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# Linear Algebra

Sage provides standard constructions from linear algebra, e.g., the characteristic polynomial, echelon form, trace, decomposition, etc., of a matrix.

Creation of matrices and matrix multiplication is easy and natural:

    sage: A = Matrix([[1,2,3],[3,2,1],[1,1,1]])
    sage: w = vector([1,1,-4])
    sage: w*A
    (0, 0, 0)
    sage: A*w
    (-9, 1, -2)
    sage: kernel(A)
    Free module of degree 3 and rank 1 over Integer Ring
    Echelon basis matrix:
    [ 1  1 -4]

Note that in Sage, the kernel of a matrix \(A\) is the “left kernel”, i.e. the space of vectors \(w\) such that \(wA=0\).

Solving matrix equations is easy, using the method `solve_right`. Evaluating `A.solve_right(Y)` returns a matrix (or vector) \(X\) so that \(AX=Y\):

    sage: Y = vector([0, -4, -1])
    sage: X = A.solve_right(Y)
    sage: X
    (-2, 1, 0)
    sage: A * X   # checking our answer...
    (0, -4, -1)

A backslash `\` can be used in the place of `solve_right`; use `A \ Y` instead of `A.solve_right(Y)`.

    sage: A \ Y
    (-2, 1, 0)

If there is no solution, Sage returns an error:

    sage: A.solve_right(w)
    Traceback (most recent call last):
    ...
    ValueError: matrix equation has no solutions

Similarly, use `A.solve_left(Y)` to solve for \(X\) in \(XA=Y\).

Sage can also compute eigenvalues and eigenvectors:

    sage: A = matrix([[0, 4], [-1, 0]])
    sage: A.eigenvalues ()
    [-2*I, 2*I]
    sage: B = matrix([[1, 3], [3, 1]])
    sage: B.eigenvectors_left()
    [(4, [
    (1, 1)
    ], 1), (-2, [
    (1, -1)
    ], 1)]

(The syntax for the output of `eigenvectors_left` is a list of triples: (eigenvalue, eigenvector, multiplicity).) Eigenvalues and eigenvectors over `QQ` or `RR` can also be computed using Maxima (see Maxima below).

As noted in Basic Rings, the ring over which a matrix is defined affects some of its properties. In the following, the first argument to the `matrix` command tells Sage to view the matrix as a matrix of integers (the `ZZ` case), a matrix of rational numbers (`QQ`), or a matrix of reals (`RR`):

    sage: AZ = matrix(ZZ, [[2,0], [0,1]])
    sage: AQ = matrix(QQ, [[2,0], [0,1]])
    sage: AR = matrix(RR, [[2,0], [0,1]])
    sage: AZ.echelon_form()
    [2 0]
    [0 1]
    sage: AQ.echelon_form()
    [1 0]
    [0 1]
    sage: AR.echelon_form()
    [ 1.00000000000000 0.000000000000000]
    [0.000000000000000  1.00000000000000]

For computing eigenvalues and eigenvectors of matrices over floating point real or complex numbers, the matrix should be defined over `RDF` (Real Double Field) or `CDF` (Complex Double Field), respectively. If no ring is specified and floating point real or complex numbers are used then by default the matrix is defined over the `RR` or `CC` fields, respectively, which do not support these computations for all the cases:

    sage: ARDF = matrix(RDF, [[1.2, 2], [2, 3]])
    sage: ARDF.eigenvalues()  # rel tol 8e-16
    [-0.09317121994613098, 4.293171219946131]
    sage: ACDF = matrix(CDF, [[1.2, I], [2, 3]])
    sage: ACDF.eigenvectors_right()  # rel tol 3e-15
    [(0.8818456983293743 - 0.8209140653434135*I, [(0.7505608183809549, -0.616145932704589 + 0.2387941530333261*I)], 1),
    (3.3181543016706256 + 0.8209140653434133*I, [(0.14559469829270957 + 0.3756690858502104*I, 0.9152458258662108)], 1)]

## Matrix spaces

We create the space \(\text{Mat}_{3\times 3}(\QQ)\) of \(3 \times 3\) matrices with rational entries:

    sage: M = MatrixSpace(QQ,3)
    sage: M
    Full MatrixSpace of 3 by 3 dense matrices over Rational Field

(To specify the space of 3 by 4 matrices, you would use `MatrixSpace(QQ,3,4)`. If the number of columns is omitted, it defaults to the number of rows, so `MatrixSpace(QQ,3)` is a synonym for `MatrixSpace(QQ,3,3)`.) The space of matrices has a basis which Sage stores as a list:

    sage: B = M.basis()
    sage: len(B)
    9
    sage: B[1]
    [0 1 0]
    [0 0 0]
    [0 0 0]

We create a matrix as an element of `M`.

    sage: A = M(range(9)); A
    [0 1 2]
    [3 4 5]
    [6 7 8]

Next we compute its reduced row echelon form and kernel.

    sage: A.echelon_form()
    [ 1  0 -1]
    [ 0  1  2]
    [ 0  0  0]
    sage: A.kernel()
    Vector space of degree 3 and dimension 1 over Rational Field
    Basis matrix:
    [ 1 -2  1]

Next we illustrate computation of matrices defined over finite fields:

    sage: M = MatrixSpace(GF(2),4,8)
    sage: A = M([1,1,0,0, 1,1,1,1, 0,1,0,0, 1,0,1,1,
    ...          0,0,1,0, 1,1,0,1, 0,0,1,1, 1,1,1,0])
    sage: A
    [1 1 0 0 1 1 1 1]
    [0 1 0 0 1 0 1 1]
    [0 0 1 0 1 1 0 1]
    [0 0 1 1 1 1 1 0]
    sage: rows = A.rows()
    sage: A.columns()
    [(1, 0, 0, 0), (1, 1, 0, 0), (0, 0, 1, 1), (0, 0, 0, 1),
     (1, 1, 1, 1), (1, 0, 1, 1), (1, 1, 0, 1), (1, 1, 1, 0)]
    sage: rows
    [(1, 1, 0, 0, 1, 1, 1, 1), (0, 1, 0, 0, 1, 0, 1, 1),
     (0, 0, 1, 0, 1, 1, 0, 1), (0, 0, 1, 1, 1, 1, 1, 0)]

We make the subspace over \(\GF{2}\) spanned by the above rows.

    sage: V = VectorSpace(GF(2),8)
    sage: S = V.subspace(rows)
    sage: S
    Vector space of degree 8 and dimension 4 over Finite Field of size 2
    Basis matrix:
    [1 0 0 0 0 1 0 0]
    [0 1 0 0 1 0 1 1]
    [0 0 1 0 1 1 0 1]
    [0 0 0 1 0 0 1 1]
    sage: A.echelon_form()
    [1 0 0 0 0 1 0 0]
    [0 1 0 0 1 0 1 1]
    [0 0 1 0 1 1 0 1]
    [0 0 0 1 0 0 1 1]

The basis of \(S\) used by Sage is obtained from the non-zero rows of the reduced row echelon form of the matrix of generators of \(S\).

## Sparse Linear Algebra

Sage has support for sparse linear algebra over PIDs.

    sage: M = MatrixSpace(QQ, 100, sparse=True)
    sage: A = M.random_element(density = 0.05)
    sage: E = A.echelon_form()

The multi-modular algorithm in Sage is good for square matrices (but not so good for non-square matrices):

    sage: M = MatrixSpace(QQ, 50, 100, sparse=True)
    sage: A = M.random_element(density = 0.05)
    sage: E = A.echelon_form()
    sage: M = MatrixSpace(GF(2), 20, 40, sparse=True)
    sage: A = M.random_element()
    sage: E = A.echelon_form()

Note that Python is case sensitive:

    sage: M = MatrixSpace(QQ, 10,10, Sparse=True)
    Traceback (most recent call last):
    ...
    TypeError: __classcall__() got an unexpected keyword argument 'Sparse'

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