_element()
186 sage: J(x.to_vector().column()) == x
190 msg
= "not a naturally-represented algebra element"
192 # The superclass implementation of random_element()
193 # needs to be able to coerce "0" into the algebra.
195 elif elt
in self
.base_ring():
196 # Ensure that no base ring -> algebra coercion is performed
197 # by this method. There's some stupidity in sage that would
198 # otherwise propagate to this method; for example, sage thinks
199 # that the integer 3 belongs to the space of 2-by-2 matrices.
200 raise ValueError(msg
)
202 natural_basis
= self
.natural_basis()
203 basis_space
= natural_basis
[0].matrix_space()
204 if elt
not in basis_space
:
205 raise ValueError(msg
)
207 # Thanks for nothing! Matrix spaces aren't vector spaces in
208 # Sage, so we have to figure out its natural-basis coordinates
209 # ourselves. We use the basis space's ring instead of the
210 # element's ring because the basis space might be an algebraic
211 # closure whereas the base ring of the 3-by-3 identity matrix
212 # could be QQ instead of QQbar.
213 V
= VectorSpace(basis_space
.base_ring(), elt
.nrows()*elt
.ncols())
214 W
= V
.span_of_basis( _mat2vec(s
) for s
in natural_basis
)
215 coords
= W
.coordinate_vector(_mat2vec(elt
))
216 return self
.from_vector(coords
)
219 def _max_test_case_size():
221 Return an integer "size" that is an upper bound on the size of
222 this algebra when it is used in a random test
223 case. Unfortunately, the term "size" is quite vague -- when
224 dealing with `R^n` under either the Hadamard or Jordan spin
225 product, the "size" refers to the dimension `n`. When dealing
226 with a matrix algebra (real symmetric or complex/quaternion
227 Hermitian), it refers to the size of the matrix, which is
228 far less than the dimension of the underlying vector space.
230 We default to five in this class, which is safe in `R^n`. The
231 matrix algebra subclasses (or any class where the "size" is
232 interpreted to be far less than the dimension) should override
233 with a smaller number.
239 Return a string representation of ``self``.
243 sage: from mjo.eja.eja_algebra import JordanSpinEJA
247 Ensure that it says what we think it says::
249 sage: JordanSpinEJA(2, field=AA)
250 Euclidean Jordan algebra of dimension 2 over Algebraic Real Field
251 sage: JordanSpinEJA(3, field=RDF)
252 Euclidean Jordan algebra of dimension 3 over Real Double Field
255 fmt
= "Euclidean Jordan algebra of dimension {} over {}"
256 return fmt
.format(self
.dimension(), self
.base_ring())
258 def product_on_basis(self
, i
, j
):
259 return self
._multiplication
_table
[i
][j
]
261 def _is_commutative(self
):
263 Whether or not this algebra's multiplication table is commutative.
265 This method should of course always return ``True``, unless
266 this algebra was constructed with ``check=False`` and passed
267 an invalid multiplication table.
269 return all( self
.product_on_basis(i
,j
) == self
.product_on_basis(i
,j
)
270 for i
in range(self
.dimension())
271 for j
in range(self
.dimension()) )
273 def _is_jordanian(self
):
275 Whether or not this algebra's multiplication table respects the
276 Jordan identity `(x^{2})(xy) = x(x^{2}y)`.
278 We only check one arrangement of `x` and `y`, so for a
279 ``True`` result to be truly true, you should also check
280 :meth:`_is_commutative`. This method should of course always
281 return ``True``, unless this algebra was constructed with
282 ``check=False`` and passed an invalid multiplication table.
284 return all( (self
.monomial(i
)**2)*(self
.monomial(i
)*self
.monomial(j
))
286 (self
.monomial(i
))*((self
.monomial(i
)**2)*self
.monomial(j
))
287 for i
in range(self
.dimension())
288 for j
in range(self
.dimension()) )
290 def _inner_product_is_associative(self
):
292 Return whether or not this algebra's inner product `B` is
293 associative; that is, whether or not `B(xy,z) = B(x,yz)`.
295 This method should of course always return ``True``, unless
296 this algebra was constructed with ``check=False`` and passed
297 an invalid multiplication table.
300 # Used to check whether or not something is zero in an inexact
301 # ring. This number is sufficient to allow the construction of
302 # QuaternionHermitianEJA(2, RDF) with check=True.
305 for i
in range(self
.dimension()):
306 for j
in range(self
.dimension()):
307 for k
in range(self
.dimension()):
311 diff
= (x
*y
).inner_product(z
) - x
.inner_product(y
*z
)
313 if self
.base_ring().is_exact():
317 if diff
.abs() > epsilon
:
323 def characteristic_polynomial_of(self
):
325 Return the algebra's "characteristic polynomial of" function,
326 which is itself a multivariate polynomial that, when evaluated
327 at the coordinates of some algebra element, returns that
328 element's characteristic polynomial.
330 The resulting polynomial has `n+1` variables, where `n` is the
331 dimension of this algebra. The first `n` variables correspond to
332 the coordinates of an algebra element: when evaluated at the
333 coordinates of an algebra element with respect to a certain
334 basis, the result is a univariate polynomial (in the one
335 remaining variable ``t``), namely the characteristic polynomial
340 sage: from mjo.eja.eja_algebra import JordanSpinEJA, TrivialEJA
344 The characteristic polynomial in the spin algebra is given in
345 Alizadeh, Example 11.11::
347 sage: J = JordanSpinEJA(3)
348 sage: p = J.characteristic_polynomial_of(); p
349 X1^2 - X2^2 - X3^2 + (-2*t)*X1 + t^2
350 sage: xvec = J.one().to_vector()
354 By definition, the characteristic polynomial is a monic
355 degree-zero polynomial in a rank-zero algebra. Note that
356 Cayley-Hamilton is indeed satisfied since the polynomial
357 ``1`` evaluates to the identity element of the algebra on
360 sage: J = TrivialEJA()
361 sage: J.characteristic_polynomial_of()
368 # The list of coefficient polynomials a_0, a_1, a_2, ..., a_(r-1).
369 a
= self
._charpoly
_coefficients
()
371 # We go to a bit of trouble here to reorder the
372 # indeterminates, so that it's easier to evaluate the
373 # characteristic polynomial at x's coordinates and get back
374 # something in terms of t, which is what we want.
375 S
= PolynomialRing(self
.base_ring(),'t')
379 S
= PolynomialRing(S
, R
.variable_names())
382 return (t
**r
+ sum( a
[k
]*(t
**k
) for k
in range(r
) ))
385 def inner_product(self
, x
, y
):
387 The inner product associated with this Euclidean Jordan algebra.
389 Defaults to the trace inner product, but can be overridden by
390 subclasses if they are sure that the necessary properties are
395 sage: from mjo.eja.eja_algebra import random_eja
399 Our inner product is "associative," which means the following for
400 a symmetric bilinear form::
402 sage: set_random_seed()
403 sage: J = random_eja()
404 sage: x,y,z = J.random_elements(3)
405 sage: (x*y).inner_product(z) == y.inner_product(x*z)
409 X
= x
.natural_representation()
410 Y
= y
.natural_representation()
411 return self
.natural_inner_product(X
,Y
)
414 def is_trivial(self
):
416 Return whether or not this algebra is trivial.
418 A trivial algebra contains only the zero element.
422 sage: from mjo.eja.eja_algebra import (ComplexHermitianEJA,
427 sage: J = ComplexHermitianEJA(3)
433 sage: J = TrivialEJA()
438 return self
.dimension() == 0
441 def multiplication_table(self
):
443 Return a visual representation of this algebra's multiplication
444 table (on basis elements).
448 sage: from mjo.eja.eja_algebra import JordanSpinEJA
452 sage: J = JordanSpinEJA(4)
453 sage: J.multiplication_table()
454 +----++----+----+----+----+
455 | * || e0 | e1 | e2 | e3 |
456 +====++====+====+====+====+
457 | e0 || e0 | e1 | e2 | e3 |
458 +----++----+----+----+----+
459 | e1 || e1 | e0 | 0 | 0 |
460 +----++----+----+----+----+
461 | e2 || e2 | 0 | e0 | 0 |
462 +----++----+----+----+----+
463 | e3 || e3 | 0 | 0 | e0 |
464 +----++----+----+----+----+
467 M
= list(self
._multiplication
_table
) # copy
468 for i
in range(len(M
)):
469 # M had better be "square"
470 M
[i
] = [self
.monomial(i
)] + M
[i
]
471 M
= [["*"] + list(self
.gens())] + M
472 return table(M
, header_row
=True, header_column
=True, frame
=True)
475 def natural_basis(self
):
477 Return a more-natural representation of this algebra's basis.
479 Every finite-dimensional Euclidean Jordan Algebra is a direct
480 sum of five simple algebras, four of which comprise Hermitian
481 matrices. This method returns the original "natural" basis
482 for our underlying vector space. (Typically, the natural basis
483 is used to construct the multiplication table in the first place.)
485 Note that this will always return a matrix. The standard basis
486 in `R^n` will be returned as `n`-by-`1` column matrices.
490 sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
491 ....: RealSymmetricEJA)
495 sage: J = RealSymmetricEJA(2)
497 Finite family {0: e0, 1: e1, 2: e2}
498 sage: J.natural_basis()
500 [1 0] [ 0 0.7071067811865475?] [0 0]
501 [0 0], [0.7071067811865475? 0], [0 1]
506 sage: J = JordanSpinEJA(2)
508 Finite family {0: e0, 1: e1}
509 sage: J.natural_basis()
516 if self
._natural
_basis
is None:
517 M
= self
.natural_basis_space()
518 return tuple( M(b
.to_vector()) for b
in self
.basis() )
520 return self
._natural
_basis
523 def natural_basis_space(self
):
525 Return the matrix space in which this algebra's natural basis
528 Generally this will be an `n`-by-`1` column-vector space,
529 except when the algebra is trivial. There it's `n`-by-`n`
530 (where `n` is zero), to ensure that two elements of the
531 natural basis space (empty matrices) can be multiplied.
533 if self
.is_trivial():
534 return MatrixSpace(self
.base_ring(), 0)
535 elif self
._natural
_basis
is None or len(self
._natural
_basis
) == 0:
536 return MatrixSpace(self
.base_ring(), self
.dimension(), 1)
538 return self
._natural
_basis
[0].matrix_space()
542 def natural_inner_product(X
,Y
):
544 Compute the inner product of two naturally-represented elements.
546 For example in the real symmetric matrix EJA, this will compute
547 the trace inner-product of two n-by-n symmetric matrices. The
548 default should work for the real cartesian product EJA, the
549 Jordan spin EJA, and the real symmetric matrices. The others
550 will have to be overridden.
552 return (X
.conjugate_transpose()*Y
).trace()
558 Return the unit element of this algebra.
562 sage: from mjo.eja.eja_algebra import (HadamardEJA,
567 sage: J = HadamardEJA(5)
569 e0 + e1 + e2 + e3 + e4
573 The identity element acts like the identity::
575 sage: set_random_seed()
576 sage: J = random_eja()
577 sage: x = J.random_element()
578 sage: J.one()*x == x and x*J.one() == x
581 The matrix of the unit element's operator is the identity::
583 sage: set_random_seed()
584 sage: J = random_eja()
585 sage: actual = J.one().operator().matrix()
586 sage: expected = matrix.identity(J.base_ring(), J.dimension())
587 sage: actual == expected
591 # We can brute-force compute the matrices of the operators
592 # that correspond to the basis elements of this algebra.
593 # If some linear combination of those basis elements is the
594 # algebra identity, then the same linear combination of
595 # their matrices has to be the identity matrix.
597 # Of course, matrices aren't vectors in sage, so we have to
598 # appeal to the "long vectors" isometry.
599 oper_vecs
= [ _mat2vec(g
.operator().matrix()) for g
in self
.gens() ]
601 # Now we use basis linear algebra to find the coefficients,
602 # of the matrices-as-vectors-linear-combination, which should
603 # work for the original algebra basis too.
604 A
= matrix
.column(self
.base_ring(), oper_vecs
)
606 # We used the isometry on the left-hand side already, but we
607 # still need to do it for the right-hand side. Recall that we
608 # wanted something that summed to the identity matrix.
609 b
= _mat2vec( matrix
.identity(self
.base_ring(), self
.dimension()) )
611 # Now if there's an identity element in the algebra, this should work.
612 coeffs
= A
.solve_right(b
)
613 return self
.linear_combination(zip(self
.gens(), coeffs
))
616 def peirce_decomposition(self
, c
):
618 The Peirce decomposition of this algebra relative to the
621 In the future, this can be extended to a complete system of
622 orthogonal idempotents.
626 - ``c`` -- an idempotent of this algebra.
630 A triple (J0, J5, J1) containing two subalgebras and one subspace
633 - ``J0`` -- the algebra on the eigenspace of ``c.operator()``
634 corresponding to the eigenvalue zero.
636 - ``J5`` -- the eigenspace (NOT a subalgebra) of ``c.operator()``
637 corresponding to the eigenvalue one-half.
639 - ``J1`` -- the algebra on the eigenspace of ``c.operator()``
640 corresponding to the eigenvalue one.
642 These are the only possible eigenspaces for that operator, and this
643 algebra is a direct sum of them. The spaces ``J0`` and ``J1`` are
644 orthogonal, and are subalgebras of this algebra with the appropriate
649 sage: from mjo.eja.eja_algebra import random_eja, RealSymmetricEJA
653 The canonical example comes from the symmetric matrices, which
654 decompose into diagonal and off-diagonal parts::
656 sage: J = RealSymmetricEJA(3)
657 sage: C = matrix(QQ, [ [1,0,0],
661 sage: J0,J5,J1 = J.peirce_decomposition(c)
663 Euclidean Jordan algebra of dimension 1...
665 Vector space of degree 6 and dimension 2...
667 Euclidean Jordan algebra of dimension 3...
668 sage: J0.one().natural_representation()
672 sage: orig_df = AA.options.display_format
673 sage: AA.options.display_format = 'radical'
674 sage: J.from_vector(J5.basis()[0]).natural_representation()
678 sage: J.from_vector(J5.basis()[1]).natural_representation()
682 sage: AA.options.display_format = orig_df
683 sage: J1.one().natural_representation()
690 Every algebra decomposes trivially with respect to its identity
693 sage: set_random_seed()
694 sage: J = random_eja()
695 sage: J0,J5,J1 = J.peirce_decomposition(J.one())
696 sage: J0.dimension() == 0 and J5.dimension() == 0
698 sage: J1.superalgebra() == J and J1.dimension() == J.dimension()
701 The decomposition is into eigenspaces, and its components are
702 therefore necessarily orthogonal. Moreover, the identity
703 elements in the two subalgebras are the projections onto their
704 respective subspaces of the superalgebra's identity element::
706 sage: set_random_seed()
707 sage: J = random_eja()
708 sage: x = J.random_element()
709 sage: if not J.is_trivial():
710 ....: while x.is_nilpotent():
711 ....: x = J.random_element()
712 sage: c = x.subalgebra_idempotent()
713 sage: J0,J5,J1 = J.peirce_decomposition(c)
715 sage: for (w,y,z) in zip(J0.basis(), J5.basis(), J1.basis()):
716 ....: w = w.superalgebra_element()
717 ....: y = J.from_vector(y)
718 ....: z = z.superalgebra_element()
719 ....: ipsum += w.inner_product(y).abs()
720 ....: ipsum += w.inner_product(z).abs()
721 ....: ipsum += y.inner_product(z).abs()
724 sage: J1(c) == J1.one()
726 sage: J0(J.one() - c) == J0.one()
730 if not c
.is_idempotent():
731 raise ValueError("element is not idempotent: %s" % c
)
733 # Default these to what they should be if they turn out to be
734 # trivial, because eigenspaces_left() won't return eigenvalues
735 # corresponding to trivial spaces (e.g. it returns only the
736 # eigenspace corresponding to lambda=1 if you take the
737 # decomposition relative to the identity element).
738 trivial
= FiniteDimensionalEuclideanJordanSubalgebra(self
, ())
739 J0
= trivial
# eigenvalue zero
740 J5
= VectorSpace(self
.base_ring(), 0) # eigenvalue one-half
741 J1
= trivial
# eigenvalue one
743 for (eigval
, eigspace
) in c
.operator().matrix().right_eigenspaces():
744 if eigval
== ~
(self
.base_ring()(2)):
747 gens
= tuple( self
.from_vector(b
) for b
in eigspace
.basis() )
748 subalg
= FiniteDimensionalEuclideanJordanSubalgebra(self
,
756 raise ValueError("unexpected eigenvalue: %s" % eigval
)
761 def random_element(self
, thorough
=False):
763 Return a random element of this algebra.
765 Our algebra superclass method only returns a linear
766 combination of at most two basis elements. We instead
767 want the vector space "random element" method that
768 returns a more diverse selection.
772 - ``thorough`` -- (boolean; default False) whether or not we
773 should generate irrational coefficients for the random
774 element when our base ring is irrational; this slows the
775 algebra operations to a crawl, but any truly random method
779 # For a general base ring... maybe we can trust this to do the
780 # right thing? Unlikely, but.
781 V
= self
.vector_space()
782 v
= V
.random_element()
784 if self
.base_ring() is AA
:
785 # The "random element" method of the algebraic reals is
786 # stupid at the moment, and only returns integers between
787 # -2 and 2, inclusive:
789 # https://trac.sagemath.org/ticket/30875
791 # Instead, we implement our own "random vector" method,
792 # and then coerce that into the algebra. We use the vector
793 # space degree here instead of the dimension because a
794 # subalgebra could (for example) be spanned by only two
795 # vectors, each with five coordinates. We need to
796 # generate all five coordinates.
798 v
*= QQbar
.random_element().real()
800 v
*= QQ
.random_element()
802 return self
.from_vector(V
.coordinate_vector(v
))
804 def random_elements(self
, count
, thorough
=False):
806 Return ``count`` random elements as a tuple.
810 - ``thorough`` -- (boolean; default False) whether or not we
811 should generate irrational coefficients for the random
812 elements when our base ring is irrational; this slows the
813 algebra operations to a crawl, but any truly random method
818 sage: from mjo.eja.eja_algebra import JordanSpinEJA
822 sage: J = JordanSpinEJA(3)
823 sage: x,y,z = J.random_elements(3)
824 sage: all( [ x in J, y in J, z in J ])
826 sage: len( J.random_elements(10) ) == 10
830 return tuple( self
.random_element(thorough
)
831 for idx
in range(count
) )
834 def random_instance(cls
, field
=AA
, **kwargs
):
836 Return a random instance of this type of algebra.
838 Beware, this will crash for "most instances" because the
839 constructor below looks wrong.
841 if cls
is TrivialEJA
:
842 # The TrivialEJA class doesn't take an "n" argument because
846 n
= ZZ
.random_element(cls
._max
_test
_case
_size
() + 1)
847 return cls(n
, field
, **kwargs
)
850 def _charpoly_coefficients(self
):
852 The `r` polynomial coefficients of the "characteristic polynomial
856 var_names
= [ "X" + str(z
) for z
in range(1,n
+1) ]
857 R
= PolynomialRing(self
.base_ring(), var_names
)
859 F
= R
.fraction_field()
862 # From a result in my book, these are the entries of the
863 # basis representation of L_x.
864 return sum( vars[k
]*self
.monomial(k
).operator().matrix()[i
,j
]
867 L_x
= matrix(F
, n
, n
, L_x_i_j
)
870 if self
.rank
.is_in_cache():
872 # There's no need to pad the system with redundant
873 # columns if we *know* they'll be redundant.
876 # Compute an extra power in case the rank is equal to
877 # the dimension (otherwise, we would stop at x^(r-1)).
878 x_powers
= [ (L_x
**k
)*self
.one().to_vector()
879 for k
in range(n
+1) ]
880 A
= matrix
.column(F
, x_powers
[:n
])
881 AE
= A
.extended_echelon_form()
888 # The theory says that only the first "r" coefficients are
889 # nonzero, and they actually live in the original polynomial
890 # ring and not the fraction field. We negate them because
891 # in the actual characteristic polynomial, they get moved
892 # to the other side where x^r lives.
893 return -A_rref
.solve_right(E
*b
).change_ring(R
)[:r
]
898 Return the rank of this EJA.
900 This is a cached method because we know the rank a priori for
901 all of the algebras we can construct. Thus we can avoid the
902 expensive ``_charpoly_coefficients()`` call unless we truly
903 need to compute the whole characteristic polynomial.
907 sage: from mjo.eja.eja_algebra import (HadamardEJA,
909 ....: RealSymmetricEJA,
910 ....: ComplexHermitianEJA,
911 ....: QuaternionHermitianEJA,
916 The rank of the Jordan spin algebra is always two::
918 sage: JordanSpinEJA(2).rank()
920 sage: JordanSpinEJA(3).rank()
922 sage: JordanSpinEJA(4).rank()
925 The rank of the `n`-by-`n` Hermitian real, complex, or
926 quaternion matrices is `n`::
928 sage: RealSymmetricEJA(4).rank()
930 sage: ComplexHermitianEJA(3).rank()
932 sage: QuaternionHermitianEJA(2).rank()
937 Ensure that every EJA that we know how to construct has a
938 positive integer rank, unless the algebra is trivial in
939 which case its rank will be zero::
941 sage: set_random_seed()
942 sage: J = random_eja()
946 sage: r > 0 or (r == 0 and J.is_trivial())
949 Ensure that computing the rank actually works, since the ranks
950 of all simple algebras are known and will be cached by default::
952 sage: J = HadamardEJA(4)
953 sage: J.rank.clear_cache()
959 sage: J = JordanSpinEJA(4)
960 sage: J.rank.clear_cache()
966 sage: J = RealSymmetricEJA(3)
967 sage: J.rank.clear_cache()
973 sage: J = ComplexHermitianEJA(2)
974 sage: J.rank.clear_cache()
980 sage: J = QuaternionHermitianEJA(2)
981 sage: J.rank.clear_cache()
985 return len(self
._charpoly
_coefficients
())
988 def vector_space(self
):
990 Return the vector space that underlies this algebra.
994 sage: from mjo.eja.eja_algebra import RealSymmetricEJA
998 sage: J = RealSymmetricEJA(2)
999 sage: J.vector_space()
1000 Vector space of dimension 3 over...
1003 return self
.zero().to_vector().parent().ambient_vector_space()
1006 Element
= FiniteDimensionalEuclideanJordanAlgebraElement
1009 class HadamardEJA(FiniteDimensionalEuclideanJordanAlgebra
):
1011 Return the Euclidean Jordan Algebra corresponding to the set
1012 `R^n` under the Hadamard product.
1014 Note: this is nothing more than the Cartesian product of ``n``
1015 copies of the spin algebra. Once Cartesian product algebras
1016 are implemented, this can go.
1020 sage: from mjo.eja.eja_algebra import HadamardEJA
1024 This multiplication table can be verified by hand::
1026 sage: J = HadamardEJA(3)
1027 sage: e0,e1,e2 = J.gens()
1043 We can change the generator prefix::
1045 sage: HadamardEJA(3, prefix='r').gens()
1049 def __init__(self
, n
, field
=AA
, **kwargs
):
1050 V
= VectorSpace(field
, n
)
1051 mult_table
= [ [ V
.gen(i
)*(i
== j
) for j
in range(n
) ]
1054 super(HadamardEJA
, self
).__init
__(field
,
1058 self
.rank
.set_cache(n
)
1060 def inner_product(self
, x
, y
):
1062 Faster to reimplement than to use natural representations.
1066 sage: from mjo.eja.eja_algebra import HadamardEJA
1070 Ensure that this is the usual inner product for the algebras
1073 sage: set_random_seed()
1074 sage: J = HadamardEJA.random_instance()
1075 sage: x,y = J.random_elements(2)
1076 sage: X = x.natural_representation()
1077 sage: Y = y.natural_representation()
1078 sage: x.inner_product(y) == J.natural_inner_product(X,Y)
1082 return x
.to_vector().inner_product(y
.to_vector())
1085 def random_eja(field
=AA
):
1087 Return a "random" finite-dimensional Euclidean Jordan Algebra.
1091 sage: from mjo.eja.eja_algebra import random_eja
1096 Euclidean Jordan algebra of dimension...
1099 classname
= choice([TrivialEJA
,
1103 ComplexHermitianEJA
,
1104 QuaternionHermitianEJA
])
1105 return classname
.random_instance(field
=field
)
1110 class MatrixEuclideanJordanAlgebra(FiniteDimensionalEuclideanJordanAlgebra
):
1112 def _max_test_case_size():
1113 # Play it safe, since this will be squared and the underlying
1114 # field can have dimension 4 (quaternions) too.
1117 def __init__(self
, field
, basis
, normalize_basis
=True, **kwargs
):
1119 Compared to the superclass constructor, we take a basis instead of
1120 a multiplication table because the latter can be computed in terms
1121 of the former when the product is known (like it is here).
1123 # Used in this class's fast _charpoly_coefficients() override.
1124 self
._basis
_normalizers
= None
1126 # We're going to loop through this a few times, so now's a good
1127 # time to ensure that it isn't a generator expression.
1128 basis
= tuple(basis
)
1130 if len(basis
) > 1 and normalize_basis
:
1131 # We'll need sqrt(2) to normalize the basis, and this
1132 # winds up in the multiplication table, so the whole
1133 # algebra needs to be over the field extension.
1134 R
= PolynomialRing(field
, 'z')
1137 if p
.is_irreducible():
1138 field
= field
.extension(p
, 'sqrt2', embedding
=RLF(2).sqrt())
1139 basis
= tuple( s
.change_ring(field
) for s
in basis
)
1140 self
._basis
_normalizers
= tuple(
1141 ~
(self
.natural_inner_product(s
,s
).sqrt()) for s
in basis
)
1142 basis
= tuple(s
*c
for (s
,c
) in zip(basis
,self
._basis
_normalizers
))
1144 Qs
= self
.multiplication_table_from_matrix_basis(basis
)
1146 super(MatrixEuclideanJordanAlgebra
, self
).__init
__(field
,
1148 natural_basis
=basis
,
1153 def _charpoly_coefficients(self
):
1155 Override the parent method with something that tries to compute
1156 over a faster (non-extension) field.
1158 if self
._basis
_normalizers
is None:
1159 # We didn't normalize, so assume that the basis we started
1160 # with had entries in a nice field.
1161 return super(MatrixEuclideanJordanAlgebra
, self
)._charpoly
_coefficients
()
1163 basis
= ( (b
/n
) for (b
,n
) in zip(self
.natural_basis(),
1164 self
._basis
_normalizers
) )
1166 # Do this over the rationals and convert back at the end.
1167 # Only works because we know the entries of the basis are
1168 # integers. The argument ``check=False`` is required
1169 # because the trace inner-product method for this
1170 # class is a stub and can't actually be checked.
1171 J
= MatrixEuclideanJordanAlgebra(QQ
,
1173 normalize_basis
=False,
1175 a
= J
._charpoly
_coefficients
()
1177 # Unfortunately, changing the basis does change the
1178 # coefficients of the characteristic polynomial, but since
1179 # these are really the coefficients of the "characteristic
1180 # polynomial of" function, everything is still nice and
1181 # unevaluated. It's therefore "obvious" how scaling the
1182 # basis affects the coordinate variables X1, X2, et
1183 # cetera. Scaling the first basis vector up by "n" adds a
1184 # factor of 1/n into every "X1" term, for example. So here
1185 # we simply undo the basis_normalizer scaling that we
1186 # performed earlier.
1188 # The a[0] access here is safe because trivial algebras
1189 # won't have any basis normalizers and therefore won't
1190 # make it to this "else" branch.
1191 XS
= a
[0].parent().gens()
1192 subs_dict
= { XS
[i
]: self
._basis
_normalizers
[i
]*XS
[i
]
1193 for i
in range(len(XS
)) }
1194 return tuple( a_i
.subs(subs_dict
) for a_i
in a
)
1198 def multiplication_table_from_matrix_basis(basis
):
1200 At least three of the five simple Euclidean Jordan algebras have the
1201 symmetric multiplication (A,B) |-> (AB + BA)/2, where the
1202 multiplication on the right is matrix multiplication. Given a basis
1203 for the underlying matrix space, this function returns a
1204 multiplication table (obtained by looping through the basis
1205 elements) for an algebra of those matrices.
1207 # In S^2, for example, we nominally have four coordinates even
1208 # though the space is of dimension three only. The vector space V
1209 # is supposed to hold the entire long vector, and the subspace W
1210 # of V will be spanned by the vectors that arise from symmetric
1211 # matrices. Thus for S^2, dim(V) == 4 and dim(W) == 3.
1215 field
= basis
[0].base_ring()
1216 dimension
= basis
[0].nrows()
1218 V
= VectorSpace(field
, dimension
**2)
1219 W
= V
.span_of_basis( _mat2vec(s
) for s
in basis
)
1221 mult_table
= [[W
.zero() for j
in range(n
)] for i
in range(n
)]
1224 mat_entry
= (basis
[i
]*basis
[j
] + basis
[j
]*basis
[i
])/2
1225 mult_table
[i
][j
] = W
.coordinate_vector(_mat2vec(mat_entry
))
1233 Embed the matrix ``M`` into a space of real matrices.
1235 The matrix ``M`` can have entries in any field at the moment:
1236 the real numbers, complex numbers, or quaternions. And although
1237 they are not a field, we can probably support octonions at some
1238 point, too. This function returns a real matrix that "acts like"
1239 the original with respect to matrix multiplication; i.e.
1241 real_embed(M*N) = real_embed(M)*real_embed(N)
1244 raise NotImplementedError
1248 def real_unembed(M
):
1250 The inverse of :meth:`real_embed`.
1252 raise NotImplementedError
1256 def natural_inner_product(cls
,X
,Y
):
1257 Xu
= cls
.real_unembed(X
)
1258 Yu
= cls
.real_unembed(Y
)
1259 tr
= (Xu
*Yu
).trace()
1262 # Works in QQ, AA, RDF, et cetera.
1264 except AttributeError:
1265 # A quaternion doesn't have a real() method, but does
1266 # have coefficient_tuple() method that returns the
1267 # coefficients of 1, i, j, and k -- in that order.
1268 return tr
.coefficient_tuple()[0]
1271 class RealMatrixEuclideanJordanAlgebra(MatrixEuclideanJordanAlgebra
):
1275 The identity function, for embedding real matrices into real
1281 def real_unembed(M
):
1283 The identity function, for unembedding real matrices from real
1289 class RealSymmetricEJA(RealMatrixEuclideanJordanAlgebra
):
1291 The rank-n simple EJA consisting of real symmetric n-by-n
1292 matrices, the usual symmetric Jordan product, and the trace inner
1293 product. It has dimension `(n^2 + n)/2` over the reals.
1297 sage: from mjo.eja.eja_algebra import RealSymmetricEJA
1301 sage: J = RealSymmetricEJA(2)
1302 sage: e0, e1, e2 = J.gens()
1310 In theory, our "field" can be any subfield of the reals::
1312 sage: RealSymmetricEJA(2, RDF)
1313 Euclidean Jordan algebra of dimension 3 over Real Double Field
1314 sage: RealSymmetricEJA(2, RR)
1315 Euclidean Jordan algebra of dimension 3 over Real Field with
1316 53 bits of precision
1320 The dimension of this algebra is `(n^2 + n) / 2`::
1322 sage: set_random_seed()
1323 sage: n_max = RealSymmetricEJA._max_test_case_size()
1324 sage: n = ZZ.random_element(1, n_max)
1325 sage: J = RealSymmetricEJA(n)
1326 sage: J.dimension() == (n^2 + n)/2
1329 The Jordan multiplication is what we think it is::
1331 sage: set_random_seed()
1332 sage: J = RealSymmetricEJA.random_instance()
1333 sage: x,y = J.random_elements(2)
1334 sage: actual = (x*y).natural_representation()
1335 sage: X = x.natural_representation()
1336 sage: Y = y.natural_representation()
1337 sage: expected = (X*Y + Y*X)/2
1338 sage: actual == expected
1340 sage: J(expected) == x*y
1343 We can change the generator prefix::
1345 sage: RealSymmetricEJA(3, prefix='q').gens()
1346 (q0, q1, q2, q3, q4, q5)
1348 Our natural basis is normalized with respect to the natural inner
1349 product unless we specify otherwise::
1351 sage: set_random_seed()
1352 sage: J = RealSymmetricEJA.random_instance()
1353 sage: all( b.norm() == 1 for b in J.gens() )
1356 Since our natural basis is normalized with respect to the natural
1357 inner product, and since we know that this algebra is an EJA, any
1358 left-multiplication operator's matrix will be symmetric because
1359 natural->EJA basis representation is an isometry and within the EJA
1360 the operator is self-adjoint by the Jordan axiom::
1362 sage: set_random_seed()
1363 sage: x = RealSymmetricEJA.random_instance().random_element()
1364 sage: x.operator().matrix().is_symmetric()
1367 We can construct the (trivial) algebra of rank zero::
1369 sage: RealSymmetricEJA(0)
1370 Euclidean Jordan algebra of dimension 0 over Algebraic Real Field
1374 def _denormalized_basis(cls
, n
, field
):
1376 Return a basis for the space of real symmetric n-by-n matrices.
1380 sage: from mjo.eja.eja_algebra import RealSymmetricEJA
1384 sage: set_random_seed()
1385 sage: n = ZZ.random_element(1,5)
1386 sage: B = RealSymmetricEJA._denormalized_basis(n,QQ)
1387 sage: all( M.is_symmetric() for M in B)
1391 # The basis of symmetric matrices, as matrices, in their R^(n-by-n)
1395 for j
in range(i
+1):
1396 Eij
= matrix(field
, n
, lambda k
,l
: k
==i
and l
==j
)
1400 Sij
= Eij
+ Eij
.transpose()
1406 def _max_test_case_size():
1407 return 4 # Dimension 10
1410 def __init__(self
, n
, field
=AA
, **kwargs
):
1411 basis
= self
._denormalized
_basis
(n
, field
)
1412 super(RealSymmetricEJA
, self
).__init
__(field
,
1416 self
.rank
.set_cache(n
)
1419 class ComplexMatrixEuclideanJordanAlgebra(MatrixEuclideanJordanAlgebra
):
1423 Embed the n-by-n complex matrix ``M`` into the space of real
1424 matrices of size 2n-by-2n via the map the sends each entry `z = a +
1425 bi` to the block matrix ``[[a,b],[-b,a]]``.
1429 sage: from mjo.eja.eja_algebra import \
1430 ....: ComplexMatrixEuclideanJordanAlgebra
1434 sage: F = QuadraticField(-1, 'I')
1435 sage: x1 = F(4 - 2*i)
1436 sage: x2 = F(1 + 2*i)
1439 sage: M = matrix(F,2,[[x1,x2],[x3,x4]])
1440 sage: ComplexMatrixEuclideanJordanAlgebra.real_embed(M)
1449 Embedding is a homomorphism (isomorphism, in fact)::
1451 sage: set_random_seed()
1452 sage: n_max = ComplexMatrixEuclideanJordanAlgebra._max_test_case_size()
1453 sage: n = ZZ.random_element(n_max)
1454 sage: F = QuadraticField(-1, 'I')
1455 sage: X = random_matrix(F, n)
1456 sage: Y = random_matrix(F, n)
1457 sage: Xe = ComplexMatrixEuclideanJordanAlgebra.real_embed(X)
1458 sage: Ye = ComplexMatrixEuclideanJordanAlgebra.real_embed(Y)
1459 sage: XYe = ComplexMatrixEuclideanJordanAlgebra.real_embed(X*Y)
1466 raise ValueError("the matrix 'M' must be square")
1468 # We don't need any adjoined elements...
1469 field
= M
.base_ring().base_ring()
1473 a
= z
.list()[0] # real part, I guess
1474 b
= z
.list()[1] # imag part, I guess
1475 blocks
.append(matrix(field
, 2, [[a
,b
],[-b
,a
]]))
1477 return matrix
.block(field
, n
, blocks
)
1481 def real_unembed(M
):
1483 The inverse of _embed_complex_matrix().
1487 sage: from mjo.eja.eja_algebra import \
1488 ....: ComplexMatrixEuclideanJordanAlgebra
1492 sage: A = matrix(QQ,[ [ 1, 2, 3, 4],
1493 ....: [-2, 1, -4, 3],
1494 ....: [ 9, 10, 11, 12],
1495 ....: [-10, 9, -12, 11] ])
1496 sage: ComplexMatrixEuclideanJordanAlgebra.real_unembed(A)
1498 [ 10*I + 9 12*I + 11]
1502 Unembedding is the inverse of embedding::
1504 sage: set_random_seed()
1505 sage: F = QuadraticField(-1, 'I')
1506 sage: M = random_matrix(F, 3)
1507 sage: Me = ComplexMatrixEuclideanJordanAlgebra.real_embed(M)
1508 sage: ComplexMatrixEuclideanJordanAlgebra.real_unembed(Me) == M
1514 raise ValueError("the matrix 'M' must be square")
1515 if not n
.mod(2).is_zero():
1516 raise ValueError("the matrix 'M' must be a complex embedding")
1518 # If "M" was normalized, its base ring might have roots
1519 # adjoined and they can stick around after unembedding.
1520 field
= M
.base_ring()
1521 R
= PolynomialRing(field
, 'z')
1524 # Sage doesn't know how to embed AA into QQbar, i.e. how
1525 # to adjoin sqrt(-1) to AA.
1528 F
= field
.extension(z
**2 + 1, 'I', embedding
=CLF(-1).sqrt())
1531 # Go top-left to bottom-right (reading order), converting every
1532 # 2-by-2 block we see to a single complex element.
1534 for k
in range(n
/2):
1535 for j
in range(n
/2):
1536 submat
= M
[2*k
:2*k
+2,2*j
:2*j
+2]
1537 if submat
[0,0] != submat
[1,1]:
1538 raise ValueError('bad on-diagonal submatrix')
1539 if submat
[0,1] != -submat
[1,0]:
1540 raise ValueError('bad off-diagonal submatrix')
1541 z
= submat
[0,0] + submat
[0,1]*i
1544 return matrix(F
, n
/2, elements
)
1548 def natural_inner_product(cls
,X
,Y
):
1550 Compute a natural inner product in this algebra directly from
1555 sage: from mjo.eja.eja_algebra import ComplexHermitianEJA
1559 This gives the same answer as the slow, default method implemented
1560 in :class:`MatrixEuclideanJordanAlgebra`::
1562 sage: set_random_seed()
1563 sage: J = ComplexHermitianEJA.random_instance()
1564 sage: x,y = J.random_elements(2)
1565 sage: Xe = x.natural_representation()
1566 sage: Ye = y.natural_representation()
1567 sage: X = ComplexHermitianEJA.real_unembed(Xe)
1568 sage: Y = ComplexHermitianEJA.real_unembed(Ye)
1569 sage: expected = (X*Y).trace().real()
1570 sage: actual = ComplexHermitianEJA.natural_inner_product(Xe,Ye)
1571 sage: actual == expected
1575 return RealMatrixEuclideanJordanAlgebra
.natural_inner_product(X
,Y
)/2
1578 class ComplexHermitianEJA(ComplexMatrixEuclideanJordanAlgebra
):
1580 The rank-n simple EJA consisting of complex Hermitian n-by-n
1581 matrices over the real numbers, the usual symmetric Jordan product,
1582 and the real-part-of-trace inner product. It has dimension `n^2` over
1587 sage: from mjo.eja.eja_algebra import ComplexHermitianEJA
1591 In theory, our "field" can be any subfield of the reals::
1593 sage: ComplexHermitianEJA(2, RDF)
1594 Euclidean Jordan algebra of dimension 4 over Real Double Field
1595 sage: ComplexHermitianEJA(2, RR)
1596 Euclidean Jordan algebra of dimension 4 over Real Field with
1597 53 bits of precision
1601 The dimension of this algebra is `n^2`::
1603 sage: set_random_seed()
1604 sage: n_max = ComplexHermitianEJA._max_test_case_size()
1605 sage: n = ZZ.random_element(1, n_max)
1606 sage: J = ComplexHermitianEJA(n)
1607 sage: J.dimension() == n^2
1610 The Jordan multiplication is what we think it is::
1612 sage: set_random_seed()
1613 sage: J = ComplexHermitianEJA.random_instance()
1614 sage: x,y = J.random_elements(2)
1615 sage: actual = (x*y).natural_representation()
1616 sage: X = x.natural_representation()
1617 sage: Y = y.natural_representation()
1618 sage: expected = (X*Y + Y*X)/2
1619 sage: actual == expected
1621 sage: J(expected) == x*y
1624 We can change the generator prefix::
1626 sage: ComplexHermitianEJA(2, prefix='z').gens()
1629 Our natural basis is normalized with respect to the natural inner
1630 product unless we specify otherwise::
1632 sage: set_random_seed()
1633 sage: J = ComplexHermitianEJA.random_instance()
1634 sage: all( b.norm() == 1 for b in J.gens() )
1637 Since our natural basis is normalized with respect to the natural
1638 inner product, and since we know that this algebra is an EJA, any
1639 left-multiplication operator's matrix will be symmetric because
1640 natural->EJA basis representation is an isometry and within the EJA
1641 the operator is self-adjoint by the Jordan axiom::
1643 sage: set_random_seed()
1644 sage: x = ComplexHermitianEJA.random_instance().random_element()
1645 sage: x.operator().matrix().is_symmetric()
1648 We can construct the (trivial) algebra of rank zero::
1650 sage: ComplexHermitianEJA(0)
1651 Euclidean Jordan algebra of dimension 0 over Algebraic Real Field
1656 def _denormalized_basis(cls
, n
, field
):
1658 Returns a basis for the space of complex Hermitian n-by-n matrices.
1660 Why do we embed these? Basically, because all of numerical linear
1661 algebra assumes that you're working with vectors consisting of `n`
1662 entries from a field and scalars from the same field. There's no way
1663 to tell SageMath that (for example) the vectors contain complex
1664 numbers, while the scalar field is real.
1668 sage: from mjo.eja.eja_algebra import ComplexHermitianEJA
1672 sage: set_random_seed()
1673 sage: n = ZZ.random_element(1,5)
1674 sage: field = QuadraticField(2, 'sqrt2')
1675 sage: B = ComplexHermitianEJA._denormalized_basis(n, field)
1676 sage: all( M.is_symmetric() for M in B)
1680 R
= PolynomialRing(field
, 'z')
1682 F
= field
.extension(z
**2 + 1, 'I')
1685 # This is like the symmetric case, but we need to be careful:
1687 # * We want conjugate-symmetry, not just symmetry.
1688 # * The diagonal will (as a result) be real.
1692 for j
in range(i
+1):
1693 Eij
= matrix(F
, n
, lambda k
,l
: k
==i
and l
==j
)
1695 Sij
= cls
.real_embed(Eij
)
1698 # The second one has a minus because it's conjugated.
1699 Sij_real
= cls
.real_embed(Eij
+ Eij
.transpose())
1701 Sij_imag
= cls
.real_embed(I
*Eij
- I
*Eij
.transpose())
1704 # Since we embedded these, we can drop back to the "field" that we
1705 # started with instead of the complex extension "F".
1706 return ( s
.change_ring(field
) for s
in S
)
1709 def __init__(self
, n
, field
=AA
, **kwargs
):
1710 basis
= self
._denormalized
_basis
(n
,field
)
1711 super(ComplexHermitianEJA
,self
).__init
__(field
,
1715 self
.rank
.set_cache(n
)
1718 class QuaternionMatrixEuclideanJordanAlgebra(MatrixEuclideanJordanAlgebra
):
1722 Embed the n-by-n quaternion matrix ``M`` into the space of real
1723 matrices of size 4n-by-4n by first sending each quaternion entry `z
1724 = a + bi + cj + dk` to the block-complex matrix ``[[a + bi,
1725 c+di],[-c + di, a-bi]]`, and then embedding those into a real
1730 sage: from mjo.eja.eja_algebra import \
1731 ....: QuaternionMatrixEuclideanJordanAlgebra
1735 sage: Q = QuaternionAlgebra(QQ,-1,-1)
1736 sage: i,j,k = Q.gens()
1737 sage: x = 1 + 2*i + 3*j + 4*k
1738 sage: M = matrix(Q, 1, [[x]])
1739 sage: QuaternionMatrixEuclideanJordanAlgebra.real_embed(M)
1745 Embedding is a homomorphism (isomorphism, in fact)::
1747 sage: set_random_seed()
1748 sage: n_max = QuaternionMatrixEuclideanJordanAlgebra._max_test_case_size()
1749 sage: n = ZZ.random_element(n_max)
1750 sage: Q = QuaternionAlgebra(QQ,-1,-1)
1751 sage: X = random_matrix(Q, n)
1752 sage: Y = random_matrix(Q, n)
1753 sage: Xe = QuaternionMatrixEuclideanJordanAlgebra.real_embed(X)
1754 sage: Ye = QuaternionMatrixEuclideanJordanAlgebra.real_embed(Y)
1755 sage: XYe = QuaternionMatrixEuclideanJordanAlgebra.real_embed(X*Y)
1760 quaternions
= M
.base_ring()
1763 raise ValueError("the matrix 'M' must be square")
1765 F
= QuadraticField(-1, 'I')
1770 t
= z
.coefficient_tuple()
1775 cplxM
= matrix(F
, 2, [[ a
+ b
*i
, c
+ d
*i
],
1776 [-c
+ d
*i
, a
- b
*i
]])
1777 realM
= ComplexMatrixEuclideanJordanAlgebra
.real_embed(cplxM
)
1778 blocks
.append(realM
)
1780 # We should have real entries by now, so use the realest field
1781 # we've got for the return value.
1782 return matrix
.block(quaternions
.base_ring(), n
, blocks
)
1787 def real_unembed(M
):
1789 The inverse of _embed_quaternion_matrix().
1793 sage: from mjo.eja.eja_algebra import \
1794 ....: QuaternionMatrixEuclideanJordanAlgebra
1798 sage: M = matrix(QQ, [[ 1, 2, 3, 4],
1799 ....: [-2, 1, -4, 3],
1800 ....: [-3, 4, 1, -2],
1801 ....: [-4, -3, 2, 1]])
1802 sage: QuaternionMatrixEuclideanJordanAlgebra.real_unembed(M)
1803 [1 + 2*i + 3*j + 4*k]
1807 Unembedding is the inverse of embedding::
1809 sage: set_random_seed()
1810 sage: Q = QuaternionAlgebra(QQ, -1, -1)
1811 sage: M = random_matrix(Q, 3)
1812 sage: Me = QuaternionMatrixEuclideanJordanAlgebra.real_embed(M)
1813 sage: QuaternionMatrixEuclideanJordanAlgebra.real_unembed(Me) == M
1819 raise ValueError("the matrix 'M' must be square")
1820 if not n
.mod(4).is_zero():
1821 raise ValueError("the matrix 'M' must be a quaternion embedding")
1823 # Use the base ring of the matrix to ensure that its entries can be
1824 # multiplied by elements of the quaternion algebra.
1825 field
= M
.base_ring()
1826 Q
= QuaternionAlgebra(field
,-1,-1)
1829 # Go top-left to bottom-right (reading order), converting every
1830 # 4-by-4 block we see to a 2-by-2 complex block, to a 1-by-1
1833 for l
in range(n
/4):
1834 for m
in range(n
/4):
1835 submat
= ComplexMatrixEuclideanJordanAlgebra
.real_unembed(
1836 M
[4*l
:4*l
+4,4*m
:4*m
+4] )
1837 if submat
[0,0] != submat
[1,1].conjugate():
1838 raise ValueError('bad on-diagonal submatrix')
1839 if submat
[0,1] != -submat
[1,0].conjugate():
1840 raise ValueError('bad off-diagonal submatrix')
1841 z
= submat
[0,0].real()
1842 z
+= submat
[0,0].imag()*i
1843 z
+= submat
[0,1].real()*j
1844 z
+= submat
[0,1].imag()*k
1847 return matrix(Q
, n
/4, elements
)
1851 def natural_inner_product(cls
,X
,Y
):
1853 Compute a natural inner product in this algebra directly from
1858 sage: from mjo.eja.eja_algebra import QuaternionHermitianEJA
1862 This gives the same answer as the slow, default method implemented
1863 in :class:`MatrixEuclideanJordanAlgebra`::
1865 sage: set_random_seed()
1866 sage: J = QuaternionHermitianEJA.random_instance()
1867 sage: x,y = J.random_elements(2)
1868 sage: Xe = x.natural_representation()
1869 sage: Ye = y.natural_representation()
1870 sage: X = QuaternionHermitianEJA.real_unembed(Xe)
1871 sage: Y = QuaternionHermitianEJA.real_unembed(Ye)
1872 sage: expected = (X*Y).trace().coefficient_tuple()[0]
1873 sage: actual = QuaternionHermitianEJA.natural_inner_product(Xe,Ye)
1874 sage: actual == expected
1878 return RealMatrixEuclideanJordanAlgebra
.natural_inner_product(X
,Y
)/4
1881 class QuaternionHermitianEJA(QuaternionMatrixEuclideanJordanAlgebra
):
1883 The rank-n simple EJA consisting of self-adjoint n-by-n quaternion
1884 matrices, the usual symmetric Jordan product, and the
1885 real-part-of-trace inner product. It has dimension `2n^2 - n` over
1890 sage: from mjo.eja.eja_algebra import QuaternionHermitianEJA
1894 In theory, our "field" can be any subfield of the reals::
1896 sage: QuaternionHermitianEJA(2, RDF)
1897 Euclidean Jordan algebra of dimension 6 over Real Double Field
1898 sage: QuaternionHermitianEJA(2, RR)
1899 Euclidean Jordan algebra of dimension 6 over Real Field with
1900 53 bits of precision
1904 The dimension of this algebra is `2*n^2 - n`::
1906 sage: set_random_seed()
1907 sage: n_max = QuaternionHermitianEJA._max_test_case_size()
1908 sage: n = ZZ.random_element(1, n_max)
1909 sage: J = QuaternionHermitianEJA(n)
1910 sage: J.dimension() == 2*(n^2) - n
1913 The Jordan multiplication is what we think it is::
1915 sage: set_random_seed()
1916 sage: J = QuaternionHermitianEJA.random_instance()
1917 sage: x,y = J.random_elements(2)
1918 sage: actual = (x*y).natural_representation()
1919 sage: X = x.natural_representation()
1920 sage: Y = y.natural_representation()
1921 sage: expected = (X*Y + Y*X)/2
1922 sage: actual == expected
1924 sage: J(expected) == x*y
1927 We can change the generator prefix::
1929 sage: QuaternionHermitianEJA(2, prefix='a').gens()
1930 (a0, a1, a2, a3, a4, a5)
1932 Our natural basis is normalized with respect to the natural inner
1933 product unless we specify otherwise::
1935 sage: set_random_seed()
1936 sage: J = QuaternionHermitianEJA.random_instance()
1937 sage: all( b.norm() == 1 for b in J.gens() )
1940 Since our natural basis is normalized with respect to the natural
1941 inner product, and since we know that this algebra is an EJA, any
1942 left-multiplication operator's matrix will be symmetric because
1943 natural->EJA basis representation is an isometry and within the EJA
1944 the operator is self-adjoint by the Jordan axiom::
1946 sage: set_random_seed()
1947 sage: x = QuaternionHermitianEJA.random_instance().random_element()
1948 sage: x.operator().matrix().is_symmetric()
1951 We can construct the (trivial) algebra of rank zero::
1953 sage: QuaternionHermitianEJA(0)
1954 Euclidean Jordan algebra of dimension 0 over Algebraic Real Field
1958 def _denormalized_basis(cls
, n
, field
):
1960 Returns a basis for the space of quaternion Hermitian n-by-n matrices.
1962 Why do we embed these? Basically, because all of numerical
1963 linear algebra assumes that you're working with vectors consisting
1964 of `n` entries from a field and scalars from the same field. There's
1965 no way to tell SageMath that (for example) the vectors contain
1966 complex numbers, while the scalar field is real.
1970 sage: from mjo.eja.eja_algebra import QuaternionHermitianEJA
1974 sage: set_random_seed()
1975 sage: n = ZZ.random_element(1,5)
1976 sage: B = QuaternionHermitianEJA._denormalized_basis(n,QQ)
1977 sage: all( M.is_symmetric() for M in B )
1981 Q
= QuaternionAlgebra(QQ
,-1,-1)
1984 # This is like the symmetric case, but we need to be careful:
1986 # * We want conjugate-symmetry, not just symmetry.
1987 # * The diagonal will (as a result) be real.
1991 for j
in range(i
+1):
1992 Eij
= matrix(Q
, n
, lambda k
,l
: k
==i
and l
==j
)
1994 Sij
= cls
.real_embed(Eij
)
1997 # The second, third, and fourth ones have a minus
1998 # because they're conjugated.
1999 Sij_real
= cls
.real_embed(Eij
+ Eij
.transpose())
2001 Sij_I
= cls
.real_embed(I
*Eij
- I
*Eij
.transpose())
2003 Sij_J
= cls
.real_embed(J
*Eij
- J
*Eij
.transpose())
2005 Sij_K
= cls
.real_embed(K
*Eij
- K
*Eij
.transpose())
2008 # Since we embedded these, we can drop back to the "field" that we
2009 # started with instead of the quaternion algebra "Q".
2010 return ( s
.change_ring(field
) for s
in S
)
2013 def __init__(self
, n
, field
=AA
, **kwargs
):
2014 basis
= self
._denormalized
_basis
(n
,field
)
2015 super(QuaternionHermitianEJA
,self
).__init
__(field
,
2019 self
.rank
.set_cache(n
)
2022 class BilinearFormEJA(FiniteDimensionalEuclideanJordanAlgebra
):
2024 The rank-2 simple EJA consisting of real vectors ``x=(x0, x_bar)``
2025 with the half-trace inner product and jordan product ``x*y =
2026 (x0*y0 + <B*x_bar,y_bar>, x0*y_bar + y0*x_bar)`` where ``B`` is a
2027 symmetric positive-definite "bilinear form" matrix. It has
2028 dimension `n` over the reals, and reduces to the ``JordanSpinEJA``
2029 when ``B`` is the identity matrix of order ``n-1``.
2033 sage: from mjo.eja.eja_algebra import (BilinearFormEJA,
2034 ....: JordanSpinEJA)
2038 When no bilinear form is specified, the identity matrix is used,
2039 and the resulting algebra is the Jordan spin algebra::
2041 sage: J0 = BilinearFormEJA(3)
2042 sage: J1 = JordanSpinEJA(3)
2043 sage: J0.multiplication_table() == J0.multiplication_table()
2048 We can create a zero-dimensional algebra::
2050 sage: J = BilinearFormEJA(0)
2054 We can check the multiplication condition given in the Jordan, von
2055 Neumann, and Wigner paper (and also discussed on my "On the
2056 symmetry..." paper). Note that this relies heavily on the standard
2057 choice of basis, as does anything utilizing the bilinear form matrix::
2059 sage: set_random_seed()
2060 sage: n = ZZ.random_element(5)
2061 sage: M = matrix.random(QQ, max(0,n-1), algorithm='unimodular')
2062 sage: B = M.transpose()*M
2063 sage: J = BilinearFormEJA(n, B=B)
2064 sage: eis = VectorSpace(M.base_ring(), M.ncols()).basis()
2065 sage: V = J.vector_space()
2066 sage: sis = [ J.from_vector(V([0] + (M.inverse()*ei).list()))
2067 ....: for ei in eis ]
2068 sage: actual = [ sis[i]*sis[j]
2069 ....: for i in range(n-1)
2070 ....: for j in range(n-1) ]
2071 sage: expected = [ J.one() if i == j else J.zero()
2072 ....: for i in range(n-1)
2073 ....: for j in range(n-1) ]
2074 sage: actual == expected
2077 def __init__(self
, n
, field
=AA
, B
=None, **kwargs
):
2079 self
._B
= matrix
.identity(field
, max(0,n
-1))
2083 V
= VectorSpace(field
, n
)
2084 mult_table
= [[V
.zero() for j
in range(n
)] for i
in range(n
)]
2093 z0
= x0
*y0
+ (self
._B
*xbar
).inner_product(ybar
)
2094 zbar
= y0
*xbar
+ x0
*ybar
2095 z
= V([z0
] + zbar
.list())
2096 mult_table
[i
][j
] = z
2098 # The rank of this algebra is two, unless we're in a
2099 # one-dimensional ambient space (because the rank is bounded
2100 # by the ambient dimension).
2101 super(BilinearFormEJA
, self
).__init
__(field
,
2105 self
.rank
.set_cache(min(n
,2))
2107 def inner_product(self
, x
, y
):
2109 Half of the trace inner product.
2111 This is defined so that the special case of the Jordan spin
2112 algebra gets the usual inner product.
2116 sage: from mjo.eja.eja_algebra import BilinearFormEJA
2120 Ensure that this is one-half of the trace inner-product when
2121 the algebra isn't just the reals (when ``n`` isn't one). This
2122 is in Faraut and Koranyi, and also my "On the symmetry..."
2125 sage: set_random_seed()
2126 sage: n = ZZ.random_element(2,5)
2127 sage: M = matrix.random(QQ, max(0,n-1), algorithm='unimodular')
2128 sage: B = M.transpose()*M
2129 sage: J = BilinearFormEJA(n, B=B)
2130 sage: x = J.random_element()
2131 sage: y = J.random_element()
2132 sage: x.inner_product(y) == (x*y).trace()/2
2136 xvec
= x
.to_vector()
2138 yvec
= y
.to_vector()
2140 return x
[0]*y
[0] + (self
._B
*xbar
).inner_product(ybar
)
2143 class JordanSpinEJA(BilinearFormEJA
):
2145 The rank-2 simple EJA consisting of real vectors ``x=(x0, x_bar)``
2146 with the usual inner product and jordan product ``x*y =
2147 (<x,y>, x0*y_bar + y0*x_bar)``. It has dimension `n` over
2152 sage: from mjo.eja.eja_algebra import JordanSpinEJA
2156 This multiplication table can be verified by hand::
2158 sage: J = JordanSpinEJA(4)
2159 sage: e0,e1,e2,e3 = J.gens()
2175 We can change the generator prefix::
2177 sage: JordanSpinEJA(2, prefix='B').gens()
2182 Ensure that we have the usual inner product on `R^n`::
2184 sage: set_random_seed()
2185 sage: J = JordanSpinEJA.random_instance()
2186 sage: x,y = J.random_elements(2)
2187 sage: X = x.natural_representation()
2188 sage: Y = y.natural_representation()
2189 sage: x.inner_product(y) == J.natural_inner_product(X,Y)
2193 def __init__(self
, n
, field
=AA
, **kwargs
):
2194 # This is a special case of the BilinearFormEJA with the identity
2195 # matrix as its bilinear form.
2196 super(JordanSpinEJA
, self
).__init
__(n
, field
, **kwargs
)
2199 class TrivialEJA(FiniteDimensionalEuclideanJordanAlgebra
):
2201 The trivial Euclidean Jordan algebra consisting of only a zero element.
2205 sage: from mjo.eja.eja_algebra import TrivialEJA
2209 sage: J = TrivialEJA()
2216 sage: 7*J.one()*12*J.one()
2218 sage: J.one().inner_product(J.one())
2220 sage: J.one().norm()
2222 sage: J.one().subalgebra_generated_by()
2223 Euclidean Jordan algebra of dimension 0 over Algebraic Real Field
2228 def __init__(self
, field
=AA
, **kwargs
):
2230 super(TrivialEJA
, self
).__init
__(field
,
2234 # The rank is zero using my definition, namely the dimension of the
2235 # largest subalgebra generated by any element.
2236 self
.rank
.set_cache(0)
2239 class DirectSumEJA(FiniteDimensionalEuclideanJordanAlgebra
):
2241 The external (orthogonal) direct sum of two other Euclidean Jordan
2242 algebras. Essentially the Cartesian product of its two factors.
2243 Every Euclidean Jordan algebra decomposes into an orthogonal
2244 direct sum of simple Euclidean Jordan algebras, so no generality
2245 is lost by providing only this construction.
2249 sage: from mjo.eja.eja_algebra import (HadamardEJA,
2250 ....: RealSymmetricEJA,
2255 sage: J1 = HadamardEJA(2)
2256 sage: J2 = RealSymmetricEJA(3)
2257 sage: J = DirectSumEJA(J1,J2)
2264 def __init__(self
, J1
, J2
, field
=AA
, **kwargs
):
2268 V
= VectorSpace(field
, n
)
2269 mult_table
= [ [ V
.zero() for j
in range(n
) ]
2273 p
= (J1
.monomial(i
)*J1
.monomial(j
)).to_vector()
2274 mult_table
[i
][j
] = V(p
.list() + [field
.zero()]*n2
)
2278 p
= (J2
.monomial(i
)*J2
.monomial(j
)).to_vector()
2279 mult_table
[n1
+i
][n1
+j
] = V([field
.zero()]*n1
+ p
.list())
2281 super(DirectSumEJA
, self
).__init
__(field
,
2285 self
.rank
.set_cache(J1
.rank() + J2
.rank())