+ EXAMPLES::
+
+ sage: A = matrix(QQ,[ [ 1, 2, 3, 4],
+ ....: [-2, 1, -4, 3],
+ ....: [ 9, 10, 11, 12],
+ ....: [-10, 9, -12, 11] ])
+ sage: ComplexMatrixEuclideanJordanAlgebra.real_unembed(A)
+ [ 2*i + 1 4*i + 3]
+ [ 10*i + 9 12*i + 11]
+
+ TESTS:
+
+ Unembedding is the inverse of embedding::
+
+ sage: set_random_seed()
+ sage: F = QuadraticField(-1, 'i')
+ sage: M = random_matrix(F, 3)
+ sage: Me = ComplexMatrixEuclideanJordanAlgebra.real_embed(M)
+ sage: ComplexMatrixEuclideanJordanAlgebra.real_unembed(Me) == M
+ True
+
+ """
+ n = ZZ(M.nrows())
+ if M.ncols() != n:
+ raise ValueError("the matrix 'M' must be square")
+ if not n.mod(2).is_zero():
+ raise ValueError("the matrix 'M' must be a complex embedding")
+
+ # If "M" was normalized, its base ring might have roots
+ # adjoined and they can stick around after unembedding.
+ field = M.base_ring()
+ R = PolynomialRing(field, 'z')
+ z = R.gen()
+ F = field.extension(z**2 + 1, 'i', embedding=CLF(-1).sqrt())
+ i = F.gen()
+
+ # Go top-left to bottom-right (reading order), converting every
+ # 2-by-2 block we see to a single complex element.
+ elements = []
+ for k in xrange(n/2):
+ for j in xrange(n/2):
+ submat = M[2*k:2*k+2,2*j:2*j+2]
+ if submat[0,0] != submat[1,1]:
+ raise ValueError('bad on-diagonal submatrix')
+ if submat[0,1] != -submat[1,0]:
+ raise ValueError('bad off-diagonal submatrix')
+ z = submat[0,0] + submat[0,1]*i
+ elements.append(z)
+
+ return matrix(F, n/2, elements)
+
+
+ @classmethod
+ def natural_inner_product(cls,X,Y):
+ """
+ Compute a natural inner product in this algebra directly from
+ its real embedding.
+
+ SETUP::
+
+ sage: from mjo.eja.eja_algebra import ComplexHermitianEJA
+
+ TESTS:
+
+ This gives the same answer as the slow, default method implemented
+ in :class:`MatrixEuclideanJordanAlgebra`::
+
+ sage: set_random_seed()
+ sage: J = ComplexHermitianEJA.random_instance()
+ sage: x,y = J.random_elements(2)
+ sage: Xe = x.natural_representation()
+ sage: Ye = y.natural_representation()
+ sage: X = ComplexHermitianEJA.real_unembed(Xe)
+ sage: Y = ComplexHermitianEJA.real_unembed(Ye)
+ sage: expected = (X*Y).trace().vector()[0]
+ sage: actual = ComplexHermitianEJA.natural_inner_product(Xe,Ye)
+ sage: actual == expected
+ True
+
+ """
+ return RealMatrixEuclideanJordanAlgebra.natural_inner_product(X,Y)/2
+
+
+class ComplexHermitianEJA(ComplexMatrixEuclideanJordanAlgebra, KnownRankEJA):