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# Morphisms defined by a matrix

A matrix morphism is a morphism that is defined by multiplication by a matrix. Elements of domain must either have a method `vector()` that returns a vector that the defining matrix can hit from the left, or be coercible into vector space of appropriate dimension.

EXAMPLES:

    sage: from sage.modules.matrix_morphism import MatrixMorphism, is_MatrixMorphism
    sage: V = QQ^3
    sage: T = End(V)
    sage: M = MatrixSpace(QQ,3)
    sage: I = M.identity_matrix()
    sage: m = MatrixMorphism(T, I); m
    Morphism defined by the matrix
    [1 0 0]
    [0 1 0]
    [0 0 1]
    sage: is_MatrixMorphism(m)
    True
    sage: m.charpoly('x')
    x^3 - 3*x^2 + 3*x - 1
    sage: m.base_ring()
    Rational Field
    sage: m.det()
    1
    sage: m.fcp('x')
    (x - 1)^3
    sage: m.matrix()
    [1 0 0]
    [0 1 0]
    [0 0 1]
    sage: m.rank()
    3
    sage: m.trace()
    3

AUTHOR:

- William Stein: initial versions
- David Joyner (2005-12-17): added examples
- William Stein (2005-01-07): added __reduce__
- Craig Citro (2008-03-18): refactored MatrixMorphism class
- Rob Beezer (2011-07-15): additional methods, bug fixes, documentation

class `sage.modules.matrix_morphism.``MatrixMorphism`(parent, A)

  Bases: `sage.modules.matrix_morphism.MatrixMorphism_abstract`

  A morphism defined by a matrix.

  INPUT:

  - `parent` - a homspace
  - `A` - matrix or a `MatrixMorphism_abstract` instance
  `is_injective`()

    Tell whether `self` is injective.

    EXAMPLE:

        sage: V1 = QQ^2
        sage: V2 = QQ^3
        sage: phi = V1.hom(Matrix([[1,2,3],[4,5,6]]),V2)
        sage: phi.is_injective()
        True
        sage: psi = V2.hom(Matrix([[1,2],[3,4],[5,6]]),V1)
        sage: psi.is_injective()
        False

    AUTHOR:

    – Simon King (2010-05)

  `is_surjective`()

    Tell whether `self` is surjective.

    EXAMPLES:

        sage: V1 = QQ^2
        sage: V2 = QQ^3
        sage: phi = V1.hom(Matrix([[1,2,3],[4,5,6]]), V2)
        sage: phi.is_surjective()
        False
        sage: psi = V2.hom(Matrix([[1,2],[3,4],[5,6]]), V1)
        sage: psi.is_surjective()
        True

    An example over a PID that is not \(\ZZ\).

        sage: R = PolynomialRing(QQ, 'x')
        sage: A = R^2
        sage: B = R^2
        sage: H = A.hom([B([x^2-1, 1]), B([x^2, 1])])
        sage: H.image()
        Free module of degree 2 and rank 2 over Univariate Polynomial Ring in x over Rational Field
        Echelon basis matrix:
        [ 1  0]
        [ 0 -1]
        sage: H.is_surjective()
        True

    This tests if Trac #11552 is fixed.

        sage: V = ZZ^2
        sage: m = matrix(ZZ, [[1,2],[0,2]])
        sage: phi = V.hom(m, V)
        sage: phi.lift(vector(ZZ, [0, 1]))
        Traceback (most recent call last):
        ...
        ValueError: element is not in the image
        sage: phi.is_surjective()
        False

    AUTHORS:

    - Simon King (2010-05)
    - Rob Beezer (2011-06-28)
  `matrix`(side='left')

    Return a matrix that defines this morphism.

    INPUT:

    - `side` - default:`'left'` - the side of the matrix where a vector is placed to effect the morphism (function).

    OUTPUT:

    A matrix which represents the morphism, relative to bases for the domain and codomain. If the modules are provided with user bases, then the representation is relative to these bases.

    Internally, Sage represents a matrix morphism with the matrix multiplying a row vector placed to the left of the matrix. If the option `side='right'` is used, then a matrix is returned that acts on a vector to the right of the matrix. These two matrices are just transposes of each other and the difference is just a preference for the style of representation.

    EXAMPLES:

        sage: V = ZZ^2; W = ZZ^3
        sage: m = column_matrix([3*V.0 - 5*V.1, 4*V.0 + 2*V.1, V.0 + V.1])
        sage: phi = V.hom(m, W)
        sage: phi.matrix()
        [ 3  4  1]
        [-5  2  1]

        sage: phi.matrix(side='right')
        [ 3 -5]
        [ 4  2]
        [ 1  1]

    TESTS:

        sage: V = ZZ^2
        sage: phi = V.hom([3*V.0, 2*V.1])
        sage: phi.matrix(side='junk')
        Traceback (most recent call last):
        ...
        ValueError: side must be 'left' or 'right', not junk

class `sage.modules.matrix_morphism.``MatrixMorphism_abstract`(parent)

  Bases: `sage.categories.morphism.Morphism`

  INPUT:

  - `parent` - a homspace
  - `A` - matrix

  EXAMPLES:

      sage: from sage.modules.matrix_morphism import MatrixMorphism
      sage: T = End(ZZ^3)
      sage: M = MatrixSpace(ZZ,3)
      sage: I = M.identity_matrix()
      sage: A = MatrixMorphism(T, I)
      sage: loads(A.dumps()) == A
      True

  `base_ring`()

    Return the base ring of self, that is, the ring over which self is given by a matrix.

    EXAMPLES:

        sage: sage.modules.matrix_morphism.MatrixMorphism((ZZ**2).endomorphism_ring(), Matrix(ZZ,2,[3..6])).base_ring()
        Integer Ring

  `characteristic_polynomial`(var='x')

    Return the characteristic polynomial of this endomorphism.

    `characteristic_polynomial` and `char_poly` are the same method.

    INPUT:
      - var – variable

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([V.0+V.1, 2*V.1])
        sage: phi.characteristic_polynomial()
        x^2 - 3*x + 2
        sage: phi.charpoly()
        x^2 - 3*x + 2
        sage: phi.matrix().charpoly()
        x^2 - 3*x + 2
        sage: phi.charpoly('T')
        T^2 - 3*T + 2

  `charpoly`(var='x')

    Return the characteristic polynomial of this endomorphism.

    `characteristic_polynomial` and `char_poly` are the same method.

    INPUT:
      - var – variable

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([V.0+V.1, 2*V.1])
        sage: phi.characteristic_polynomial()
        x^2 - 3*x + 2
        sage: phi.charpoly()
        x^2 - 3*x + 2
        sage: phi.matrix().charpoly()
        x^2 - 3*x + 2
        sage: phi.charpoly('T')
        T^2 - 3*T + 2

  `decomposition`(*args, **kwds)

    Return decomposition of this endomorphism, i.e., sequence of subspaces obtained by finding invariant subspaces of self.

    See the documentation for self.matrix().decomposition for more details. All inputs to this function are passed onto the matrix one.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([V.0+V.1, 2*V.1])
        sage: phi.decomposition()
        [
        Free module of degree 2 and rank 1 over Integer Ring
        Echelon basis matrix:
        [0 1],
        Free module of degree 2 and rank 1 over Integer Ring
        Echelon basis matrix:
        [ 1 -1]
        ]

  `det`()

    Return the determinant of this endomorphism.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([V.0+V.1, 2*V.1])
        sage: phi.det()
        2

  `fcp`(var='x')

    Return the factorization of the characteristic polynomial.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([V.0+V.1, 2*V.1])
        sage: phi.fcp()
        (x - 2) * (x - 1)
        sage: phi.fcp('T')
        (T - 2) * (T - 1)

  `image`()

    Compute the image of this morphism.

    EXAMPLES:

        sage: V = VectorSpace(QQ,3)
        sage: phi = V.Hom(V)(matrix(QQ, 3, range(9)))
        sage: phi.image()
        Vector space of degree 3 and dimension 2 over Rational Field
        Basis matrix:
        [ 1  0 -1]
        [ 0  1  2]
        sage: hom(GF(7)^3, GF(7)^2, zero_matrix(GF(7), 3, 2)).image()
        Vector space of degree 2 and dimension 0 over Finite Field of size 7
        Basis matrix:
        []

    Compute the image of the identity map on a ZZ-submodule:

        sage: V = (ZZ^2).span([[1,2],[3,4]])
        sage: phi = V.Hom(V)(identity_matrix(ZZ,2))
        sage: phi(V.0) == V.0
        True
        sage: phi(V.1) == V.1
        True
        sage: phi.image()
        Free module of degree 2 and rank 2 over Integer Ring
        Echelon basis matrix:
        [1 0]
        [0 2]
        sage: phi.image() == V
        True

  `inverse`()

    Returns the inverse of this matrix morphism, if the inverse exists.

    Raises a `ZeroDivisionError` if the inverse does not exist.

    EXAMPLES:

    An invertible morphism created as a restriction of a non-invertible morphism, and which has an unequal domain and codomain.

        sage: V = QQ^4
        sage: W = QQ^3
        sage: m = matrix(QQ, [[2, 0, 3], [-6, 1, 4], [1, 2, -4], [1, 0, 1]])
        sage: phi = V.hom(m, W)
        sage: rho = phi.restrict_domain(V.span([V.0, V.3]))
        sage: zeta = rho.restrict_codomain(W.span([W.0, W.2]))
        sage: x = vector(QQ, [2, 0, 0, -7])
        sage: y = zeta(x); y
        (-3, 0, -1)
        sage: inv = zeta.inverse(); inv
        Vector space morphism represented by the matrix:
        [-1  3]
        [ 1 -2]
        Domain: Vector space of degree 3 and dimension 2 over Rational Field
        Basis matrix:
        [1 0 0]
        [0 0 1]
        Codomain: Vector space of degree 4 and dimension 2 over Rational Field
        Basis matrix:
        [1 0 0 0]
        [0 0 0 1]
        sage: inv(y) == x
        True

    An example of an invertible morphism between modules, (rather than between vector spaces).

        sage: M = ZZ^4
        sage: p = matrix(ZZ, [[ 0, -1,  1, -2],
        ...                   [ 1, -3,  2, -3],
        ...                   [ 0,  4, -3,  4],
        ...                   [-2,  8, -4,  3]])
        sage: phi = M.hom(p, M)
        sage: x = vector(ZZ, [1, -3, 5, -2])
        sage: y = phi(x); y
        (1, 12, -12, 21)
        sage: rho = phi.inverse(); rho
        Free module morphism defined by the matrix
        [ -5   3  -1   1]
        [ -9   4  -3   2]
        [-20   8  -7   4]
        [ -6   2  -2   1]
        Domain: Ambient free module of rank 4 over the principal ideal domain ...
        Codomain: Ambient free module of rank 4 over the principal ideal domain ...
        sage: rho(y) == x
        True

    A non-invertible morphism, despite having an appropriate domain and codomain.

        sage: V = QQ^2
        sage: m = matrix(QQ, [[1, 2], [20, 40]])
        sage: phi = V.hom(m, V)
        sage: phi.is_bijective()
        False
        sage: phi.inverse()
        Traceback (most recent call last):
        ...
        ZeroDivisionError: matrix morphism not invertible

    The matrix representation of this morphism is invertible over the rationals, but not over the integers, thus the morphism is not invertible as a map between modules. It is easy to notice from the definition that every vector of the image will have a second entry that is an even integer.

        sage: V = ZZ^2
        sage: q = matrix(ZZ, [[1, 2], [3, 4]])
        sage: phi = V.hom(q, V)
        sage: phi.matrix().change_ring(QQ).inverse()
        [  -2    1]
        [ 3/2 -1/2]
        sage: phi.is_bijective()
        False
        sage: phi.image()
        Free module of degree 2 and rank 2 over Integer Ring
        Echelon basis matrix:
        [1 0]
        [0 2]
        sage: phi.lift(vector(ZZ, [1, 1]))
        Traceback (most recent call last):
        ...
        ValueError: element is not in the image
        sage: phi.inverse()
        Traceback (most recent call last):
        ...
        ZeroDivisionError: matrix morphism not invertible

    The unary invert operator (~, tilde, “wiggle”) is synonymous with the `inverse()` method (and a lot easier to type).

        sage: V = QQ^2
        sage: r = matrix(QQ, [[4, 3], [-2, 5]])
        sage: phi = V.hom(r, V)
        sage: rho = phi.inverse()
        sage: zeta = ~phi
        sage: rho.is_equal_function(zeta)
        True

    TESTS:

        sage: V = QQ^2
        sage: W = QQ^3
        sage: U = W.span([W.0, W.1])
        sage: m = matrix(QQ, [[2, 1], [3, 4]])
        sage: phi = V.hom(m, U)
        sage: inv = phi.inverse()
        sage: (inv*phi).is_identity()
        True
        sage: (phi*inv).is_identity()
        True

  `is_bijective`()

    Tell whether `self` is bijective.

    EXAMPLES:

    Two morphisms that are obviously not bijective, simply on considerations of the dimensions. However, each fullfills half of the requirements to be a bijection.

        sage: V1 = QQ^2
        sage: V2 = QQ^3
        sage: m = matrix(QQ, [[1, 2, 3], [4, 5, 6]])
        sage: phi = V1.hom(m, V2)
        sage: phi.is_injective()
        True
        sage: phi.is_bijective()
        False
        sage: rho = V2.hom(m.transpose(), V1)
        sage: rho.is_surjective()
        True
        sage: rho.is_bijective()
        False

    We construct a simple bijection between two one-dimensional vector spaces.

        sage: V1 = QQ^3
        sage: V2 = QQ^2
        sage: phi = V1.hom(matrix(QQ, [[1, 2], [3, 4], [5, 6]]), V2)
        sage: x = vector(QQ, [1, -1, 4])
        sage: y = phi(x); y
        (18, 22)
        sage: rho = phi.restrict_domain(V1.span([x]))
        sage: zeta = rho.restrict_codomain(V2.span([y]))
        sage: zeta.is_bijective()
        True

    AUTHOR:

    - Rob Beezer (2011-06-28)
  `is_equal_function`(other)

    Determines if two morphisms are equal functions.

    INPUT:

    - `other` - a morphism to compare with `self`

    OUTPUT:

    Returns `True` precisely when the two morphisms have equal domains and codomains (as sets) and produce identical output when given the same input. Otherwise returns `False`.

    This is useful when `self` and `other` may have different representations.

    Sage’s default comparison of matrix morphisms requires the domains to have the same bases and the codomains to have the same bases, and then compares the matrix representations. This notion of equality is more permissive (it will return `True` “more often”), but is more correct mathematically.

    EXAMPLES:

    Three morphisms defined by combinations of different bases for the domain and codomain and different functions. Two are equal, the third is different from both of the others.

        sage: B = matrix(QQ, [[-3,  5, -4,  2],
        ...                   [-1,  2, -1,  4],
        ...                   [ 4, -6,  5, -1],
        ...                   [-5,  7, -6,  1]])
        sage: U = (QQ^4).subspace_with_basis(B.rows())
        sage: C = matrix(QQ, [[-1, -6, -4],
        ...                   [ 3, -5,  6],
        ...                   [ 1,  2,  3]])
        sage: V = (QQ^3).subspace_with_basis(C.rows())
        sage: H = Hom(U, V)

        sage: D = matrix(QQ, [[-7, -2, -5,  2],
        ...                   [-5,  1, -4, -8],
        ...                   [ 1, -1,  1,  4],
        ...                   [-4, -1, -3,   1]])
        sage: X = (QQ^4).subspace_with_basis(D.rows())
        sage: E = matrix(QQ, [[ 4, -1,  4],
        ...                   [ 5, -4, -5],
        ...                   [-1,  0, -2]])
        sage: Y = (QQ^3).subspace_with_basis(E.rows())
        sage: K = Hom(X, Y)

        sage: f = lambda x: vector(QQ, [x[0]+x[1], 2*x[1]-4*x[2], 5*x[3]])
        sage: g = lambda x: vector(QQ, [x[0]-x[2], 2*x[1]-4*x[2], 5*x[3]])

        sage: rho = H(f)
        sage: phi = K(f)
        sage: zeta = H(g)

        sage: rho.is_equal_function(phi)
        True
        sage: phi.is_equal_function(rho)
        True
        sage: zeta.is_equal_function(rho)
        False
        sage: phi.is_equal_function(zeta)
        False

    TEST:

        sage: H = Hom(ZZ^2, ZZ^2)
        sage: phi = H(matrix(ZZ, 2, range(4)))
        sage: phi.is_equal_function('junk')
        Traceback (most recent call last):
        ...
        TypeError: can only compare to a matrix morphism, not junk

    AUTHOR:

    - Rob Beezer (2011-07-15)
  `is_identity`()

    Determines if this morphism is an identity function or not.

    EXAMPLES:

    A homomorphism that cannot possibly be the identity due to an unequal domain and codomain.

        sage: V = QQ^3
        sage: W = QQ^2
        sage: m = matrix(QQ, [[1, 2], [3, 4], [5, 6]])
        sage: phi = V.hom(m, W)
        sage: phi.is_identity()
        False

    A bijection, but not the identity.

        sage: V = QQ^3
        sage: n = matrix(QQ, [[3, 1, -8], [5, -4, 6], [1, 1, -5]])
        sage: phi = V.hom(n, V)
        sage: phi.is_bijective()
        True
        sage: phi.is_identity()
        False

    A restriction that is the identity.

        sage: V = QQ^3
        sage: p = matrix(QQ, [[1, 0, 0], [5, 8, 3], [0, 0, 1]])
        sage: phi = V.hom(p, V)
        sage: rho = phi.restrict(V.span([V.0, V.2]))
        sage: rho.is_identity()
        True

    An identity linear transformation that is defined with a domain and codomain with wildly different bases, so that the matrix representation is not simply the identity matrix.

        sage: A = matrix(QQ, [[1, 1, 0], [2, 3, -4], [2, 4, -7]])
        sage: B = matrix(QQ, [[2, 7, -2], [-1, -3, 1], [-1, -6, 2]])
        sage: U = (QQ^3).subspace_with_basis(A.rows())
        sage: V = (QQ^3).subspace_with_basis(B.rows())
        sage: H = Hom(U, V)
        sage: id = lambda x: x
        sage: phi = H(id)
        sage: phi([203, -179, 34])
        (203, -179, 34)
        sage: phi.matrix()
        [  1   0   1]
        [ -9 -18  -2]
        [-17 -31  -5]
        sage: phi.is_identity()
        True

    TEST:

        sage: V = QQ^10
        sage: H = Hom(V, V)
        sage: id = H.identity()
        sage: id.is_identity()
        True

    AUTHOR:

    - Rob Beezer (2011-06-28)
  `is_zero`()

    Determines if this morphism is a zero function or not.

    EXAMPLES:

    A zero morphism created from a function.

        sage: V = ZZ^5
        sage: W = ZZ^3
        sage: z = lambda x: zero_vector(ZZ, 3)
        sage: phi = V.hom(z, W)
        sage: phi.is_zero()
        True

    An image list that just barely makes a non-zero morphism.

        sage: V = ZZ^4
        sage: W = ZZ^6
        sage: z = zero_vector(ZZ, 6)
        sage: images = [z, z, W.5, z]
        sage: phi = V.hom(images, W)
        sage: phi.is_zero()
        False

    TEST:

        sage: V = QQ^10
        sage: W = QQ^3
        sage: H = Hom(V, W)
        sage: rho = H.zero()
        sage: rho.is_zero()
        True

    AUTHOR:

    - Rob Beezer (2011-07-15)
  `kernel`()

    Compute the kernel of this morphism.

    EXAMPLES:

        sage: V = VectorSpace(QQ,3)
        sage: id = V.Hom(V)(identity_matrix(QQ,3))
        sage: null = V.Hom(V)(0*identity_matrix(QQ,3))
        sage: id.kernel()
        Vector space of degree 3 and dimension 0 over Rational Field
        Basis matrix:
        []
        sage: phi = V.Hom(V)(matrix(QQ,3,range(9)))
        sage: phi.kernel()
        Vector space of degree 3 and dimension 1 over Rational Field
        Basis matrix:
        [ 1 -2  1]
        sage: hom(CC^2, CC^2, matrix(CC, [[1,0], [0,1]])).kernel()
        Vector space of degree 2 and dimension 0 over Complex Field with 53 bits of precision
        Basis matrix:
        []

  `matrix`()

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom(V.basis())
        sage: phi.matrix()
        [1 0]
        [0 1]
        sage: sage.modules.matrix_morphism.MatrixMorphism_abstract.matrix(phi)
        Traceback (most recent call last):
        ...
        NotImplementedError: this method must be overridden in the extension class

  `nullity`()

    Returns the nullity of the matrix representing this morphism, which is the dimension of its kernel.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom(V.basis())
        sage: phi.nullity()
        0
        sage: V = ZZ^2; phi = V.hom([V.0, V.0])
        sage: phi.nullity()
        1

  `rank`()

    Returns the rank of the matrix representing this morphism.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom(V.basis())
        sage: phi.rank()
        2
        sage: V = ZZ^2; phi = V.hom([V.0, V.0])
        sage: phi.rank()
        1

  `restrict`(sub)

    Restrict this matrix morphism to a subspace sub of the domain.

    The codomain and domain of the resulting matrix are both sub.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([3*V.0, 2*V.1])
        sage: phi.restrict(V.span([V.0]))
        Free module morphism defined by the matrix
        [3]
        Domain: Free module of degree 2 and rank 1 over Integer Ring
        Echelon ...
        Codomain: Free module of degree 2 and rank 1 over Integer Ring
        Echelon ...

        sage: V = (QQ^2).span_of_basis([[1,2],[3,4]])
        sage: phi = V.hom([V.0+V.1, 2*V.1])
        sage: phi(V.1) == 2*V.1
        True
        sage: W = span([V.1])
        sage: phi(W)
        Vector space of degree 2 and dimension 1 over Rational Field
        Basis matrix:
        [  1 4/3]
        sage: psi = phi.restrict(W); psi
        Vector space morphism represented by the matrix:
        [2]
        Domain: Vector space of degree 2 and dimension 1 over Rational Field
        Basis matrix:
        [  1 4/3]
        Codomain: Vector space of degree 2 and dimension 1 over Rational Field
        Basis matrix:
        [  1 4/3]
        sage: psi.domain() == W
        True
        sage: psi(W.0) == 2*W.0
        True

  `restrict_codomain`(sub)

    Restrict this matrix morphism to a subspace sub of the codomain.

    The resulting morphism has the same domain as before, but a new codomain.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([4*(V.0+V.1),0])
        sage: W = V.span([2*(V.0+V.1)])
        sage: phi
        Free module morphism defined by the matrix
        [4 4]
        [0 0]
        Domain: Ambient free module of rank 2 over the principal ideal domain ...
        Codomain: Ambient free module of rank 2 over the principal ideal domain ...
        sage: psi = phi.restrict_codomain(W); psi
        Free module morphism defined by the matrix
        [2]
        [0]
        Domain: Ambient free module of rank 2 over the principal ideal domain ...
        Codomain: Free module of degree 2 and rank 1 over Integer Ring
        Echelon ...

    An example in which the codomain equals the full ambient space, but with a different basis:

        sage: V = QQ^2
        sage: W = V.span_of_basis([[1,2],[3,4]])
        sage: phi = V.hom(matrix(QQ,2,[1,0,2,0]),W)
        sage: phi.matrix()
        [1 0]
        [2 0]
        sage: phi(V.0)
        (1, 2)
        sage: phi(V.1)
        (2, 4)
        sage: X = V.span([[1,2]]); X
        Vector space of degree 2 and dimension 1 over Rational Field
        Basis matrix:
        [1 2]
        sage: phi(V.0) in X
        True
        sage: phi(V.1) in X
        True
        sage: psi = phi.restrict_codomain(X); psi
        Vector space morphism represented by the matrix:
        [1]
        [2]
        Domain: Vector space of dimension 2 over Rational Field
        Codomain: Vector space of degree 2 and dimension 1 over Rational Field
        Basis matrix:
        [1 2]
        sage: psi(V.0)
        (1, 2)
        sage: psi(V.1)
        (2, 4)
        sage: psi(V.0).parent() is X
        True

  `restrict_domain`(sub)

    Restrict this matrix morphism to a subspace sub of the domain. The subspace sub should have a basis() method and elements of the basis should be coercible into domain.

    The resulting morphism has the same codomain as before, but a new domain.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([3*V.0, 2*V.1])
        sage: phi.restrict_domain(V.span([V.0]))
        Free module morphism defined by the matrix
        [3 0]
        Domain: Free module of degree 2 and rank 1 over Integer Ring
        Echelon ...
        Codomain: Ambient free module of rank 2 over the principal ideal domain ...
        sage: phi.restrict_domain(V.span([V.1]))
        Free module morphism defined by the matrix
        [0 2]...

  `trace`()

    Return the trace of this endomorphism.

    EXAMPLES:

        sage: V = ZZ^2; phi = V.hom([V.0+V.1, 2*V.1])
        sage: phi.trace()
        3

`sage.modules.matrix_morphism.``is_MatrixMorphism`(x)

  Return True if x is a Matrix morphism of free modules.

  EXAMPLES:

      sage: V = ZZ^2; phi = V.hom([3*V.0, 2*V.1])
      sage: sage.modules.matrix_morphism.is_MatrixMorphism(phi)
      True
      sage: sage.modules.matrix_morphism.is_MatrixMorphism(3)
      False

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