def parity_check_matrix(self):
        r"""
        Return a parity check matrix of ``self``.

        The construction of the parity check matrix in case ``self``
        is not a binary code is not really well documented.
        Regarding the choice of projective geometry, one might check:

        - the note over section 2.3 in [Rot2006]_, pages 47-48
        - the dedicated paragraph in [HP2003]_, page 30

        EXAMPLES::

            sage: C = codes.HammingCode(GF(3), 3)
            sage: C.parity_check_matrix()
            [1 0 1 1 0 1 0 1 1 1 0 1 1]
            [0 1 1 2 0 0 1 1 2 0 1 1 2]
            [0 0 0 0 1 1 1 1 1 2 2 2 2]

        """
        n = self.length()
        F = self.base_field()
        m = n - self.dimension()
        MS = MatrixSpace(F, n, m)
        X = ProjectiveSpace(m - 1, F)
        PFn = [list(p) for p in X.point_set(F).points()]

        H = MS(PFn).transpose()
        H = H[::-1, :]
        H.set_immutable()
        return H

def HammingCode(r,F):
    r"""
    Implements the Hamming codes.

    The `r^{th}` Hamming code over `F=GF(q)` is an
    `[n,k,d]` code with length `n=(q^r-1)/(q-1)`,
    dimension `k=(q^r-1)/(q-1) - r` and minimum distance
    `d=3`. The parity check matrix of a Hamming code has rows
    consisting of all nonzero vectors of length r in its columns,
    modulo a scalar factor so no parallel columns arise. A Hamming code
    is a single error-correcting code.

    INPUT:


    -  ``r`` - an integer 2

    -  ``F`` - a finite field.


    OUTPUT: Returns the r-th q-ary Hamming code.

    EXAMPLES::

        sage: codes.HammingCode(3,GF(2))
        Linear code of length 7, dimension 4 over Finite Field of size 2
        sage: C = codes.HammingCode(3,GF(3)); C
        Linear code of length 13, dimension 10 over Finite Field of size 3
        sage: C.minimum_distance()
        3
        sage: C.minimum_distance(algorithm='gap') # long time, check d=3
        3
        sage: C = codes.HammingCode(3,GF(4,'a')); C
        Linear code of length 21, dimension 18 over Finite Field in a of size 2^2

    While the ``codes`` object now gathers all code constructors,
    ``HammingCode`` is still available in the global namespace::

        sage: HammingCode(3,GF(2))
        doctest:...: DeprecationWarning: This method soon will not be available in that way anymore. To use it, you can now call it by typing codes.HammingCode
        See http://trac.sagemath.org/15445 for details.
        Linear code of length 7, dimension 4 over Finite Field of size 2

    """
    q = F.order()
    n =  (q**r-1)/(q-1)
    k = n-r
    MS = MatrixSpace(F,n,r)
    X = ProjectiveSpace(r-1,F)
    PFn = [list(p) for p in X.point_set(F).points(F)]
    H = MS(PFn).transpose()
    Cd = LinearCode(H)
    # Hamming code always has distance 3, so we provide the distance.
    return LinearCode(Cd.dual_code().gen_mat(), d=3)

 def __init__(self,J):
     """
     """
     R = J.base_ring()
     PP = ProjectiveSpace(3,R,["X0","X1","X2","X3"])
     X0, X1, X2, X3 = PP.gens()
     C = J.curve()
     f, h = C.hyperelliptic_polynomials()
     a12 = f[0]; a10 = f[1]; a8 = f[2];
     a6 = f[3]; a4 = f[4]; a2 = f[5]; a0 = f[6]
     if h != 0:
         c6 = h[0]; c4 = h[1]; c2 = h[2]; c0 = h[3]
         a12, a10, a8, a6, a4, a2, a0 = \
              (4*a12 + c6**2,
               4*a10 + 2*c4*c6,
               4*a8 + 2*c2*c6 + c4**2,
               4*a6 + 2*c0*c6 + 2*c2*c4,
               4*a4 + 2*c0*c4 + c2**2,
               4*a2 + 2*c0*c2,
               4*a0 + c0**2)
     F = \
       (-4*a8*a12 + a10**2)*X0**4 + \
       -4*a6*a12*X0**3*X1 + \
       -2*a6*a10*X0**3*X2 + \
       -4*a12*X0**3*X3 + \
       -4*a4*a12*X0**2*X1**2 + \
       (4*a2*a12 - 4*a4*a10)*X0**2*X1*X2 + \
       -2*a10*X0**2*X1*X3 + \
       (-4*a0*a12 + 2*a2*a10 - 4*a4*a8 + a6**2)*X0**2*X2**2 + \
       -4*a8*X0**2*X2*X3 + \
       -4*a2*a12*X0*X1**3 + \
       (8*a0*a12 - 4*a2*a10)*X0*X1**2*X2 + \
       (4*a0*a10 - 4*a2*a8)*X0*X1*X2**2 + \
       -2*a6*X0*X1*X2*X3 + \
       -2*a2*a6*X0*X2**3 + \
       -4*a4*X0*X2**2*X3 + \
       -4*X0*X2*X3**2 + \
       -4*a0*a12*X1**4 + \
       -4*a0*a10*X1**3*X2 + \
       -4*a0*a8*X1**2*X2**2 + \
       X1**2*X3**2 + \
       -4*a0*a6*X1*X2**3 + \
       -2*a2*X1*X2**2*X3 + \
       (-4*a0*a4 + a2**2)*X2**4 + \
       -4*a0*X2**3*X3
     AlgebraicScheme_subscheme_projective.__init__(self, PP, F)
     X, Y, Z = C.ambient_space().gens()
     if a0 ==0:
         a0 = a2
     phi = Hom(C,self)([0,Z**2,X*Z,a0*X**2],Schemes())
     C._kummer_morphism = phi
     J._kummer_surface = self

def HammingCode(r,F):
    r"""
    Implements the Hamming codes.

    The `r^{th}` Hamming code over `F=GF(q)` is an
    `[n,k,d]` code with length `n=(q^r-1)/(q-1)`,
    dimension `k=(q^r-1)/(q-1) - r` and minimum distance
    `d=3`. The parity check matrix of a Hamming code has rows
    consisting of all nonzero vectors of length r in its columns,
    modulo a scalar factor so no parallel columns arise. A Hamming code
    is a single error-correcting code.

    INPUT:


    -  ``r`` - an integer 2

    -  ``F`` - a finite field.


    OUTPUT: Returns the r-th q-ary Hamming code.

    EXAMPLES::

        sage: codes.HammingCode(3,GF(2))
        Linear code of length 7, dimension 4 over Finite Field of size 2
        sage: C = codes.HammingCode(3,GF(3)); C
        Linear code of length 13, dimension 10 over Finite Field of size 3
        sage: C.minimum_distance()
        3
        sage: C.minimum_distance(algorithm='gap') # long time, check d=3
        3
        sage: C = codes.HammingCode(3,GF(4,'a')); C
        Linear code of length 21, dimension 18 over Finite Field in a of size 2^2
    """
    q = F.order()
    n =  (q**r-1)/(q-1)
    k = n-r
    MS = MatrixSpace(F,n,r)
    X = ProjectiveSpace(r-1,F)
    PFn = [list(p) for p in X.point_set(F).points(F)]
    H = MS(PFn).transpose()
    Cd = LinearCode(H)
    # Hamming code always has distance 3, so we provide the distance.
    return LinearCode(Cd.dual_code().generator_matrix(), d=3)

    def __init__(self, poly, ambient=None):
        """
        Return the projective hypersurface in the space ambient
        defined by the polynomial poly.

        If ambient is not given, it will be constructed based on
        poly.

        EXAMPLES::

            sage: P.<x, y, z> = ProjectiveSpace(ZZ, 2)
            sage: ProjectiveHypersurface(x-y, P)
            Projective hypersurface defined by x - y in Projective Space of dimension 2 over Integer Ring

        ::

            sage: R.<x, y, z> = QQ[]
            sage: ProjectiveHypersurface(x-y)
            Projective hypersurface defined by x - y in Projective Space of dimension 2 over Rational Field

        TESTS::

            sage: H = ProjectiveHypersurface(x-y)
            sage: H == loads(dumps(H))
            True
        """
        if not is_MPolynomial(poly):
            raise TypeError, \
                  "Defining polynomial (=%s) must be a multivariate polynomial."%poly
        if not poly.is_homogeneous():
            raise TypeError, "Defining polynomial (=%s) must be homogeneous."%poly
        if ambient == None:
            R = poly.parent()
            from sage.schemes.projective.projective_space import ProjectiveSpace
            ambient = ProjectiveSpace(R.base_ring(), R.ngens()-1)
            ambient._coordinate_ring = R
        AlgebraicScheme_subscheme_projective.__init__(self, ambient, [poly])

    def parametrization(self, point=None, morphism=True):
        r"""
        Return a parametrization `f` of ``self`` together with the
        inverse of `f`.

        If ``point`` is specified, then that point is used
        for the parametrization. Otherwise, use ``self.rational_point()``
        to find a point.

        If ``morphism`` is True, then `f` is returned in the form
        of a Scheme morphism. Otherwise, it is a tuple of polynomials
        that gives the parametrization.

        ALGORITHM:

        Uses the PARI/GP function ``qfparam``.

        EXAMPLES ::

            sage: c = Conic([1,1,-1])
            sage: c.parametrization()
            (Scheme morphism:
              From: Projective Space of dimension 1 over Rational Field
              To:   Projective Conic Curve over Rational Field defined by x^2 + y^2 - z^2
              Defn: Defined on coordinates by sending (x : y) to
                    (2*x*y : x^2 - y^2 : x^2 + y^2),
             Scheme morphism:
              From: Projective Conic Curve over Rational Field defined by x^2 + y^2 - z^2
              To:   Projective Space of dimension 1 over Rational Field
              Defn: Defined on coordinates by sending (x : y : z) to
                    (1/2*x : -1/2*y + 1/2*z))

        An example with ``morphism = False`` ::

            sage: R.<x,y,z> = QQ[]
            sage: C = Curve(7*x^2 + 2*y*z + z^2)
            sage: (p, i) = C.parametrization(morphism = False); (p, i)
            ([-2*x*y, x^2 + 7*y^2, -2*x^2], [-1/2*x, 1/7*y + 1/14*z])
            sage: C.defining_polynomial()(p)
            0
            sage: i[0](p) / i[1](p)
            x/y

        A ``ValueError`` is raised if ``self`` has no rational point ::

            sage: C = Conic(x^2 + 2*y^2 + z^2)
            sage: C.parametrization()
            Traceback (most recent call last):
            ...
            ValueError: Conic Projective Conic Curve over Rational Field defined by x^2 + 2*y^2 + z^2 has no rational points over Rational Field!

        A ``ValueError`` is raised if ``self`` is not smooth ::

            sage: C = Conic(x^2 + y^2)
            sage: C.parametrization()
            Traceback (most recent call last):
            ...
            ValueError: The conic self (=Projective Conic Curve over Rational Field defined by x^2 + y^2) is not smooth, hence does not have a parametrization.
        """
        if (not self._parametrization is None) and not point:
            par = self._parametrization
        else:
            if not self.is_smooth():
                raise ValueError("The conic self (=%s) is not smooth, hence does not have a parametrization." % self)
            if point is None:
                point = self.rational_point()
            point = Sequence(point)
            Q = PolynomialRing(QQ, 'x,y')
            [x, y] = Q.gens()
            gens = self.ambient_space().gens()
            M = self.symmetric_matrix()
            M *= lcm([ t.denominator() for t in M.list() ])
            par1 = qfparam(M, point)
            B = Matrix([[par1[i][j] for j in range(3)] for i in range(3)])
            # self is in the image of B and does not lie on a line,
            # hence B is invertible
            A = B.inverse()
            par2 = [sum([A[i,j]*gens[j] for j in range(3)]) for i in [1,0]]
            par = ([Q(pol(x/y)*y**2) for pol in par1], par2)
            if self._parametrization is None:
                self._parametrization = par
        if not morphism:
            return par
        P1 = ProjectiveSpace(self.base_ring(), 1, 'x,y')
        return P1.hom(par[0],self), self.Hom(P1)(par[1], check = False)

#.........这里部分代码省略.........
        sage: cubic = x^2*y + 4*x*y^2 + x^2*z + 8*x*y*z + 4*y^2*z + 9*x*z^2 + 9*y*z^2
        sage: EllipticCurve_from_cubic(cubic, [1,-1,1], morphism=False)
        Elliptic Curve defined by y^2 - 882*x*y - 2560000*y = x^3 - 127281*x^2 over Rational Field
    """
    import sage.matrix.all as matrix

    # check the input
    R = F.parent()
    if not is_MPolynomialRing(R):
        raise TypeError('equation must be a polynomial')
    if R.ngens() != 3:
        raise TypeError('equation must be a polynomial in three variables')
    if not F.is_homogeneous():
        raise TypeError('equation must be a homogeneous polynomial')
    K = F.parent().base_ring()
    try:
        P = [K(c) for c in P]
    except TypeError:
        raise TypeError('cannot convert %s into %s'%(P,K))
    if F(P) != 0:
        raise ValueError('%s is not a point on %s'%(P,F))
    if len(P) != 3:
        raise TypeError('%s is not a projective point'%P)
    x, y, z = R.gens()

    # First case: if P = P2 then P is a flex
    P2 = chord_and_tangent(F, P)
    if are_projectively_equivalent(P, P2, base_ring=K):
        # find the tangent to F in P
        dx = K(F.derivative(x)(P))
        dy = K(F.derivative(y)(P))
        dz = K(F.derivative(z)(P))
        # find a second point Q on the tangent line but not on the cubic
        for tangent in [[dy, -dx, K.zero()], [dz, K.zero(), -dx], [K.zero(), -dz, dx]]:
            tangent = projective_point(tangent)
            Q = [tangent[0]+P[0], tangent[1]+P[1], tangent[2]+P[2]]
            F_Q = F(Q)
            if F_Q != 0:  # At most one further point may accidentally be on the cubic
                break
        assert F_Q != 0
        # pick linearly independent third point
        for third_point in [(1,0,0), (0,1,0), (0,0,1)]:
            M = matrix.matrix(K, [Q, P, third_point]).transpose()
            if M.is_invertible():
                break
        F2 = R(M.act_on_polynomial(F))
        # scale and dehomogenise
        a = K(F2.coefficient(x**3))
        F3 = F2/a
        b = K(F3.coefficient(y*y*z))
        S = rings.PolynomialRing(K, 'x,y,z')
        # elliptic curve coordinates
        X, Y, Z = S.gen(0), S.gen(1), S(-1/b)*S.gen(2)
        F4 = F3(X, Y, Z)
        E = EllipticCurve(F4.subs(z=1))
        if not morphism:
            return E
        inv_defining_poly = [ M[i,0]*X + M[i,1]*Y + M[i,2]*Z for i in range(3) ]
        inv_post = -1/a
        M = M.inverse()
        trans_x, trans_y, trans_z = [ M[i,0]*x + M[i,1]*y + M[i,2]*z for i in range(3) ]
        fwd_defining_poly = [trans_x, trans_y, -b*trans_z]
        fwd_post = -a

    # Second case: P is not a flex, then P, P2, P3 are different
    else:
        P3 = chord_and_tangent(F, P2)
        # send P, P2, P3 to (1:0:0), (0:1:0), (0:0:1) respectively
        M = matrix.matrix(K, [P, P2, P3]).transpose()
        F2 = M.act_on_polynomial(F)
        # substitute x = U^2, y = V*W, z = U*W, and rename (x,y,z)=(U,V,W)
        F3 = F2.substitute({x:x**2, y:y*z, z:x*z}) // (x**2*z)
        # scale and dehomogenise
        a = K(F3.coefficient(x**3))
        F4 = F3/a
        b = K(F4.coefficient(y*y*z))
        # change to a polynomial in only two variables
        S = rings.PolynomialRing(K, 'x,y,z')
        # elliptic curve coordinates
        X, Y, Z = S.gen(0), S.gen(1), S(-1/b)*S.gen(2)
        F5 = F4(X, Y, Z)
        E = EllipticCurve(F5.subs(z=1))
        if not morphism:
            return E
        inv_defining_poly = [ M[i,0]*X*X + M[i,1]*Y*Z + M[i,2]*X*Z for i in range(3) ]
        inv_post = -1/a/(X**2)/Z
        M = M.inverse()
        trans_x, trans_y, trans_z = [
            (M[i,0]*x + M[i,1]*y + M[i,2]*z) for i in range(3) ]
        fwd_defining_poly = [ trans_x*trans_z, trans_x*trans_y, -b*trans_z*trans_z ]
        fwd_post = -a/(trans_x*trans_z*trans_z)

    # Construct the morphism
    from sage.schemes.projective.projective_space import ProjectiveSpace
    P2 = ProjectiveSpace(2, K, names=map(str, R.gens()))
    cubic = P2.subscheme(F)
    from sage.schemes.elliptic_curves.weierstrass_transform import \
        WeierstrassTransformationWithInverse
    return WeierstrassTransformationWithInverse(
        cubic, E, fwd_defining_poly, fwd_post, inv_defining_poly, inv_post)

    def projective_embedding(self, i=None, PP=None):
        """
        Returns a morphism from this space into an ambient projective space
        of the same dimension.

        INPUT:


        -  ``i`` -- integer (default: dimension of self = last
           coordinate) determines which projective embedding to compute. The
           embedding is that which has a 1 in the i-th coordinate, numbered
           from 0.

        -  ``PP`` -- (default: None) ambient projective space, i.e.,
           codomain of morphism; this is constructed if it is not
           given.

        EXAMPLES::

            sage: AA = AffineSpace(2, QQ, 'x')
            sage: pi = AA.projective_embedding(0); pi
            Scheme morphism:
              From: Affine Space of dimension 2 over Rational Field
              To:   Projective Space of dimension 2 over Rational Field
              Defn: Defined on coordinates by sending (x0, x1) to
                    (1 : x0 : x1)
            sage: z = AA(3, 4)
            sage: pi(z)
            (1/4 : 3/4 : 1)
            sage: pi(AA(0,2))
            (1/2 : 0 : 1)
            sage: pi = AA.projective_embedding(1); pi
            Scheme morphism:
              From: Affine Space of dimension 2 over Rational Field
              To:   Projective Space of dimension 2 over Rational Field
              Defn: Defined on coordinates by sending (x0, x1) to
                    (x0 : 1 : x1)
            sage: pi(z)
            (3/4 : 1/4 : 1)
            sage: pi = AA.projective_embedding(2)
            sage: pi(z)
            (3 : 4 : 1)

        ::

            sage: A.<x,y> = AffineSpace(ZZ, 2)
            sage: A.projective_embedding(2).codomain().affine_patch(2) == A
            True
        """
        n = self.dimension_relative()
        if i is None:
            try:
                i = self._default_embedding_index
            except AttributeError:
                i = int(n)
        else:
            i = int(i)

        try:
            phi = self.__projective_embedding[i]
            #assume that if you've passed in a new codomain you want to override
            #the existing embedding
            if PP is None or phi.codomain() == PP:
                return(phi)
        except AttributeError:
            self.__projective_embedding = {}
        except KeyError:
            pass

        #if no i-th embedding exists, we may still be here with PP==None
        if PP is None:
            from sage.schemes.projective.projective_space import ProjectiveSpace
            PP = ProjectiveSpace(n, self.base_ring())
        elif PP.dimension_relative() != n:
            raise ValueError("projective Space must be of dimension %s"%(n))

        R = self.coordinate_ring()
        v = list(R.gens())
        if n < 0 or n >self.dimension_relative():
            raise ValueError("argument i (=%s) must be between 0 and %s, inclusive"%(i,n))
        v.insert(i, R(1))
        phi = self.hom(v, PP)
        self.__projective_embedding[i] = phi
        #make affine patch and projective embedding match
        PP.affine_patch(i,self)
        return phi

    def homogenize(self,n,newvar='h'):
        r"""
        Return the homogenization of ``self``. If ``self.domain()`` is a subscheme, the domain of
        the homogenized map is the projective embedding of ``self.domain()``

        INPUT:

        - ``newvar`` -- the name of the homogenization variable (only used when ``self.domain()`` is affine space)

        - ``n`` -- the n-th projective embedding into projective space

        OUTPUT:

        - :class:`SchemMorphism_polynomial_projective_space`

        EXAMPLES::

            sage: A.<x,y>=AffineSpace(ZZ,2)
            sage: H=Hom(A,A)
            sage: f=H([(x^2-2)/x^5,y^2])
            sage: f.homogenize(2,'z')
            Scheme endomorphism of Projective Space of dimension 2 over Integer Ring
              Defn: Defined on coordinates by sending (x : y : z) to
                    (x^2*z^5 - 2*z^7 : x^5*y^2 : x^5*z^2)

        ::

            sage: A.<x,y>=AffineSpace(CC,2)
            sage: H=Hom(A,A)
            sage: f=H([(x^2-2)/(x*y),y^2-x])
            sage: f.homogenize(0,'z')
            Scheme endomorphism of Projective Space of dimension 2 over Complex
            Field with 53 bits of precision
              Defn: Defined on coordinates by sending (x : y : z) to
                    (x*y*z^2 : x^2*z^2 + (-2.00000000000000)*z^4 : x*y^3 - x^2*y*z)

        ::

            sage: A.<x,y>=AffineSpace(ZZ,2)
            sage: X=A.subscheme([x-y^2])
            sage: H=Hom(X,X)
            sage: f=H([9*y^2,3*y])
            sage: f.homogenize(2)
            Scheme endomorphism of Closed subscheme of Projective Space of dimension 2 over Integer Ring defined by:
              -x1^2 + x0*x2
              Defn: Defined on coordinates by sending (x0 : x1 : x2) to
                    (9*x0*x2 : 3*x1*x2 : x2^2)

        ::

            sage: R.<t>=PolynomialRing(ZZ)
            sage: A.<x,y>=AffineSpace(R,2)
            sage: H=Hom(A,A)
            sage: f=H([(x^2-2)/y,y^2-x])
            sage: f.homogenize(0,'z')
            Scheme endomorphism of Projective Space of dimension 2 over Univariate
            Polynomial Ring in t over Integer Ring
              Defn: Defined on coordinates by sending (x : y : z) to
                    (y*z^2 : x^2*z + (-2)*z^3 : y^3 - x*y*z)
        """
        A=self.domain()
        B=self.codomain()
        N=A.ambient_space().dimension_relative()
        NB=B.ambient_space().dimension_relative()
        Vars=list(A.ambient_space().variable_names())+[newvar]
        S=PolynomialRing(A.base_ring(),Vars)
        try:
            l=lcm([self[i].denominator() for i in range(N)])
        except Exception:  #no lcm
            l=prod([self[i].denominator() for i in range(N)])

        from sage.rings.polynomial.polynomial_ring import PolynomialRing_general
        from sage.rings.polynomial.multi_polynomial_ring_generic import MPolynomialRing_generic
        if self.domain().base_ring()==RealField() or self.domain().base_ring()==ComplexField():
            F=[S(((self[i]*l).numerator())._maxima_().divide(self[i].denominator())[0].sage()) for i in range(N)]
        elif isinstance(self.domain().base_ring(),(PolynomialRing_general,MPolynomialRing_generic)):
            F=[S(((self[i]*l).numerator())._maxima_().divide(self[i].denominator())[0].sage()) for i in range(N)]
        else:
            F=[S(self[i]*l) for i in range(N)]
        F.insert(n,S(l))
        d=max([F[i].degree() for i in range(N+1)])
        F=[F[i].homogenize(newvar)*S.gen(N)**(d-F[i].degree()) for i in range(N+1)]
        from sage.schemes.affine.affine_space import is_AffineSpace
        if is_AffineSpace(A)==True:
            from sage.schemes.projective.projective_space import ProjectiveSpace
            X=ProjectiveSpace(A.base_ring(),NB,Vars)
        else:
            X=A.projective_embedding(n).codomain()
            phi=S.hom(X.ambient_space().gens(),X.ambient_space().coordinate_ring())
            F=[phi(f) for f in F]
        H=Hom(X,X)
        return(H(F))

    def segre_embedding(self, PP=None, var='u'):
        r"""
        Return the Segre embedding of ``self`` into the appropriate
        projective space.

        INPUT:

        -  ``PP`` -- (default: ``None``) ambient image projective space;
            this is constructed if it is not given.

        - ``var`` -- string, variable name of the image projective space, default `u` (optional)

        OUTPUT:

        Hom -- from ``self`` to the appropriate subscheme of projective space

        .. TODO::

            Cartesian products with more than two components

        EXAMPLES::

            sage: X.<y0,y1,y2,y3,y4,y5> = ProductProjectiveSpaces(ZZ,[2,2])
            sage: phi = X.segre_embedding(); phi
            Scheme morphism:
              From: Product of projective spaces P^2 x P^2 over Integer Ring
              To:   Closed subscheme of Projective Space of dimension 8 over Integer Ring defined by:
              -u5*u7 + u4*u8,
              -u5*u6 + u3*u8,
              -u4*u6 + u3*u7,
              -u2*u7 + u1*u8,
              -u2*u4 + u1*u5,
              -u2*u6 + u0*u8,
              -u1*u6 + u0*u7,
              -u2*u3 + u0*u5,
              -u1*u3 + u0*u4
              Defn: Defined by sending (y0 : y1 : y2 , y3 : y4 : y5) to
                    (y0*y3 : y0*y4 : y0*y5 : y1*y3 : y1*y4 : y1*y5 : y2*y3 : y2*y4 : y2*y5).

            ::

            sage: T = ProductProjectiveSpaces([1,2],CC,'z')
            sage: T.segre_embedding()
            Scheme morphism:
              From: Product of projective spaces P^1 x P^2 over Complex Field with 53 bits of precision
              To:   Closed subscheme of Projective Space of dimension 5 over Complex Field with 53 bits of precision defined by:
              -u2*u4 + u1*u5,
              -u2*u3 + u0*u5,
              -u1*u3 + u0*u4
              Defn: Defined by sending (z0 : z1 , z2 : z3 : z4) to
                    (z0*z2 : z0*z3 : z0*z4 : z1*z2 : z1*z3 : z1*z4).
        """
        N = self._dims
        if len(N) > 2:
            raise NotImplementedError("Cannot have more than two components.")
        M = (N[0]+1)*(N[1]+1)-1

        vars = list(self.coordinate_ring().variable_names()) + [var + str(i) for i in range(M+1)]
        R = PolynomialRing(self.base_ring(),self.ngens()+M+1, vars, order='lex')

        #set-up the elimination for the segre embedding
        mapping = []
        k = self.ngens()
        for i in range(N[0]+1):
            for j in range(N[0]+1,N[0]+N[1]+2):
                mapping.append(R.gen(k)-R(self.gen(i)*self.gen(j)))
                k+=1

        #change the defining ideal of the subscheme into the variables
        I = R.ideal(list(self.defining_polynomials()) + mapping)
        J = I.groebner_basis()
        s = set(R.gens()[:self.ngens()])
        n = len(J)-1
        L = []
        while s.isdisjoint(J[n].variables()):
            L.append(J[n])
            n = n-1

        #create new subscheme
        if PP is None:
            PS = ProjectiveSpace(self.base_ring(),M,R.gens()[self.ngens():])
            Y = PS.subscheme(L)
        else:
            if PP.dimension_relative()!= M:
                raise ValueError("Projective Space %s must be dimension %s")%(PP, M)
            S = PP.coordinate_ring()
            psi = R.hom([0]*(N[0]+N[1]+2) + list(S.gens()),S)
            L = [psi(l) for l in L]
            Y = PP.subscheme(L)

        #create embedding for points
        mapping = []
        for i in range(N[0]+1):
            for j in range(N[0]+1,N[0]+N[1]+2):
                mapping.append(self.gen(i)*self.gen(j))
        phi = self.hom(mapping,Y)

        return phi

    def parametrization(self, point=None, morphism=True):
        r"""
        Return a parametrization `f` of ``self`` together with the
        inverse of `f`.

        If ``point`` is specified, then that point is used
        for the parametrization. Otherwise, use ``self.rational_point()``
        to find a point.

        If ``morphism`` is True, then `f` is returned in the form
        of a Scheme morphism. Otherwise, it is a tuple of polynomials
        that gives the parametrization.

        EXAMPLES:

        An example over a finite field ::

            sage: c = Conic(GF(2), [1,1,1,1,1,0])
            sage: c.parametrization()
            (Scheme morphism:
              From: Projective Space of dimension 1 over Finite Field of size 2
              To:   Projective Conic Curve over Finite Field of size 2 defined by x^2 + x*y
            + y^2 + x*z + y*z
              Defn: Defined on coordinates by sending (x : y) to
                    (x*y + y^2 : x^2 + x*y : x^2 + x*y + y^2),
             Scheme morphism:
              From: Projective Conic Curve over Finite Field of size 2 defined by x^2 + x*y
            + y^2 + x*z + y*z
              To:   Projective Space of dimension 1 over Finite Field of size 2
              Defn: Defined on coordinates by sending (x : y : z) to
                    (y : x))

        An example with ``morphism = False`` ::

            sage: R.<x,y,z> = QQ[]
            sage: C = Curve(7*x^2 + 2*y*z + z^2)
            sage: (p, i) = C.parametrization(morphism = False); (p, i)
            ([-2*x*y, 7*x^2 + y^2, -2*y^2], [-1/2*x, -1/2*z])
            sage: C.defining_polynomial()(p)
            0
            sage: i[0](p) / i[1](p)
            x/y

        A ``ValueError`` is raised if ``self`` has no rational point ::

            sage: C = Conic(x^2 + y^2 + 7*z^2)
            sage: C.parametrization()
            Traceback (most recent call last):
            ...
            ValueError: Conic Projective Conic Curve over Rational Field defined by x^2 + y^2 + 7*z^2 has no rational points over Rational Field!

        A ``ValueError`` is raised if ``self`` is not smooth ::

            sage: C = Conic(x^2 + y^2)
            sage: C.parametrization()
            Traceback (most recent call last):
            ...
            ValueError: The conic self (=Projective Conic Curve over Rational Field defined by x^2 + y^2) is not smooth, hence does not have a parametrization.
        """
        if (not self._parametrization is None) and not point:
            par = self._parametrization
        else:
            if not self.is_smooth():
                raise ValueError("The conic self (=%s) is not smooth, hence does not have a parametrization." % self)
            if point is None:
                point = self.rational_point()
            point = Sequence(point)
            B = self.base_ring()
            Q = PolynomialRing(B, 'x,y')
            [x, y] = Q.gens()
            gens = self.ambient_space().gens()
            P = PolynomialRing(B, 4, ['X', 'Y', 'T0', 'T1'])
            [X, Y, T0, T1] = P.gens()
            c3 = [j for j in range(2,-1,-1) if point[j] != 0][0]
            c1 = [j for j in range(3) if j != c3][0]
            c2 = [j for j in range(3) if j != c3 and j != c1][0]
            L = [0,0,0]
            L[c1] = Y*T1*point[c1] + Y*T0
            L[c2] = Y*T1*point[c2] + X*T0
            L[c3] = Y*T1*point[c3]
            bezout = P(self.defining_polynomial()(L) / T0)
            t = [bezout([x,y,0,-1]),bezout([x,y,1,0])]
            par = (tuple([Q(p([x,y,t[0],t[1]])/y) for  p in L]),
                   tuple([gens[m]*point[c3]-gens[c3]*point[m]
                       for m in [c2,c1]]))
            if self._parametrization is None:
                self._parametrization = par
        if not morphism:
            return par
        P1 = ProjectiveSpace(self.base_ring(), 1, 'x,y')
        return P1.hom(par[0],self), self.Hom(P1)(par[1], check = False)

    def segre_embedding(self, PP=None, var='u'):
        r"""
        Return the Segre embedding of this space into the appropriate
        projective space.

        INPUT:

        -  ``PP`` -- (default: ``None``) ambient image projective space;
            this is constructed if it is not given.

        - ``var`` -- string, variable name of the image projective space, default `u` (optional).

        OUTPUT:

        Hom -- from this space to the appropriate subscheme of projective space.

        .. TODO::

            Cartesian products with more than two components.

        EXAMPLES::

            sage: X.<y0,y1,y2,y3,y4,y5> = ProductProjectiveSpaces(ZZ, [2, 2])
            sage: phi = X.segre_embedding(); phi
            Scheme morphism:
              From: Product of projective spaces P^2 x P^2 over Integer Ring
              To:   Closed subscheme of Projective Space of dimension 8 over Integer Ring defined by:
              -u5*u7 + u4*u8,
              -u5*u6 + u3*u8,
              -u4*u6 + u3*u7,
              -u2*u7 + u1*u8,
              -u2*u4 + u1*u5,
              -u2*u6 + u0*u8,
              -u1*u6 + u0*u7,
              -u2*u3 + u0*u5,
              -u1*u3 + u0*u4
              Defn: Defined by sending (y0 : y1 : y2 , y3 : y4 : y5) to
                    (y0*y3 : y0*y4 : y0*y5 : y1*y3 : y1*y4 : y1*y5 : y2*y3 : y2*y4 : y2*y5).

            ::

            sage: T = ProductProjectiveSpaces([1, 2], CC, 'z')
            sage: T.segre_embedding()
            Scheme morphism:
              From: Product of projective spaces P^1 x P^2 over Complex Field with 53 bits of precision
              To:   Closed subscheme of Projective Space of dimension 5 over Complex Field with 53 bits of precision defined by:
              -u2*u4 + u1*u5,
              -u2*u3 + u0*u5,
              -u1*u3 + u0*u4
              Defn: Defined by sending (z0 : z1 , z2 : z3 : z4) to
                    (z0*z2 : z0*z3 : z0*z4 : z1*z2 : z1*z3 : z1*z4).

            ::

            sage: T = ProductProjectiveSpaces([1, 2, 1], QQ, 'z')
            sage: T.segre_embedding()
            Scheme morphism:
              From: Product of projective spaces P^1 x P^2 x P^1 over Rational Field
              To:   Closed subscheme of Projective Space of dimension 11 over
            Rational Field defined by:
              -u9*u10 + u8*u11,
              -u7*u10 + u6*u11,
              -u7*u8 + u6*u9,
              -u5*u10 + u4*u11,
              -u5*u8 + u4*u9,
              -u5*u6 + u4*u7,
              -u5*u9 + u3*u11,
              -u5*u8 + u3*u10,
              -u5*u8 + u2*u11,
              -u4*u8 + u2*u10,
              -u3*u8 + u2*u9,
              -u3*u6 + u2*u7,
              -u3*u4 + u2*u5,
              -u5*u7 + u1*u11,
              -u5*u6 + u1*u10,
              -u3*u7 + u1*u9,
              -u3*u6 + u1*u8,
              -u5*u6 + u0*u11,
              -u4*u6 + u0*u10,
              -u3*u6 + u0*u9,
              -u2*u6 + u0*u8,
              -u1*u6 + u0*u7,
              -u1*u4 + u0*u5,
              -u1*u2 + u0*u3
              Defn: Defined by sending (z0 : z1 , z2 : z3 : z4 , z5 : z6) to
                    (z0*z2*z5 : z0*z2*z6 : z0*z3*z5 : z0*z3*z6 : z0*z4*z5 : z0*z4*z6
            : z1*z2*z5 : z1*z2*z6 : z1*z3*z5 : z1*z3*z6 : z1*z4*z5 : z1*z4*z6).
        """
        N = self._dims
        M = prod([n+1 for n in N]) - 1
        CR = self.coordinate_ring()

        vars = list(self.coordinate_ring().variable_names()) + [var + str(i) for i in range(M+1)]
        R = PolynomialRing(self.base_ring(), self.ngens()+M+1, vars, order='lex')

        #set-up the elimination for the segre embedding
        mapping = []
        k = self.ngens()
        index = self.num_components()*[0]
        for count in range(M + 1):
#.........这里部分代码省略.........

    def dual(self):
        r"""
        Return the projective dual of the given subscheme of projective space.

        INPUT:

        - ``X`` -- A subscheme of projective space. At present, ``X`` is
          required to be an irreducible and reduced hypersurface defined
          over `\QQ` or a finite field.

        OUTPUT:

        - The dual of ``X`` as a subscheme of the dual projective space.

        EXAMPLES:

        The dual of a smooth conic in the plane is also a smooth conic::

            sage: R.<x, y, z> = QQ[]
            sage: P.<x, y, z> = ProjectiveSpace(2, QQ)
            sage: I = R.ideal(x^2 + y^2 + z^2)
            sage: X = P.subscheme(I)
            sage: X.dual()
            Closed subscheme of Projective Space of dimension 2 over Rational Field defined by:
              y0^2 + y1^2 + y2^2

        The dual of the twisted cubic curve in projective 3-space is a singular
        quartic surface. In the following example, we compute the dual of this
        surface, which by double duality is equal to the twisted cubic itself.
        The output is the twisted cubic as an intersection of three quadrics::

            sage: R.<x, y, z, w> = QQ[]
            sage: P.<x, y, z, w> = ProjectiveSpace(3, QQ)
            sage: I = R.ideal(y^2*z^2 - 4*x*z^3 - 4*y^3*w + 18*x*y*z*w - 27*x^2*w^2)
            sage: X = P.subscheme(I)
            sage: X.dual()
            Closed subscheme of Projective Space of dimension 3 over
            Rational Field defined by:
              y2^2 - y1*y3,
              y1*y2 - y0*y3,
              y1^2 - y0*y2

        The singular locus of the quartic surface in the last example
        is itself supported on a twisted cubic::

            sage: X.Jacobian().radical()
            Ideal (z^2 - 3*y*w, y*z - 9*x*w, y^2 - 3*x*z) of Multivariate
            Polynomial Ring in x, y, z, w over Rational Field

        An example over a finite field::

            sage: R = PolynomialRing(GF(61), 'a,b,c')
            sage: P.<a, b, c> = ProjectiveSpace(2, R.base_ring())
            sage: X = P.subscheme(R.ideal(a*a+2*b*b+3*c*c))
            sage: X.dual()
            Closed subscheme of Projective Space of dimension 2 over
            Finite Field of size 61 defined by:
            y0^2 - 30*y1^2 - 20*y2^2

        TESTS::

            sage: R = PolynomialRing(Qp(3), 'a,b,c')
            sage: P.<a, b, c> = ProjectiveSpace(2, R.base_ring())
            sage: X = P.subscheme(R.ideal(a*a+2*b*b+3*c*c))
            sage: X.dual()
            Traceback (most recent call last):
            ...
            NotImplementedError: base ring must be QQ or a finite field
        """
        from sage.libs.singular.function_factory import ff

        K = self.base_ring()
        if not(is_RationalField(K) or is_FiniteField(K)):
            raise NotImplementedError("base ring must be QQ or a finite field")
        I = self.defining_ideal()
        m = I.ngens()
        n = I.ring().ngens() - 1
        if (m != 1 or (n < 1) or I.is_zero()
            or I.is_trivial() or not I.is_prime()):
            raise NotImplementedError("At the present, the method is only"
                                      " implemented for irreducible and"
                                      " reduced hypersurfaces and the given"
                                      " list of generators for the ideal must"
                                      " have exactly one element.")
        R = PolynomialRing(K, 'x', n + 1)
        from sage.schemes.projective.projective_space import ProjectiveSpace
        Pd = ProjectiveSpace(n, K, 'y')
        Rd = Pd.coordinate_ring()
        x = R.variable_names()
        y = Rd.variable_names()
        S = PolynomialRing(K, x + y + ('t',))
        if S.has_coerce_map_from(I.ring()):
            T = PolynomialRing(K, 'w', n + 1)
            I_S = (I.change_ring(T)).change_ring(S)
        else:
            I_S = I.change_ring(S)
        f_S = I_S.gens()[0]
        z = S.gens()
        J = I_S
        for i in range(n + 1):
#.........这里部分代码省略.........

#.........这里部分代码省略.........

            sage: A.<x,y>=AffineSpace(ZZ,2)
            sage: H=Hom(A,A)
            sage: f=H([(x^2-2)/x^5,y^2])
            sage: f.homogenize(2,'z')
            Scheme endomorphism of Projective Space of dimension 2 over Integer Ring
              Defn: Defined on coordinates by sending (x : y : z) to
                    (x^2*z^5 - 2*z^7 : x^5*y^2 : x^5*z^2)

        ::

            sage: A.<x,y>=AffineSpace(CC,2)
            sage: H=Hom(A,A)
            sage: f=H([(x^2-2)/(x*y),y^2-x])
            sage: f.homogenize((2,0),'z')
            Scheme endomorphism of Projective Space of dimension 2 over Complex
            Field with 53 bits of precision
              Defn: Defined on coordinates by sending (x : y : z) to
                    (x*y*z^2 : x^2*z^2 + (-2.00000000000000)*z^4 : x*y^3 - x^2*y*z)

        ::

            sage: A.<x,y>=AffineSpace(ZZ,2)
            sage: X=A.subscheme([x-y^2])
            sage: H=Hom(X,X)
            sage: f=H([9*y^2,3*y])
            sage: f.homogenize(2)
            Scheme endomorphism of Closed subscheme of Projective Space of dimension 2 over Integer Ring defined by:
              -x1^2 + x0*x2
              Defn: Defined on coordinates by sending (x0 : x1 : x2) to
                    (9*x0*x2 : 3*x1*x2 : x2^2)

        ::

            sage: R.<t>=PolynomialRing(ZZ)
            sage: A.<x,y>=AffineSpace(R,2)
            sage: H=Hom(A,A)
            sage: f=H([(x^2-2)/y,y^2-x])
            sage: f.homogenize((2,0),'z')
            Scheme endomorphism of Projective Space of dimension 2 over Univariate
            Polynomial Ring in t over Integer Ring
              Defn: Defined on coordinates by sending (x : y : z) to
                    (y*z^2 : x^2*z + (-2)*z^3 : y^3 - x*y*z)

        ::

            sage: A.<x>=AffineSpace(QQ,1)
            sage: H=End(A)
            sage: f=H([x^2-1])
            sage: f.homogenize((1,0),'y')
            Scheme endomorphism of Projective Space of dimension 1 over Rational
            Field
              Defn: Defined on coordinates by sending (x : y) to
                    (y^2 : x^2 - y^2)
        """
        A=self.domain()
        B=self.codomain()
        N=A.ambient_space().dimension_relative()
        NB=B.ambient_space().dimension_relative()

        #it is possible to homogenize the domain and codomain at different coordinates
        if isinstance(n,(tuple,list)):
            ind=tuple(n)
        else:
            ind=(n,n)

        #homogenize the domain
        Vars=list(A.ambient_space().variable_names())
        Vars.insert(ind[0],newvar)
        S=PolynomialRing(A.base_ring(),Vars)

        #find the denominators if a rational function
        try:
            l=lcm([self[i].denominator() for i in range(N)])
        except Exception:  #no lcm
            l=prod([self[i].denominator() for i in range(N)])

        from sage.rings.polynomial.polynomial_ring import PolynomialRing_general
        from sage.rings.polynomial.multi_polynomial_ring_generic import MPolynomialRing_generic
        if self.domain().base_ring()==RealField() or self.domain().base_ring()==ComplexField():
            F=[S(((self[i]*l).numerator())._maxima_().divide(self[i].denominator())[0].sage()) for i in range(N)]
        elif isinstance(self.domain().base_ring(),(PolynomialRing_general,MPolynomialRing_generic)):
            F=[S(((self[i]*l).numerator())._maxima_().divide(self[i].denominator())[0].sage()) for i in range(N)]
        else:
            F=[S(self[i]*l) for i in range(N)]

        #homogenize the codomain
        F.insert(ind[1],S(l))
        d=max([F[i].degree() for i in range(N+1)])
        F=[F[i].homogenize(newvar)*S.gen(N)**(d-F[i].degree()) for i in range(N+1)]
        from sage.schemes.affine.affine_space import is_AffineSpace
        if is_AffineSpace(A)==True:
            from sage.schemes.projective.projective_space import ProjectiveSpace
            X=ProjectiveSpace(A.base_ring(),NB,Vars)
        else:
            X=A.projective_embedding(ind[1]).codomain()
            phi=S.hom(X.ambient_space().gens(),X.ambient_space().coordinate_ring())
            F=[phi(f) for f in F]
        H=Hom(X,X)
        return(H(F))