mjo/eja/eja_element.py

   1 # -*- coding: utf-8 -*-
   2
   3 from sage.matrix.constructor import matrix
   4 from sage.modules.free_module import VectorSpace
   5 from sage.modules.with_basis.indexed_element import IndexedFreeModuleElement
   6
   7 # TODO: make this unnecessary somehow.
   8 from sage.misc.lazy_import import lazy_import
   9 lazy_import('mjo.eja.eja_algebra', 'FiniteDimensionalEuclideanJordanAlgebra')
  10 lazy_import('mjo.eja.eja_element_subalgebra',
  11             'FiniteDimensionalEuclideanJordanElementSubalgebra')
  12 from mjo.eja.eja_operator import FiniteDimensionalEuclideanJordanAlgebraOperator
  13 from mjo.eja.eja_utils import _mat2vec
  14
  15 class FiniteDimensionalEuclideanJordanAlgebraElement(IndexedFreeModuleElement):
  16     """
  17     An element of a Euclidean Jordan algebra.
  18     """
  19
  20     def __dir__(self):
  21         """
  22         Oh man, I should not be doing this. This hides the "disabled"
  23         methods ``left_matrix`` and ``matrix`` from introspection;
  24         in particular it removes them from tab-completion.
  25         """
  26         return filter(lambda s: s not in ['left_matrix', 'matrix'],
  27                       dir(self.__class__) )
  28
  29
  30
  31
  32     def __pow__(self, n):
  33         """
  34         Return ``self`` raised to the power ``n``.
  35
  36         Jordan algebras are always power-associative; see for
  37         example Faraut and Korányi, Proposition II.1.2 (ii).
  38
  39         We have to override this because our superclass uses row
  40         vectors instead of column vectors! We, on the other hand,
  41         assume column vectors everywhere.
  42
  43         SETUP::
  44
  45             sage: from mjo.eja.eja_algebra import random_eja
  46
  47         TESTS:
  48
  49         The definition of `x^2` is the unambiguous `x*x`::
  50
  51             sage: set_random_seed()
  52             sage: x = random_eja().random_element()
  53             sage: x*x == (x^2)
  54             True
  55
  56         A few examples of power-associativity::
  57
  58             sage: set_random_seed()
  59             sage: x = random_eja().random_element()
  60             sage: x*(x*x)*(x*x) == x^5
  61             True
  62             sage: (x*x)*(x*x*x) == x^5
  63             True
  64
  65         We also know that powers operator-commute (Koecher, Chapter
  66         III, Corollary 1)::
  67
  68             sage: set_random_seed()
  69             sage: x = random_eja().random_element()
  70             sage: m = ZZ.random_element(0,10)
  71             sage: n = ZZ.random_element(0,10)
  72             sage: Lxm = (x^m).operator()
  73             sage: Lxn = (x^n).operator()
  74             sage: Lxm*Lxn == Lxn*Lxm
  75             True
  76
  77         """
  78         if n == 0:
  79             return self.parent().one()
  80         elif n == 1:
  81             return self
  82         else:
  83             return (self**(n-1))*self
  84
  85
  86     def apply_univariate_polynomial(self, p):
  87         """
  88         Apply the univariate polynomial ``p`` to this element.
  89
  90         A priori, SageMath won't allow us to apply a univariate
  91         polynomial to an element of an EJA, because we don't know
  92         that EJAs are rings (they are usually not associative). Of
  93         course, we know that EJAs are power-associative, so the
  94         operation is ultimately kosher. This function sidesteps
  95         the CAS to get the answer we want and expect.
  96
  97         SETUP::
  98
  99             sage: from mjo.eja.eja_algebra import (HadamardEJA,
 100             ....:                                  random_eja)
 101
 102         EXAMPLES::
 103
 104             sage: R = PolynomialRing(QQ, 't')
 105             sage: t = R.gen(0)
 106             sage: p = t^4 - t^3 + 5*t - 2
 107             sage: J = HadamardEJA(5)
 108             sage: J.one().apply_univariate_polynomial(p) == 3*J.one()
 109             True
 110
 111         TESTS:
 112
 113         We should always get back an element of the algebra::
 114
 115             sage: set_random_seed()
 116             sage: p = PolynomialRing(AA, 't').random_element()
 117             sage: J = random_eja()
 118             sage: x = J.random_element()
 119             sage: x.apply_univariate_polynomial(p) in J
 120             True
 121
 122         """
 123         if len(p.variables()) > 1:
 124             raise ValueError("not a univariate polynomial")
 125         P = self.parent()
 126         R = P.base_ring()
 127         # Convert the coeficcients to the parent's base ring,
 128         # because a priori they might live in an (unnecessarily)
 129         # larger ring for which P.sum() would fail below.
 130         cs = [ R(c) for c in p.coefficients(sparse=False) ]
 131         return P.sum( cs[k]*(self**k) for k in range(len(cs)) )
 132
 133
 134     def characteristic_polynomial(self):
 135         """
 136         Return the characteristic polynomial of this element.
 137
 138         SETUP::
 139
 140             sage: from mjo.eja.eja_algebra import HadamardEJA
 141
 142         EXAMPLES:
 143
 144         The rank of `R^3` is three, and the minimal polynomial of
 145         the identity element is `(t-1)` from which it follows that
 146         the characteristic polynomial should be `(t-1)^3`::
 147
 148             sage: J = HadamardEJA(3)
 149             sage: J.one().characteristic_polynomial()
 150             t^3 - 3*t^2 + 3*t - 1
 151
 152         Likewise, the characteristic of the zero element in the
 153         rank-three algebra `R^{n}` should be `t^{3}`::
 154
 155             sage: J = HadamardEJA(3)
 156             sage: J.zero().characteristic_polynomial()
 157             t^3
 158
 159         TESTS:
 160
 161         The characteristic polynomial of an element should evaluate
 162         to zero on that element::
 163
 164             sage: set_random_seed()
 165             sage: x = HadamardEJA(3).random_element()
 166             sage: p = x.characteristic_polynomial()
 167             sage: x.apply_univariate_polynomial(p)
 168             0
 169
 170         The characteristic polynomials of the zero and unit elements
 171         should be what we think they are in a subalgebra, too::
 172
 173             sage: J = HadamardEJA(3)
 174             sage: p1 = J.one().characteristic_polynomial()
 175             sage: q1 = J.zero().characteristic_polynomial()
 176             sage: e0,e1,e2 = J.gens()
 177             sage: A = (e0 + 2*e1 + 3*e2).subalgebra_generated_by() # dim 3
 178             sage: p2 = A.one().characteristic_polynomial()
 179             sage: q2 = A.zero().characteristic_polynomial()
 180             sage: p1 == p2
 181             True
 182             sage: q1 == q2
 183             True
 184
 185         """
 186         p = self.parent().characteristic_polynomial()
 187         return p(*self.to_vector())
 188
 189
 190     def inner_product(self, other):
 191         """
 192         Return the parent algebra's inner product of myself and ``other``.
 193
 194         SETUP::
 195
 196             sage: from mjo.eja.eja_algebra import (
 197             ....:   ComplexHermitianEJA,
 198             ....:   JordanSpinEJA,
 199             ....:   QuaternionHermitianEJA,
 200             ....:   RealSymmetricEJA,
 201             ....:   random_eja)
 202
 203         EXAMPLES:
 204
 205         The inner product in the Jordan spin algebra is the usual
 206         inner product on `R^n` (this example only works because the
 207         basis for the Jordan algebra is the standard basis in `R^n`)::
 208
 209             sage: J = JordanSpinEJA(3)
 210             sage: x = vector(QQ,[1,2,3])
 211             sage: y = vector(QQ,[4,5,6])
 212             sage: x.inner_product(y)
 213             32
 214             sage: J.from_vector(x).inner_product(J.from_vector(y))
 215             32
 216
 217         The inner product on `S^n` is `<X,Y> = trace(X*Y)`, where
 218         multiplication is the usual matrix multiplication in `S^n`,
 219         so the inner product of the identity matrix with itself
 220         should be the `n`::
 221
 222             sage: J = RealSymmetricEJA(3)
 223             sage: J.one().inner_product(J.one())
 224             3
 225
 226         Likewise, the inner product on `C^n` is `<X,Y> =
 227         Re(trace(X*Y))`, where we must necessarily take the real
 228         part because the product of Hermitian matrices may not be
 229         Hermitian::
 230
 231             sage: J = ComplexHermitianEJA(3)
 232             sage: J.one().inner_product(J.one())
 233             3
 234
 235         Ditto for the quaternions::
 236
 237             sage: J = QuaternionHermitianEJA(3)
 238             sage: J.one().inner_product(J.one())
 239             3
 240
 241         TESTS:
 242
 243         Ensure that we can always compute an inner product, and that
 244         it gives us back a real number::
 245
 246             sage: set_random_seed()
 247             sage: J = random_eja()
 248             sage: x,y = J.random_elements(2)
 249             sage: x.inner_product(y) in RLF
 250             True
 251
 252         """
 253         P = self.parent()
 254         if not other in P:
 255             raise TypeError("'other' must live in the same algebra")
 256
 257         return P.inner_product(self, other)
 258
 259
 260     def operator_commutes_with(self, other):
 261         """
 262         Return whether or not this element operator-commutes
 263         with ``other``.
 264
 265         SETUP::
 266
 267             sage: from mjo.eja.eja_algebra import random_eja
 268
 269         EXAMPLES:
 270
 271         The definition of a Jordan algebra says that any element
 272         operator-commutes with its square::
 273
 274             sage: set_random_seed()
 275             sage: x = random_eja().random_element()
 276             sage: x.operator_commutes_with(x^2)
 277             True
 278
 279         TESTS:
 280
 281         Test Lemma 1 from Chapter III of Koecher::
 282
 283             sage: set_random_seed()
 284             sage: u,v = random_eja().random_elements(2)
 285             sage: lhs = u.operator_commutes_with(u*v)
 286             sage: rhs = v.operator_commutes_with(u^2)
 287             sage: lhs == rhs
 288             True
 289
 290         Test the first polarization identity from my notes, Koecher
 291         Chapter III, or from Baes (2.3)::
 292
 293             sage: set_random_seed()
 294             sage: x,y = random_eja().random_elements(2)
 295             sage: Lx = x.operator()
 296             sage: Ly = y.operator()
 297             sage: Lxx = (x*x).operator()
 298             sage: Lxy = (x*y).operator()
 299             sage: bool(2*Lx*Lxy + Ly*Lxx == 2*Lxy*Lx + Lxx*Ly)
 300             True
 301
 302         Test the second polarization identity from my notes or from
 303         Baes (2.4)::
 304
 305             sage: set_random_seed()
 306             sage: x,y,z = random_eja().random_elements(3)
 307             sage: Lx = x.operator()
 308             sage: Ly = y.operator()
 309             sage: Lz = z.operator()
 310             sage: Lzy = (z*y).operator()
 311             sage: Lxy = (x*y).operator()
 312             sage: Lxz = (x*z).operator()
 313             sage: bool(Lx*Lzy + Lz*Lxy + Ly*Lxz == Lzy*Lx + Lxy*Lz + Lxz*Ly)
 314             True
 315
 316         Test the third polarization identity from my notes or from
 317         Baes (2.5)::
 318
 319             sage: set_random_seed()
 320             sage: u,y,z = random_eja().random_elements(3)
 321             sage: Lu = u.operator()
 322             sage: Ly = y.operator()
 323             sage: Lz = z.operator()
 324             sage: Lzy = (z*y).operator()
 325             sage: Luy = (u*y).operator()
 326             sage: Luz = (u*z).operator()
 327             sage: Luyz = (u*(y*z)).operator()
 328             sage: lhs = Lu*Lzy + Lz*Luy + Ly*Luz
 329             sage: rhs = Luyz + Ly*Lu*Lz + Lz*Lu*Ly
 330             sage: bool(lhs == rhs)
 331             True
 332
 333         """
 334         if not other in self.parent():
 335             raise TypeError("'other' must live in the same algebra")
 336
 337         A = self.operator()
 338         B = other.operator()
 339         return (A*B == B*A)
 340
 341
 342     def det(self):
 343         """
 344         Return my determinant, the product of my eigenvalues.
 345
 346         SETUP::
 347
 348             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
 349             ....:                                  TrivialEJA,
 350             ....:                                  random_eja)
 351
 352         EXAMPLES::
 353
 354             sage: J = JordanSpinEJA(2)
 355             sage: e0,e1 = J.gens()
 356             sage: x = sum( J.gens() )
 357             sage: x.det()
 358             0
 359
 360         ::
 361
 362             sage: J = JordanSpinEJA(3)
 363             sage: e0,e1,e2 = J.gens()
 364             sage: x = sum( J.gens() )
 365             sage: x.det()
 366             -1
 367
 368         The determinant of the sole element in the rank-zero trivial
 369         algebra is ``1``, by three paths of reasoning. First, its
 370         characteristic polynomial is a constant ``1``, so the constant
 371         term in that polynomial is ``1``. Second, the characteristic
 372         polynomial evaluated at zero is again ``1``. And finally, the
 373         (empty) product of its eigenvalues is likewise just unity::
 374
 375             sage: J = TrivialEJA()
 376             sage: J.zero().det()
 377             1
 378
 379         TESTS:
 380
 381         An element is invertible if and only if its determinant is
 382         non-zero::
 383
 384             sage: set_random_seed()
 385             sage: x = random_eja().random_element()
 386             sage: x.is_invertible() == (x.det() != 0)
 387             True
 388
 389         Ensure that the determinant is multiplicative on an associative
 390         subalgebra as in Faraut and Korányi's Proposition II.2.2::
 391
 392             sage: set_random_seed()
 393             sage: J = random_eja().random_element().subalgebra_generated_by()
 394             sage: x,y = J.random_elements(2)
 395             sage: (x*y).det() == x.det()*y.det()
 396             True
 397         """
 398         P = self.parent()
 399         r = P.rank()
 400
 401         if r == 0:
 402             # Special case, since we don't get the a0=1
 403             # coefficient when the rank of the algebra
 404             # is zero.
 405             return P.base_ring().one()
 406
 407         p = P._charpoly_coefficients()[0]
 408         # The _charpoly_coeff function already adds the factor of -1
 409         # to ensure that _charpoly_coefficients()[0] is really what
 410         # appears in front of t^{0} in the charpoly. However, we want
 411         # (-1)^r times THAT for the determinant.
 412         return ((-1)**r)*p(*self.to_vector())
 413
 414
 415     def inverse(self):
 416         """
 417         Return the Jordan-multiplicative inverse of this element.
 418
 419         ALGORITHM:
 420
 421         We appeal to the quadratic representation as in Koecher's
 422         Theorem 12 in Chapter III, Section 5.
 423
 424         SETUP::
 425
 426             sage: from mjo.eja.eja_algebra import (ComplexHermitianEJA,
 427             ....:                                  JordanSpinEJA,
 428             ....:                                  random_eja)
 429
 430         EXAMPLES:
 431
 432         The inverse in the spin factor algebra is given in Alizadeh's
 433         Example 11.11::
 434
 435             sage: set_random_seed()
 436             sage: J = JordanSpinEJA.random_instance()
 437             sage: x = J.random_element()
 438             sage: while not x.is_invertible():
 439             ....:     x = J.random_element()
 440             sage: x_vec = x.to_vector()
 441             sage: x0 = x_vec[0]
 442             sage: x_bar = x_vec[1:]
 443             sage: coeff = ~(x0^2 - x_bar.inner_product(x_bar))
 444             sage: inv_vec = x_vec.parent()([x0] + (-x_bar).list())
 445             sage: x_inverse = coeff*inv_vec
 446             sage: x.inverse() == J.from_vector(x_inverse)
 447             True
 448
 449         Trying to invert a non-invertible element throws an error:
 450
 451             sage: JordanSpinEJA(3).zero().inverse()
 452             Traceback (most recent call last):
 453             ...
 454             ValueError: element is not invertible
 455
 456         TESTS:
 457
 458         The identity element is its own inverse::
 459
 460             sage: set_random_seed()
 461             sage: J = random_eja()
 462             sage: J.one().inverse() == J.one()
 463             True
 464
 465         If an element has an inverse, it acts like one::
 466
 467             sage: set_random_seed()
 468             sage: J = random_eja()
 469             sage: x = J.random_element()
 470             sage: (not x.is_invertible()) or (x.inverse()*x == J.one())
 471             True
 472
 473         The inverse of the inverse is what we started with::
 474
 475             sage: set_random_seed()
 476             sage: J = random_eja()
 477             sage: x = J.random_element()
 478             sage: (not x.is_invertible()) or (x.inverse().inverse() == x)
 479             True
 480
 481         Proposition II.2.3 in Faraut and Korányi says that the inverse
 482         of an element is the inverse of its left-multiplication operator
 483         applied to the algebra's identity, when that inverse exists::
 484
 485             sage: set_random_seed()
 486             sage: J = random_eja()
 487             sage: x = J.random_element()
 488             sage: (not x.operator().is_invertible()) or (
 489             ....:    x.operator().inverse()(J.one()) == x.inverse() )
 490             True
 491
 492         Proposition II.2.4 in Faraut and Korányi gives a formula for
 493         the inverse based on the characteristic polynomial and the
 494         Cayley-Hamilton theorem for Euclidean Jordan algebras::
 495
 496             sage: set_random_seed()
 497             sage: J = ComplexHermitianEJA(3)
 498             sage: x = J.random_element()
 499             sage: while not x.is_invertible():
 500             ....:     x = J.random_element()
 501             sage: r = J.rank()
 502             sage: a = x.characteristic_polynomial().coefficients(sparse=False)
 503             sage: expected  = (-1)^(r+1)/x.det()
 504             sage: expected *= sum( a[i+1]*x^i for i in range(r) )
 505             sage: x.inverse() == expected
 506             True
 507
 508         """
 509         if not self.is_invertible():
 510             raise ValueError("element is not invertible")
 511
 512         return (~self.quadratic_representation())(self)
 513
 514
 515     def is_invertible(self):
 516         """
 517         Return whether or not this element is invertible.
 518
 519         ALGORITHM:
 520
 521         The usual way to do this is to check if the determinant is
 522         zero, but we need the characteristic polynomial for the
 523         determinant. The minimal polynomial is a lot easier to get,
 524         so we use Corollary 2 in Chapter V of Koecher to check
 525         whether or not the paren't algebra's zero element is a root
 526         of this element's minimal polynomial.
 527
 528         Beware that we can't use the superclass method, because it
 529         relies on the algebra being associative.
 530
 531         SETUP::
 532
 533             sage: from mjo.eja.eja_algebra import random_eja
 534
 535         TESTS:
 536
 537         The identity element is always invertible::
 538
 539             sage: set_random_seed()
 540             sage: J = random_eja()
 541             sage: J.one().is_invertible()
 542             True
 543
 544         The zero element is never invertible in a non-trivial algebra::
 545
 546             sage: set_random_seed()
 547             sage: J = random_eja()
 548             sage: (not J.is_trivial()) and J.zero().is_invertible()
 549             False
 550
 551         """
 552         if self.is_zero():
 553             if self.parent().is_trivial():
 554                 return True
 555             else:
 556                 return False
 557
 558         # In fact, we only need to know if the constant term is non-zero,
 559         # so we can pass in the field's zero element instead.
 560         zero = self.base_ring().zero()
 561         p = self.minimal_polynomial()
 562         return not (p(zero) == zero)
 563
 564
 565     def is_primitive_idempotent(self):
 566         """
 567         Return whether or not this element is a primitive (or minimal)
 568         idempotent.
 569
 570         A primitive idempotent is a non-zero idempotent that is not
 571         the sum of two other non-zero idempotents. Remark 2.7.15 in
 572         Baes shows that this is what he refers to as a "minimal
 573         idempotent."
 574
 575         An element of a Euclidean Jordan algebra is a minimal idempotent
 576         if it :meth:`is_idempotent` and if its Peirce subalgebra
 577         corresponding to the eigenvalue ``1`` has dimension ``1`` (Baes,
 578         Proposition 2.7.17).
 579
 580         SETUP::
 581
 582             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
 583             ....:                                  RealSymmetricEJA,
 584             ....:                                  TrivialEJA,
 585             ....:                                  random_eja)
 586
 587         WARNING::
 588
 589         This method is sloooooow.
 590
 591         EXAMPLES:
 592
 593         The spectral decomposition of a non-regular element should always
 594         contain at least one non-minimal idempotent::
 595
 596             sage: J = RealSymmetricEJA(3)
 597             sage: x = sum(J.gens())
 598             sage: x.is_regular()
 599             False
 600             sage: [ c.is_primitive_idempotent()
 601             ....:   for (l,c) in x.spectral_decomposition() ]
 602             [False, True]
 603
 604         On the other hand, the spectral decomposition of a regular
 605         element should always be in terms of minimal idempotents::
 606
 607             sage: J = JordanSpinEJA(4)
 608             sage: x = sum( i*J.gens()[i] for i in range(len(J.gens())) )
 609             sage: x.is_regular()
 610             True
 611             sage: [ c.is_primitive_idempotent()
 612             ....:   for (l,c) in x.spectral_decomposition() ]
 613             [True, True]
 614
 615         TESTS:
 616
 617         The identity element is minimal only in an EJA of rank one::
 618
 619             sage: set_random_seed()
 620             sage: J = random_eja()
 621             sage: J.rank() == 1 or not J.one().is_primitive_idempotent()
 622             True
 623
 624         A non-idempotent cannot be a minimal idempotent::
 625
 626             sage: set_random_seed()
 627             sage: J = JordanSpinEJA(4)
 628             sage: x = J.random_element()
 629             sage: (not x.is_idempotent()) and x.is_primitive_idempotent()
 630             False
 631
 632         Proposition 2.7.19 in Baes says that an element is a minimal
 633         idempotent if and only if it's idempotent with trace equal to
 634         unity::
 635
 636             sage: set_random_seed()
 637             sage: J = JordanSpinEJA(4)
 638             sage: x = J.random_element()
 639             sage: expected = (x.is_idempotent() and x.trace() == 1)
 640             sage: actual = x.is_primitive_idempotent()
 641             sage: actual == expected
 642             True
 643
 644         Primitive idempotents must be non-zero::
 645
 646             sage: set_random_seed()
 647             sage: J = random_eja()
 648             sage: J.zero().is_idempotent()
 649             True
 650             sage: J.zero().is_primitive_idempotent()
 651             False
 652
 653         As a consequence of the fact that primitive idempotents must
 654         be non-zero, there are no primitive idempotents in a trivial
 655         Euclidean Jordan algebra::
 656
 657             sage: J = TrivialEJA()
 658             sage: J.one().is_idempotent()
 659             True
 660             sage: J.one().is_primitive_idempotent()
 661             False
 662
 663         """
 664         if not self.is_idempotent():
 665             return False
 666
 667         if self.is_zero():
 668             return False
 669
 670         (_,_,J1) = self.parent().peirce_decomposition(self)
 671         return (J1.dimension() == 1)
 672
 673
 674     def is_nilpotent(self):
 675         """
 676         Return whether or not some power of this element is zero.
 677
 678         ALGORITHM:
 679
 680         We use Theorem 5 in Chapter III of Koecher, which says that
 681         an element ``x`` is nilpotent if and only if ``x.operator()``
 682         is nilpotent. And it is a basic fact of linear algebra that
 683         an operator on an `n`-dimensional space is nilpotent if and
 684         only if, when raised to the `n`th power, it equals the zero
 685         operator (for example, see Axler Corollary 8.8).
 686
 687         SETUP::
 688
 689             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
 690             ....:                                  random_eja)
 691
 692         EXAMPLES::
 693
 694             sage: J = JordanSpinEJA(3)
 695             sage: x = sum(J.gens())
 696             sage: x.is_nilpotent()
 697             False
 698
 699         TESTS:
 700
 701         The identity element is never nilpotent, except in a trivial EJA::
 702
 703             sage: set_random_seed()
 704             sage: J = random_eja()
 705             sage: J.one().is_nilpotent() and not J.is_trivial()
 706             False
 707
 708         The additive identity is always nilpotent::
 709
 710             sage: set_random_seed()
 711             sage: random_eja().zero().is_nilpotent()
 712             True
 713
 714         """
 715         P = self.parent()
 716         zero_operator = P.zero().operator()
 717         return self.operator()**P.dimension() == zero_operator
 718
 719
 720     def is_regular(self):
 721         """
 722         Return whether or not this is a regular element.
 723
 724         SETUP::
 725
 726             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
 727             ....:                                  random_eja)
 728
 729         EXAMPLES:
 730
 731         The identity element always has degree one, but any element
 732         linearly-independent from it is regular::
 733
 734             sage: J = JordanSpinEJA(5)
 735             sage: J.one().is_regular()
 736             False
 737             sage: e0, e1, e2, e3, e4 = J.gens() # e0 is the identity
 738             sage: for x in J.gens():
 739             ....:     (J.one() + x).is_regular()
 740             False
 741             True
 742             True
 743             True
 744             True
 745
 746         TESTS:
 747
 748         The zero element should never be regular, unless the parent
 749         algebra has dimension less than or equal to one::
 750
 751             sage: set_random_seed()
 752             sage: J = random_eja()
 753             sage: J.dimension() <= 1 or not J.zero().is_regular()
 754             True
 755
 756         The unit element isn't regular unless the algebra happens to
 757         consist of only its scalar multiples::
 758
 759             sage: set_random_seed()
 760             sage: J = random_eja()
 761             sage: J.dimension() <= 1 or not J.one().is_regular()
 762             True
 763
 764         """
 765         return self.degree() == self.parent().rank()
 766
 767
 768     def degree(self):
 769         """
 770         Return the degree of this element, which is defined to be
 771         the degree of its minimal polynomial.
 772
 773         ALGORITHM:
 774
 775         For now, we skip the messy minimal polynomial computation
 776         and instead return the dimension of the vector space spanned
 777         by the powers of this element. The latter is a bit more
 778         straightforward to compute.
 779
 780         SETUP::
 781
 782             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
 783             ....:                                  random_eja)
 784
 785         EXAMPLES::
 786
 787             sage: J = JordanSpinEJA(4)
 788             sage: J.one().degree()
 789             1
 790             sage: e0,e1,e2,e3 = J.gens()
 791             sage: (e0 - e1).degree()
 792             2
 793
 794         In the spin factor algebra (of rank two), all elements that
 795         aren't multiples of the identity are regular::
 796
 797             sage: set_random_seed()
 798             sage: J = JordanSpinEJA.random_instance()
 799             sage: x = J.random_element()
 800             sage: x == x.coefficient(0)*J.one() or x.degree() == 2
 801             True
 802
 803         TESTS:
 804
 805         The zero and unit elements are both of degree one in nontrivial
 806         algebras::
 807
 808             sage: set_random_seed()
 809             sage: J = random_eja()
 810             sage: d = J.zero().degree()
 811             sage: (J.is_trivial() and d == 0) or d == 1
 812             True
 813             sage: d = J.one().degree()
 814             sage: (J.is_trivial() and d == 0) or d == 1
 815             True
 816
 817         Our implementation agrees with the definition::
 818
 819             sage: set_random_seed()
 820             sage: x = random_eja().random_element()
 821             sage: x.degree() == x.minimal_polynomial().degree()
 822             True
 823
 824         """
 825         if self.is_zero() and not self.parent().is_trivial():
 826             # The minimal polynomial of zero in a nontrivial algebra
 827             # is "t"; in a trivial algebra it's "1" by convention
 828             # (it's an empty product).
 829             return 1
 830         return self.subalgebra_generated_by().dimension()
 831
 832
 833     def left_matrix(self):
 834         """
 835         Our parent class defines ``left_matrix`` and ``matrix``
 836         methods whose names are misleading. We don't want them.
 837         """
 838         raise NotImplementedError("use operator().matrix() instead")
 839
 840     matrix = left_matrix
 841
 842
 843     def minimal_polynomial(self):
 844         """
 845         Return the minimal polynomial of this element,
 846         as a function of the variable `t`.
 847
 848         ALGORITHM:
 849
 850         We restrict ourselves to the associative subalgebra
 851         generated by this element, and then return the minimal
 852         polynomial of this element's operator matrix (in that
 853         subalgebra). This works by Baes Proposition 2.3.16.
 854
 855         SETUP::
 856
 857             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
 858             ....:                                  RealSymmetricEJA,
 859             ....:                                  TrivialEJA,
 860             ....:                                  random_eja)
 861
 862         EXAMPLES:
 863
 864         Keeping in mind that the polynomial ``1`` evaluates the identity
 865         element (also the zero element) of the trivial algebra, it is clear
 866         that the polynomial ``1`` is the minimal polynomial of the only
 867         element in a trivial algebra::
 868
 869             sage: J = TrivialEJA()
 870             sage: J.one().minimal_polynomial()
 871             1
 872             sage: J.zero().minimal_polynomial()
 873             1
 874
 875         TESTS:
 876
 877         The minimal polynomial of the identity and zero elements are
 878         always the same::
 879
 880             sage: set_random_seed()
 881             sage: J = random_eja(nontrivial=True)
 882             sage: J.one().minimal_polynomial()
 883             t - 1
 884             sage: J.zero().minimal_polynomial()
 885             t
 886
 887         The degree of an element is (by one definition) the degree
 888         of its minimal polynomial::
 889
 890             sage: set_random_seed()
 891             sage: x = random_eja().random_element()
 892             sage: x.degree() == x.minimal_polynomial().degree()
 893             True
 894
 895         The minimal polynomial and the characteristic polynomial coincide
 896         and are known (see Alizadeh, Example 11.11) for all elements of
 897         the spin factor algebra that aren't scalar multiples of the
 898         identity. We require the dimension of the algebra to be at least
 899         two here so that said elements actually exist::
 900
 901             sage: set_random_seed()
 902             sage: n_max = max(2, JordanSpinEJA._max_test_case_size())
 903             sage: n = ZZ.random_element(2, n_max)
 904             sage: J = JordanSpinEJA(n)
 905             sage: y = J.random_element()
 906             sage: while y == y.coefficient(0)*J.one():
 907             ....:     y = J.random_element()
 908             sage: y0 = y.to_vector()[0]
 909             sage: y_bar = y.to_vector()[1:]
 910             sage: actual = y.minimal_polynomial()
 911             sage: t = PolynomialRing(J.base_ring(),'t').gen(0)
 912             sage: expected = t^2 - 2*y0*t + (y0^2 - norm(y_bar)^2)
 913             sage: bool(actual == expected)
 914             True
 915
 916         The minimal polynomial should always kill its element::
 917
 918             sage: set_random_seed()
 919             sage: x = random_eja().random_element()
 920             sage: p = x.minimal_polynomial()
 921             sage: x.apply_univariate_polynomial(p)
 922             0
 923
 924         The minimal polynomial is invariant under a change of basis,
 925         and in particular, a re-scaling of the basis::
 926
 927             sage: set_random_seed()
 928             sage: n_max = RealSymmetricEJA._max_test_case_size()
 929             sage: n = ZZ.random_element(1, n_max)
 930             sage: J1 = RealSymmetricEJA(n)
 931             sage: J2 = RealSymmetricEJA(n,normalize_basis=False)
 932             sage: X = random_matrix(AA,n)
 933             sage: X = X*X.transpose()
 934             sage: x1 = J1(X)
 935             sage: x2 = J2(X)
 936             sage: x1.minimal_polynomial() == x2.minimal_polynomial()
 937             True
 938
 939         """
 940         if self.is_zero():
 941             # We would generate a zero-dimensional subalgebra
 942             # where the minimal polynomial would be constant.
 943             # That might be correct, but only if *this* algebra
 944             # is trivial too.
 945             if not self.parent().is_trivial():
 946                 # Pretty sure we know what the minimal polynomial of
 947                 # the zero operator is going to be. This ensures
 948                 # consistency of e.g. the polynomial variable returned
 949                 # in the "normal" case without us having to think about it.
 950                 return self.operator().minimal_polynomial()
 951
 952         A = self.subalgebra_generated_by()
 953         return A(self).operator().minimal_polynomial()
 954
 955
 956
 957     def natural_representation(self):
 958         """
 959         Return a more-natural representation of this element.
 960
 961         Every finite-dimensional Euclidean Jordan Algebra is a
 962         direct sum of five simple algebras, four of which comprise
 963         Hermitian matrices. This method returns the original
 964         "natural" representation of this element as a Hermitian
 965         matrix, if it has one. If not, you get the usual representation.
 966
 967         SETUP::
 968
 969             sage: from mjo.eja.eja_algebra import (ComplexHermitianEJA,
 970             ....:                                  QuaternionHermitianEJA)
 971
 972         EXAMPLES::
 973
 974             sage: J = ComplexHermitianEJA(3)
 975             sage: J.one()
 976             e0 + e3 + e8
 977             sage: J.one().natural_representation()
 978             [1 0 0 0 0 0]
 979             [0 1 0 0 0 0]
 980             [0 0 1 0 0 0]
 981             [0 0 0 1 0 0]
 982             [0 0 0 0 1 0]
 983             [0 0 0 0 0 1]
 984
 985         ::
 986
 987             sage: J = QuaternionHermitianEJA(3)
 988             sage: J.one()
 989             e0 + e5 + e14
 990             sage: J.one().natural_representation()
 991             [1 0 0 0 0 0 0 0 0 0 0 0]
 992             [0 1 0 0 0 0 0 0 0 0 0 0]
 993             [0 0 1 0 0 0 0 0 0 0 0 0]
 994             [0 0 0 1 0 0 0 0 0 0 0 0]
 995             [0 0 0 0 1 0 0 0 0 0 0 0]
 996             [0 0 0 0 0 1 0 0 0 0 0 0]
 997             [0 0 0 0 0 0 1 0 0 0 0 0]
 998             [0 0 0 0 0 0 0 1 0 0 0 0]
 999             [0 0 0 0 0 0 0 0 1 0 0 0]
1000             [0 0 0 0 0 0 0 0 0 1 0 0]
1001             [0 0 0 0 0 0 0 0 0 0 1 0]
1002             [0 0 0 0 0 0 0 0 0 0 0 1]
1003
1004         """
1005         B = self.parent().natural_basis()
1006         W = self.parent().natural_basis_space()
1007         return W.linear_combination(zip(B,self.to_vector()))
1008
1009
1010     def norm(self):
1011         """
1012         The norm of this element with respect to :meth:`inner_product`.
1013
1014         SETUP::
1015
1016             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
1017             ....:                                  HadamardEJA)
1018
1019         EXAMPLES::
1020
1021             sage: J = HadamardEJA(2)
1022             sage: x = sum(J.gens())
1023             sage: x.norm()
1024             1.414213562373095?
1025
1026         ::
1027
1028             sage: J = JordanSpinEJA(4)
1029             sage: x = sum(J.gens())
1030             sage: x.norm()
1031             2
1032
1033         """
1034         return self.inner_product(self).sqrt()
1035
1036
1037     def operator(self):
1038         """
1039         Return the left-multiplication-by-this-element
1040         operator on the ambient algebra.
1041
1042         SETUP::
1043
1044             sage: from mjo.eja.eja_algebra import random_eja
1045
1046         TESTS::
1047
1048             sage: set_random_seed()
1049             sage: J = random_eja()
1050             sage: x,y = J.random_elements(2)
1051             sage: x.operator()(y) == x*y
1052             True
1053             sage: y.operator()(x) == x*y
1054             True
1055
1056         """
1057         P = self.parent()
1058         left_mult_by_self = lambda y: self*y
1059         L = P.module_morphism(function=left_mult_by_self, codomain=P)
1060         return FiniteDimensionalEuclideanJordanAlgebraOperator(
1061                  P,
1062                  P,
1063                  L.matrix() )
1064
1065
1066     def quadratic_representation(self, other=None):
1067         """
1068         Return the quadratic representation of this element.
1069
1070         SETUP::
1071
1072             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
1073             ....:                                  random_eja)
1074
1075         EXAMPLES:
1076
1077         The explicit form in the spin factor algebra is given by
1078         Alizadeh's Example 11.12::
1079
1080             sage: set_random_seed()
1081             sage: x = JordanSpinEJA.random_instance().random_element()
1082             sage: x_vec = x.to_vector()
1083             sage: n = x_vec.degree()
1084             sage: x0 = x_vec[0]
1085             sage: x_bar = x_vec[1:]
1086             sage: A = matrix(AA, 1, [x_vec.inner_product(x_vec)])
1087             sage: B = 2*x0*x_bar.row()
1088             sage: C = 2*x0*x_bar.column()
1089             sage: D = matrix.identity(AA, n-1)
1090             sage: D = (x0^2 - x_bar.inner_product(x_bar))*D
1091             sage: D = D + 2*x_bar.tensor_product(x_bar)
1092             sage: Q = matrix.block(2,2,[A,B,C,D])
1093             sage: Q == x.quadratic_representation().matrix()
1094             True
1095
1096         Test all of the properties from Theorem 11.2 in Alizadeh::
1097
1098             sage: set_random_seed()
1099             sage: J = random_eja()
1100             sage: x,y = J.random_elements(2)
1101             sage: Lx = x.operator()
1102             sage: Lxx = (x*x).operator()
1103             sage: Qx = x.quadratic_representation()
1104             sage: Qy = y.quadratic_representation()
1105             sage: Qxy = x.quadratic_representation(y)
1106             sage: Qex = J.one().quadratic_representation(x)
1107             sage: n = ZZ.random_element(10)
1108             sage: Qxn = (x^n).quadratic_representation()
1109
1110         Property 1:
1111
1112             sage: 2*Qxy == (x+y).quadratic_representation() - Qx - Qy
1113             True
1114
1115         Property 2 (multiply on the right for :trac:`28272`):
1116
1117             sage: alpha = J.base_ring().random_element()
1118             sage: (alpha*x).quadratic_representation() == Qx*(alpha^2)
1119             True
1120
1121         Property 3:
1122
1123             sage: not x.is_invertible() or ( Qx(x.inverse()) == x )
1124             True
1125
1126             sage: not x.is_invertible() or (
1127             ....:   ~Qx
1128             ....:   ==
1129             ....:   x.inverse().quadratic_representation() )
1130             True
1131
1132             sage: Qxy(J.one()) == x*y
1133             True
1134
1135         Property 4:
1136
1137             sage: not x.is_invertible() or (
1138             ....:   x.quadratic_representation(x.inverse())*Qx
1139             ....:   == Qx*x.quadratic_representation(x.inverse()) )
1140             True
1141
1142             sage: not x.is_invertible() or (
1143             ....:   x.quadratic_representation(x.inverse())*Qx
1144             ....:   ==
1145             ....:   2*Lx*Qex - Qx )
1146             True
1147
1148             sage: 2*Lx*Qex - Qx == Lxx
1149             True
1150
1151         Property 5:
1152
1153             sage: Qy(x).quadratic_representation() == Qy*Qx*Qy
1154             True
1155
1156         Property 6:
1157
1158             sage: Qxn == (Qx)^n
1159             True
1160
1161         Property 7:
1162
1163             sage: not x.is_invertible() or (
1164             ....:   Qx*x.inverse().operator() == Lx )
1165             True
1166
1167         Property 8:
1168
1169             sage: not x.operator_commutes_with(y) or (
1170             ....:   Qx(y)^n == Qxn(y^n) )
1171             True
1172
1173         """
1174         if other is None:
1175             other=self
1176         elif not other in self.parent():
1177             raise TypeError("'other' must live in the same algebra")
1178
1179         L = self.operator()
1180         M = other.operator()
1181         return ( L*M + M*L - (self*other).operator() )
1182
1183
1184
1185     def spectral_decomposition(self):
1186         """
1187         Return the unique spectral decomposition of this element.
1188
1189         ALGORITHM:
1190
1191         Following Faraut and Korányi's Theorem III.1.1, we restrict this
1192         element's left-multiplication-by operator to the subalgebra it
1193         generates. We then compute the spectral decomposition of that
1194         operator, and the spectral projectors we get back must be the
1195         left-multiplication-by operators for the idempotents we
1196         seek. Thus applying them to the identity element gives us those
1197         idempotents.
1198
1199         Since the eigenvalues are required to be distinct, we take
1200         the spectral decomposition of the zero element to be zero
1201         times the identity element of the algebra (which is idempotent,
1202         obviously).
1203
1204         SETUP::
1205
1206             sage: from mjo.eja.eja_algebra import RealSymmetricEJA
1207
1208         EXAMPLES:
1209
1210         The spectral decomposition of the identity is ``1`` times itself,
1211         and the spectral decomposition of zero is ``0`` times the identity::
1212
1213             sage: J = RealSymmetricEJA(3)
1214             sage: J.one()
1215             e0 + e2 + e5
1216             sage: J.one().spectral_decomposition()
1217             [(1, e0 + e2 + e5)]
1218             sage: J.zero().spectral_decomposition()
1219             [(0, e0 + e2 + e5)]
1220
1221         TESTS::
1222
1223             sage: J = RealSymmetricEJA(4)
1224             sage: x = sum(J.gens())
1225             sage: sd = x.spectral_decomposition()
1226             sage: l0 = sd[0][0]
1227             sage: l1 = sd[1][0]
1228             sage: c0 = sd[0][1]
1229             sage: c1 = sd[1][1]
1230             sage: c0.inner_product(c1) == 0
1231             True
1232             sage: c0.is_idempotent()
1233             True
1234             sage: c1.is_idempotent()
1235             True
1236             sage: c0 + c1 == J.one()
1237             True
1238             sage: l0*c0 + l1*c1 == x
1239             True
1240
1241         The spectral decomposition should work in subalgebras, too::
1242
1243             sage: J = RealSymmetricEJA(4)
1244             sage: (e0, e1, e2, e3, e4, e5, e6, e7, e8, e9) = J.gens()
1245             sage: A = 2*e5 - 2*e8
1246             sage: (lambda1, c1) = A.spectral_decomposition()[1]
1247             sage: (J0, J5, J1) = J.peirce_decomposition(c1)
1248             sage: (f0, f1, f2) = J1.gens()
1249             sage: f0.spectral_decomposition()
1250             [(0, 1.000000000000000?*f2), (1, 1.000000000000000?*f0)]
1251
1252         """
1253         A = self.subalgebra_generated_by(orthonormalize_basis=True)
1254         result = []
1255         for (evalue, proj) in A(self).operator().spectral_decomposition():
1256             result.append( (evalue, proj(A.one()).superalgebra_element()) )
1257         return result
1258
1259     def subalgebra_generated_by(self, orthonormalize_basis=False):
1260         """
1261         Return the associative subalgebra of the parent EJA generated
1262         by this element.
1263
1264         Since our parent algebra is unital, we want "subalgebra" to mean
1265         "unital subalgebra" as well; thus the subalgebra that an element
1266         generates will itself be a Euclidean Jordan algebra after
1267         restricting the algebra operations appropriately. This is the
1268         subalgebra that Faraut and Korányi work with in section II.2, for
1269         example.
1270
1271         SETUP::
1272
1273             sage: from mjo.eja.eja_algebra import random_eja
1274
1275         TESTS:
1276
1277         This subalgebra, being composed of only powers, is associative::
1278
1279             sage: set_random_seed()
1280             sage: x0 = random_eja().random_element()
1281             sage: A = x0.subalgebra_generated_by()
1282             sage: x,y,z = A.random_elements(3)
1283             sage: (x*y)*z == x*(y*z)
1284             True
1285
1286         Squaring in the subalgebra should work the same as in
1287         the superalgebra::
1288
1289             sage: set_random_seed()
1290             sage: x = random_eja().random_element()
1291             sage: A = x.subalgebra_generated_by()
1292             sage: A(x^2) == A(x)*A(x)
1293             True
1294
1295         By definition, the subalgebra generated by the zero element is
1296         the one-dimensional algebra generated by the identity
1297         element... unless the original algebra was trivial, in which
1298         case the subalgebra is trivial too::
1299
1300             sage: set_random_seed()
1301             sage: A = random_eja().zero().subalgebra_generated_by()
1302             sage: (A.is_trivial() and A.dimension() == 0) or A.dimension() == 1
1303             True
1304
1305         """
1306         return FiniteDimensionalEuclideanJordanElementSubalgebra(self, orthonormalize_basis)
1307
1308
1309     def subalgebra_idempotent(self):
1310         """
1311         Find an idempotent in the associative subalgebra I generate
1312         using Proposition 2.3.5 in Baes.
1313
1314         SETUP::
1315
1316             sage: from mjo.eja.eja_algebra import random_eja
1317
1318         TESTS:
1319
1320         Ensure that we can find an idempotent in a non-trivial algebra
1321         where there are non-nilpotent elements, or that we get the dumb
1322         solution in the trivial algebra::
1323
1324             sage: set_random_seed()
1325             sage: J = random_eja()
1326             sage: x = J.random_element()
1327             sage: while x.is_nilpotent() and not J.is_trivial():
1328             ....:     x = J.random_element()
1329             sage: c = x.subalgebra_idempotent()
1330             sage: c^2 == c
1331             True
1332
1333         """
1334         if self.parent().is_trivial():
1335             return self
1336
1337         if self.is_nilpotent():
1338             raise ValueError("this only works with non-nilpotent elements!")
1339
1340         J = self.subalgebra_generated_by()
1341         u = J(self)
1342
1343         # The image of the matrix of left-u^m-multiplication
1344         # will be minimal for some natural number s...
1345         s = 0
1346         minimal_dim = J.dimension()
1347         for i in range(1, minimal_dim):
1348             this_dim = (u**i).operator().matrix().image().dimension()
1349             if this_dim < minimal_dim:
1350                 minimal_dim = this_dim
1351                 s = i
1352
1353         # Now minimal_matrix should correspond to the smallest
1354         # non-zero subspace in Baes's (or really, Koecher's)
1355         # proposition.
1356         #
1357         # However, we need to restrict the matrix to work on the
1358         # subspace... or do we? Can't we just solve, knowing that
1359         # A(c) = u^(s+1) should have a solution in the big space,
1360         # too?
1361         #
1362         # Beware, solve_right() means that we're using COLUMN vectors.
1363         # Our FiniteDimensionalAlgebraElement superclass uses rows.
1364         u_next = u**(s+1)
1365         A = u_next.operator().matrix()
1366         c = J.from_vector(A.solve_right(u_next.to_vector()))
1367
1368         # Now c is the idempotent we want, but it still lives in the subalgebra.
1369         return c.superalgebra_element()
1370
1371
1372     def trace(self):
1373         """
1374         Return my trace, the sum of my eigenvalues.
1375
1376         In a trivial algebra, however you want to look at it, the trace is
1377         an empty sum for which we declare the result to be zero.
1378
1379         SETUP::
1380
1381             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
1382             ....:                                  HadamardEJA,
1383             ....:                                  TrivialEJA,
1384             ....:                                  random_eja)
1385
1386         EXAMPLES::
1387
1388             sage: J = TrivialEJA()
1389             sage: J.zero().trace()
1390             0
1391
1392         ::
1393             sage: J = JordanSpinEJA(3)
1394             sage: x = sum(J.gens())
1395             sage: x.trace()
1396             2
1397
1398         ::
1399
1400             sage: J = HadamardEJA(5)
1401             sage: J.one().trace()
1402             5
1403
1404         TESTS:
1405
1406         The trace of an element is a real number::
1407
1408             sage: set_random_seed()
1409             sage: J = random_eja()
1410             sage: J.random_element().trace() in RLF
1411             True
1412
1413         """
1414         P = self.parent()
1415         r = P.rank()
1416
1417         if r == 0:
1418             # Special case for the trivial algebra where
1419             # the trace is an empty sum.
1420             return P.base_ring().zero()
1421
1422         p = P._charpoly_coefficients()[r-1]
1423         # The _charpoly_coeff function already adds the factor of
1424         # -1 to ensure that _charpoly_coeff(r-1) is really what
1425         # appears in front of t^{r-1} in the charpoly. However,
1426         # we want the negative of THAT for the trace.
1427         return -p(*self.to_vector())
1428
1429
1430     def trace_inner_product(self, other):
1431         """
1432         Return the trace inner product of myself and ``other``.
1433
1434         SETUP::
1435
1436             sage: from mjo.eja.eja_algebra import random_eja
1437
1438         TESTS:
1439
1440         The trace inner product is commutative, bilinear, and associative::
1441
1442             sage: set_random_seed()
1443             sage: J = random_eja()
1444             sage: x,y,z = J.random_elements(3)
1445             sage: # commutative
1446             sage: x.trace_inner_product(y) == y.trace_inner_product(x)
1447             True
1448             sage: # bilinear
1449             sage: a = J.base_ring().random_element();
1450             sage: actual = (a*(x+z)).trace_inner_product(y)
1451             sage: expected = ( a*x.trace_inner_product(y) +
1452             ....:              a*z.trace_inner_product(y) )
1453             sage: actual == expected
1454             True
1455             sage: actual = x.trace_inner_product(a*(y+z))
1456             sage: expected = ( a*x.trace_inner_product(y) +
1457             ....:              a*x.trace_inner_product(z) )
1458             sage: actual == expected
1459             True
1460             sage: # associative
1461             sage: (x*y).trace_inner_product(z) == y.trace_inner_product(x*z)
1462             True
1463
1464         """
1465         if not other in self.parent():
1466             raise TypeError("'other' must live in the same algebra")
1467
1468         return (self*other).trace()
1469
1470
1471     def trace_norm(self):
1472         """
1473         The norm of this element with respect to :meth:`trace_inner_product`.
1474
1475         SETUP::
1476
1477             sage: from mjo.eja.eja_algebra import (JordanSpinEJA,
1478             ....:                                  HadamardEJA)
1479
1480         EXAMPLES::
1481
1482             sage: J = HadamardEJA(2)
1483             sage: x = sum(J.gens())
1484             sage: x.trace_norm()
1485             1.414213562373095?
1486
1487         ::
1488
1489             sage: J = JordanSpinEJA(4)
1490             sage: x = sum(J.gens())
1491             sage: x.trace_norm()
1492             2.828427124746190?
1493
1494         """
1495         return self.trace_inner_product(self).sqrt()