Matrices

From

    >>> from sympy import *
    >>> init_printing(use_unicode=True)

In Julia:

• In SymPy, matrices can be store using Julia's genericMatrix{T} type where T <: Symor using SymPy's matrix type, wrapped in a SymMatrix type by SymPy. This tutorial shows how to use the underlying SymMatrix values. To construct a matrix of symbolic values is identical to construction a matrix of numeric values within Julia, and will be illustrated at the end.

• methods for SymMatrix objects use the dot call syntax. As a convenience, this will also work for Array{Sym} objects. The returned value may be a SymMatrix, not an Array{Sym}.

• The matrix constructor in SymPy using a vector of row vectors does not work in SymPy, as of newer versions (it does not work with version 1.5.1 of sympy and PyCall). This style is used in this document. The user of SymPy can eaesily avoid this specification, using Julia's matrix construction techniques.

julia> using SymPy

julia> using LinearAlgebra

To make a matrix in SymPy, use the Matrix object. A matrix is constructed by providing a list of row vectors that make up the matrix. For example, to construct the matrix

use

    >>> Matrix([[1, -1], [3, 4], [0, 2]])
    ⎡1  -1⎤
    ⎢     ⎥
    ⎢3  4 ⎥
    ⎢     ⎥
    ⎣0  2 ⎦

In Julia:

• In Julia, the Matrix constructor is not exported, so must be qualified. Here we avoid the vector of row vectors above:

julia> sympy.Matrix([1 -1; 3 4; 0 2])
3×2 Array{Sym,2}:
 1  -1
 3   4
 0   2

However, through the magic of PyCall, such matrices are converted into Julia matrices, of type Array{Sym}, so the familiar matrix operations for Julia users are available.

In fact, the above could be done in the more Julian manner through

julia> Sym[1 -1; 3 4; 0 2]
3×2 Array{Sym,2}:
 1  -1
 3   4
 0   2

using an annotation to ensure the type. Alternatively, through promotion, just a single symbolic object will result in the same:

julia> [Sym(1) -1; 3 4; 0 2]
3×2 Array{Sym,2}:
 1  -1
 3   4
 0   2

To make it easy to make column vectors, a list of elements is considered to be a column vector.

    >>> Matrix([1, 2, 3])
    ⎡1⎤
    ⎢ ⎥
    ⎢2⎥
    ⎢ ⎥
    ⎣3⎦

In Julia:

For this use, sympy.Matrix does work, but again its usage is discouraged:

julia> sympy.Matrix([1, 2, 3])
3×1 Array{Sym,2}:
 1
 2
 3

• Again, this is converted into a Vector{Sym} object or entered directly:

julia> Sym[1,2,3]
3-element Array{Sym,1}:
 1
 2
 3

Matrices are manipulated just like any other object in SymPy or Python.

    >>> M = Matrix([[1, 2, 3], [3, 2, 1]])
    >>> N = Matrix([0, 1, 1])
    >>> M*N
    ⎡5⎤
    ⎢ ⎥
    ⎣3⎦

In Julia:

• In Julia, matrices are just matrices, and inherit all of the operations defined on them:

julia> M = Sym[1 2 3; 3 2 1]
2×3 Array{Sym,2}:
 1  2  3
 3  2  1

julia> N = Sym[0, 1, 1]
3-element Array{Sym,1}:
 0
 1
 1

julia> M*N
2-element Array{Sym,1}:
 5
 3

One important thing to note about SymPy matrices is that, unlike every other object in SymPy, they are mutable. This means that they can be modified in place, as we will see below. The downside to this is that Matrix cannot be used in places that require immutability, such as inside other SymPy expressions or as keys to dictionaries. If you need an immutable version of Matrix, use ImmutableMatrix.

In Julia:

A distinction is made between ImmutableMatrix and a mutable one. Mutable ones are mapped to Julia arrays, immutable ones are left as a symbolic object of type SymMatrix. The usual infix mathematical operations (but not dot broadcasting), 0-based indexing, and dot call syntax for methods maay be used with these objects.

Basic Operations

Shape

Here are some basic operations on Matrix. To get the shape of a matrix use shape

    >>> M = Matrix([[1, 2, 3], [-2, 0, 4]])
    >>> M
    ⎡1   2  3⎤
    ⎢        ⎥
    ⎣-2  0  4⎦
    >>> M.shape
    (2, 3)

In Julia:

julia> M = Sym[1 2 3; -2 0 4]
2×3 Array{Sym,2}:
  1  2  3
 -2  0  4

julia> M.shape
(2, 3)

Or, the Julian counterpart:

julia> size(M)
(2, 3)

Accessing Rows and Columns

To get an individual row or column of a matrix, use row or col. For example, M.row(0) will get the first row. M.col(-1) will get the last column.

    >>> M.row(0)
    [1  2  3]
    >>> M.col(-1)
    ⎡3⎤
    ⎢ ⎥
    ⎣4⎦

In Julia:

• these 0-based operations are supported:

julia> M.row(0)
1×3 Array{Sym,2}:
 1  2  3

julia> M.col(-1)
2×1 Array{Sym,2}:
 3
 4

The more familiar counterparts would be:

julia> M[1,:], M[:, end]
(Sym[1, 2, 3], Sym[3, 4])

Deleting and Inserting Rows and Columns

To delete a row or column, use row_del or col_del. These operations will modify the Matrix in place.

    >>> M.col_del(0)
    >>> M
    ⎡2  3⎤
    ⎢    ⎥
    ⎣0  4⎦
    >>> M.row_del(1)
    >>> M
    [2  3]

In Julia:

These methods do not work on Array{Sym} objects, use Julia's indexing notation to remove a row or column.

However, these methods do work on the ImmutableMatrix class:

julia> M = sympy.ImmutableMatrix([1 2 3; -2 0 4])  # avoid vector of row vector construction
⎡1   2  3⎤
⎢        ⎥
⎣-2  0  4⎦

julia> M.col_del(0)
⎡2  3⎤
⎢    ⎥
⎣0  4⎦

julia> M.row_del(1)
[1  2  3]

julia> @vars x
(x,)

julia> sympy.ImmutableMatrix([x 1;  1  x])
⎡x  1⎤
⎢    ⎥
⎣1  x⎦

To insert rows or columns, use row_insert or col_insert. These operations do not operate in place.

    >>> M
    [2  3]
    >>> M = M.row_insert(1, Matrix([[0, 4]]))
    >>> M
    ⎡2  3⎤
    ⎢    ⎥
    ⎣0  4⎦
    >>> M = M.col_insert(0, Matrix([1, -2]))
    >>> M
    ⎡1   2  3⎤
    ⎢        ⎥
    ⎣-2  0  4⎦

In Julia:

julia> M = sympy.ImmutableMatrix([2 3])
[2  3]

julia> M = M.row_insert(1, Sym[0 4])
⎡2  3⎤
⎢    ⎥
⎣0  4⎦

julia> M = M.col_insert(0, Sym[1, -2])
⎡1   2  3⎤
⎢        ⎥
⎣-2  0  4⎦

Unless explicitly stated, the methods mentioned below do not operate in place. In general, a method that does not operate in place will return a new Matrix and a method that does operate in place will return None.

In Julia

This would be the case for the immutable matrices.

Basic Methods

As noted above, simple operations like addition and multiplication are done just by using +, *, and **. To find the inverse of a matrix, just raise it to the -1 power.

    >>> M = Matrix([[1, 3], [-2, 3]])
    >>> N = Matrix([[0, 3], [0, 7]])
    >>> M + N
    ⎡1   6 ⎤
    ⎢      ⎥
    ⎣-2  10⎦
    >>> M*N
    ⎡0  24⎤
    ⎢     ⎥
    ⎣0  15⎦
    >>> 3*M
    ⎡3   9⎤
    ⎢     ⎥
    ⎣-6  9⎦
    >>> M**2
    ⎡-5  12⎤
    ⎢      ⎥
    ⎣-8  3 ⎦
    >>> M**-1
    ⎡1/3  -1/3⎤
    ⎢         ⎥
    ⎣2/9  1/9 ⎦
    >>> N**-1
    Traceback (most recent call last):
    ...
    ValueError: Matrix det == 0; not invertible.

In Julia:

In Julia, we use M1 instead of N, an exported symbol of SymPy. Otherise, it all looks similar:

julia> M = Sym[1 3; -2 3]
2×2 Array{Sym,2}:
  1  3
 -2  3

julia> M1 = Sym[0 3; 0 7]
2×2 Array{Sym,2}:
 0  3
 0  7

julia> M + M1
2×2 Array{Sym,2}:
  1   6
 -2  10

julia> M*M1
2×2 Array{Sym,2}:
 0  24
 0  15

julia> 3*M
2×2 Array{Sym,2}:
  3  9
 -6  9

julia> M^2
2×2 Array{Sym,2}:
 -5  12
 -8   3

julia> M^-1
2×2 Array{Sym,2}:
 1/3  -1/3
 2/9   1/9

Attempting to find the inverse of M1 will error (we suppress its lengthy output)

julia> using Test

julia> @test_throws  Exception M1^-1
Test Passed
      Thrown: PyCall.PyError

The above (except for the inverses) are using generic Julia definitions. For immutable matrices, we would have:

julia> M = sympy.ImmutableMatrix([1 3; -2 3])
⎡1   3⎤
⎢     ⎥
⎣-2  3⎦

julia> M1 = sympy.ImmutableMatrix([0 3; 0 7])
⎡0  3⎤
⎢    ⎥
⎣0  7⎦

julia> M + M1
⎡1   6 ⎤
⎢      ⎥
⎣-2  10⎦

julia> M*M1
⎡0  24⎤
⎢     ⎥
⎣0  15⎦

julia> 3*M
⎡3   9⎤
⎢     ⎥
⎣-6  9⎦

julia> M^2
         2
2

julia> M^-1
⎡1/3  -1/3⎤
⎢         ⎥
⎣2/9  1/9 ⎦

Similarly, M1^(-1) would yield an error for the non-invertible matrix

• There is no broadcasting defined for the SymMatrix type.

To take the transpose of a Matrix, use T.

    >>> M = Matrix([[1, 2, 3], [4, 5, 6]])
    >>> M
    ⎡1  2  3⎤
    ⎢       ⎥
    ⎣4  5  6⎦
    >>> M.T
    ⎡1  4⎤
    ⎢    ⎥
    ⎢2  5⎥
    ⎢    ⎥
    ⎣3  6⎦

In Julia:

julia> M = Sym[1 2 3; 4 5 6]
2×3 Array{Sym,2}:
 1  2  3
 4  5  6

julia> M.T
3×2 Array{Sym,2}:
 1  4
 2  5
 3  6

Matrix Constructors

Several constructors exist for creating common matrices. To create an identity matrix, use eye. The command eye(n) will create an n x n identity matrix:

    >>> eye(3)
    ⎡1  0  0⎤
    ⎢       ⎥
    ⎢0  1  0⎥
    ⎢       ⎥
    ⎣0  0  1⎦
    >>> eye(4)
    ⎡1  0  0  0⎤
    ⎢          ⎥
    ⎢0  1  0  0⎥
    ⎢          ⎥
    ⎢0  0  1  0⎥
    ⎢          ⎥
    ⎣0  0  0  1⎦

In Julia:

• eye is not exported so must qualified:

julia> sympy.eye(3)
3×3 Array{Sym,2}:
 1  0  0
 0  1  0
 0  0  1

julia> sympy.eye(4)
4×4 Array{Sym,2}:
 1  0  0  0
 0  1  0  0
 0  0  1  0
 0  0  0  1

To create a matrix of all zeros, use zeros. zeros(n, m) creates an n x m matrix of 0s.

    >>> zeros(2, 3)
    ⎡0  0  0⎤
    ⎢       ⎥
    ⎣0  0  0⎦

In Julia:

• zeros is extended but the method expects a symbolic first argument. Either qualify it:

julia> sympy.zeros(2, 3)
2×3 Array{Sym,2}:
 0  0  0
 0  0  0

or create a symbolic first value:

julia> zeros(Sym(2), 3)
2×3 Array{Sym,2}:
 0  0  0
 0  0  0

or use the Julia constructor:

julia> zeros(Sym, 2, 3)
2×3 Array{Sym,2}:
 0  0  0
 0  0  0

Similarly, ones creates a matrix of ones.

    >>> ones(3, 2)
    ⎡1  1⎤
    ⎢    ⎥
    ⎢1  1⎥
    ⎢    ⎥
    ⎣1  1⎦

In Julia:

• Similarly with ones:

julia> sympy.ones(3, 2)
3×2 Array{Sym,2}:
 1  1
 1  1
 1  1

To create diagonal matrices, use diag. The arguments to diag can be either numbers or matrices. A number is interpreted as a 1 x 1 matrix. The matrices are stacked diagonally. The remaining elements are filled with 0\ s.

    >>> diag(1, 2, 3)
    ⎡1  0  0⎤
    ⎢       ⎥
    ⎢0  2  0⎥
    ⎢       ⎥
    ⎣0  0  3⎦
    >>> diag(-1, ones(2, 2), Matrix([5, 7, 5]))
    ⎡-1  0  0  0⎤
    ⎢           ⎥
    ⎢0   1  1  0⎥
    ⎢           ⎥
    ⎢0   1  1  0⎥
    ⎢           ⎥
    ⎢0   0  0  5⎥
    ⎢           ⎥
    ⎢0   0  0  7⎥
    ⎢           ⎥
    ⎣0   0  0  5⎦

In Julia:

• similarly with diag:

julia> sympy.diag(1, 2, 3)
3×3 Array{Sym,2}:
 1  0  0
 0  2  0
 0  0  3

julia> sympy.diag(-1, sympy.ones(2, 2), sympy.Matrix([5, 7, 5]))
6×4 Array{Sym,2}:
 -1  0  0  0
  0  1  1  0
  0  1  1  0
  0  0  0  5
  0  0  0  7
  0  0  0  5

• The first one, could also use Julia's diagm function from the LinearAlgebra package:

julia> diagm(0 => Sym[1,2,3])
3×3 Array{Sym,2}:
 1  0  0
 0  2  0
 0  0  3

Advanced Methods

Determinant

To compute the determinant of a matrix, use det.

    >>> M = Matrix([[1, 0, 1], [2, -1, 3], [4, 3, 2]])
    >>> M
    ⎡1  0   1⎤
    ⎢        ⎥
    ⎢2  -1  3⎥
    ⎢        ⎥
    ⎣4  3   2⎦
    >>> M.det()
    -1

In Julia:

julia> M = Sym[1 0 1; 2 -1 3; 4 3 2]
3×3 Array{Sym,2}:
 1   0  1
 2  -1  3
 4   3  2

julia> M.det()
-1

Let

julia> @vars x
(x,)

julia> A = Sym[x 1; 1 x]
2×2 Array{Sym,2}:
 x  1
 1  x

The method for det falls back the sympy method:

julia> det(A)
 2
x  - 1

There is no reason to, but generic Julia methods could be used:

julia> out  = lu(A)
LU{Sym,Array{Sym,2}}
L factor:
2×2 Array{Sym,2}:
 1  0
 x  1
U factor:
2×2 Array{Sym,2}:
 1        x
 0  1 - x^2

julia> prod(diag(out.L)) * prod(diag(out.U))
     2
1 - x 

RREF

To put a matrix into reduced row echelon form, use rref. rref returns a tuple of two elements. The first is the reduced row echelon form, and the second is a tuple of indices of the pivot columns.

    >>> M = Matrix([[1, 0, 1, 3], [2, 3, 4, 7], [-1, -3, -3, -4]])
    >>> M
    ⎡1   0   1   3 ⎤
    ⎢              ⎥
    ⎢2   3   4   7 ⎥
    ⎢              ⎥
    ⎣-1  -3  -3  -4⎦
    >>> M.rref()
    ⎛⎡1  0   1    3 ⎤        ⎞
    ⎜⎢              ⎥        ⎟
    ⎜⎢0  1  2/3  1/3⎥, (0, 1)⎟
    ⎜⎢              ⎥        ⎟
    ⎝⎣0  0   0    0 ⎦        ⎠

In Julia:

julia> M = Sym[1 0 1 3; 2 3 4 7; -1 -3 -3 -4]
3×4 Array{Sym,2}:
  1   0   1   3
  2   3   4   7
 -1  -3  -3  -4

julia> M.rref()
(Sym[1 0 1 3; 0 1 2/3 1/3; 0 0 0 0], (0, 1))

Nullspace

To find the nullspace of a matrix, use nullspace. nullspace returns a list of column vectors that span the nullspace of the matrix.

    >>> M = Matrix([[1, 2, 3, 0, 0], [4, 10, 0, 0, 1]])
    >>> M
    ⎡1  2   3  0  0⎤
    ⎢              ⎥
    ⎣4  10  0  0  1⎦
    >>> M.nullspace()
    ⎡⎡-15⎤  ⎡0⎤  ⎡ 1  ⎤⎤
    ⎢⎢   ⎥  ⎢ ⎥  ⎢    ⎥⎥
    ⎢⎢ 6 ⎥  ⎢0⎥  ⎢-1/2⎥⎥
    ⎢⎢   ⎥  ⎢ ⎥  ⎢    ⎥⎥
    ⎢⎢ 1 ⎥, ⎢0⎥, ⎢ 0  ⎥⎥
    ⎢⎢   ⎥  ⎢ ⎥  ⎢    ⎥⎥
    ⎢⎢ 0 ⎥  ⎢1⎥  ⎢ 0  ⎥⎥
    ⎢⎢   ⎥  ⎢ ⎥  ⎢    ⎥⎥
    ⎣⎣ 0 ⎦  ⎣0⎦  ⎣ 1  ⎦⎦

In Julia:

• the list is mapped to an array of vectors, otherwise this is identical:

julia> M = Sym[1 2 3 0 0; 4 10 0 0 1]
2×5 Array{Sym,2}:
 1   2  3  0  0
 4  10  0  0  1

julia> M.nullspace()
3-element Array{Array{Sym,2},1}:
 [-15; 6; … ; 0; 0]
 [0; 0; … ; 1; 0]
 [1; -1/2; … ; 0; 1]

Columnspace

To find the columnspace of a matrix, use columnspace. columnspace returns a list of column vectors that span the columnspace of the matrix.

    >>> M = Matrix([[1, 1, 2], [2 ,1 , 3], [3 , 1, 4]])
    >>> M
    ⎡1  1  2⎤
    ⎢       ⎥
    ⎢2  1  3⎥
    ⎢       ⎥
    ⎣3  1  4⎦
    >>> M.columnspace()
    ⎡⎡1⎤  ⎡1⎤⎤
    ⎢⎢ ⎥  ⎢ ⎥⎥
    ⎢⎢2⎥, ⎢1⎥⎥
    ⎢⎢ ⎥  ⎢ ⎥⎥
    ⎣⎣3⎦  ⎣1⎦⎦

In Julia:

• as with nullspace, the return value is a vector of vectors:

julia> M = Sym[1 1 2; 2 1 3; 3 1 4]
3×3 Array{Sym,2}:
 1  1  2
 2  1  3
 3  1  4

julia> M.columnspace()
2-element Array{Array{Sym,2},1}:
 [1; 2; 3]
 [1; 1; 1]

Eigenvalues, Eigenvectors, and Diagonalization

To find the eigenvalues of a matrix, use eigenvals. eigenvals returns a dictionary of eigenvalue:algebraic multiplicity pairs (similar to the output of :ref:roots <tutorial-roots>).

    >>> M = Matrix([[3, -2,  4, -2], [5,  3, -3, -2], [5, -2,  2, -2], [5, -2, -3,  3]])
    >>> M
    ⎡3  -2  4   -2⎤
    ⎢             ⎥
    ⎢5  3   -3  -2⎥
    ⎢             ⎥
    ⎢5  -2  2   -2⎥
    ⎢             ⎥
    ⎣5  -2  -3  3 ⎦
    >>> M.eigenvals()
    {-2: 1, 3: 1, 5: 2}

In Julia:

julia> M = Sym[3 -2  4 -2; 5  3 -3 -2; 5 -2  2 -2; 5 -2 -3  3]
4×4 Array{Sym,2}:
 3  -2   4  -2
 5   3  -3  -2
 5  -2   2  -2
 5  -2  -3   3

julia> M.eigenvals()
Dict{Any,Any} with 3 entries:
  3  => 1
  -2 => 1
  5  => 2

This means that M has eigenvalues -2, 3, and 5, and that the eigenvalues -2 and 3 have algebraic multiplicity 1 and that the eigenvalue 5 has algebraic multiplicity 2.

To find the eigenvectors of a matrix, use eigenvects. eigenvects returns a list of tuples of the form (eigenvalue:algebraic multiplicity, [eigenvectors]).

    >>> M.eigenvects()
    ⎡⎛       ⎡⎡0⎤⎤⎞  ⎛      ⎡⎡1⎤⎤⎞  ⎛      ⎡⎡1⎤  ⎡0 ⎤⎤⎞⎤
    ⎢⎜       ⎢⎢ ⎥⎥⎟  ⎜      ⎢⎢ ⎥⎥⎟  ⎜      ⎢⎢ ⎥  ⎢  ⎥⎥⎟⎥
    ⎢⎜       ⎢⎢1⎥⎥⎟  ⎜      ⎢⎢1⎥⎥⎟  ⎜      ⎢⎢1⎥  ⎢-1⎥⎥⎟⎥
    ⎢⎜-2, 1, ⎢⎢ ⎥⎥⎟, ⎜3, 1, ⎢⎢ ⎥⎥⎟, ⎜5, 2, ⎢⎢ ⎥, ⎢  ⎥⎥⎟⎥
    ⎢⎜       ⎢⎢1⎥⎥⎟  ⎜      ⎢⎢1⎥⎥⎟  ⎜      ⎢⎢1⎥  ⎢0 ⎥⎥⎟⎥
    ⎢⎜       ⎢⎢ ⎥⎥⎟  ⎜      ⎢⎢ ⎥⎥⎟  ⎜      ⎢⎢ ⎥  ⎢  ⎥⎥⎟⎥
    ⎣⎝       ⎣⎣1⎦⎦⎠  ⎝      ⎣⎣1⎦⎦⎠  ⎝      ⎣⎣0⎦  ⎣1 ⎦⎦⎠⎦

In Julia:

• the output is less than desirable, as there is no special show method

• the eigvals and eigvecs methods present the output in the manner that Julia's generic functions do:

julia> M.eigenvects()
3-element Array{Tuple{Sym,Int64,Array{Array{Sym,2},1}},1}:
 (-2, 1, [[0; 1; 1; 1]])
 (3, 1, [[1; 1; 1; 1]])
 (5, 2, [[1; 1; 1; 0], [0; -1; 0; 1]])

compare with

julia> eigvecs(M)
4×4 Array{Sym,2}:
 0  1  1   0
 1  1  1  -1
 1  1  1   0
 1  1  0   1

This shows us that, for example, the eigenvalue 5 also has geometric multiplicity 2, because it has two eigenvectors. Because the algebraic and geometric multiplicities are the same for all the eigenvalues, M is diagonalizable.

To diagonalize a matrix, use diagonalize. diagonalize returns a tuple (P, D), where D is diagonal and M = PDP^{-1}.

    >>> P, D = M.diagonalize()
    >>> P
    ⎡0  1  1  0 ⎤
    ⎢           ⎥
    ⎢1  1  1  -1⎥
    ⎢           ⎥
    ⎢1  1  1  0 ⎥
    ⎢           ⎥
    ⎣1  1  0  1 ⎦
    >>> D
    ⎡-2  0  0  0⎤
    ⎢           ⎥
    ⎢0   3  0  0⎥
    ⎢           ⎥
    ⎢0   0  5  0⎥
    ⎢           ⎥
    ⎣0   0  0  5⎦
    >>> P*D*P**-1
    ⎡3  -2  4   -2⎤
    ⎢             ⎥
    ⎢5  3   -3  -2⎥
    ⎢             ⎥
    ⎢5  -2  2   -2⎥
    ⎢             ⎥
    ⎣5  -2  -3  3 ⎦
    >>> P*D*P**-1 == M
    True

In Julia:

julia> P, D = M.diagonalize()
(Sym[0 1 1 0; 1 1 1 -1; 1 1 1 0; 1 1 0 1], Sym[-2 0 0 0; 0 3 0 0; 0 0 5 0; 0 0 0 5])

julia> P
4×4 Array{Sym,2}:
 0  1  1   0
 1  1  1  -1
 1  1  1   0
 1  1  0   1

julia> D
4×4 Array{Sym,2}:
 -2  0  0  0
  0  3  0  0
  0  0  5  0
  0  0  0  5

julia> P*D*P^-1
4×4 Array{Sym,2}:
 3  -2   4  -2
 5   3  -3  -2
 5  -2   2  -2
 5  -2  -3   3

julia> P*D*P^-1 == M
true

As lambda is a reserved keyword in Python, so to create a Symbol called λ, while using the same names for SymPy Symbols and Python variables, use lamda (without the b). It will still pretty print as λ.

Note that since eigenvects also includes the eigenvalues, you should use it instead of eigenvals if you also want the eigenvectors. However, as computing the eigenvectors may often be costly, eigenvals should be preferred if you only wish to find the eigenvalues.

If all you want is the characteristic polynomial, use charpoly. This is more efficient than eigenvals, because sometimes symbolic roots can be expensive to calculate.

    >>> lamda = symbols('lamda')
    >>> p = M.charpoly(lamda)
    >>> factor(p)
           2
    (λ - 5) ⋅(λ - 3)⋅(λ + 2)

In Julia:

• note missing b is not needed with Julia:

julia> lambda = symbols("lambda")
λ

julia> p = M.charpoly(lambda)
PurePoly(lambda**4 - 11*lambda**3 + 29*lambda**2 + 35*lambda - 150, lambda, domain='ZZ')

julia> factor(p) |>  string
"(lambda - 5)^2*(lambda - 3)*(lambda + 2)"

Possible Issues

Zero Testing

If your matrix operations are failing or returning wrong answers, the common reasons would likely be from zero testing. If there is an expression not properly zero-tested, it can possibly bring issues in finding pivots for gaussian elimination, or deciding whether the matrix is inversible, or any high level functions which relies on the prior procedures.

Currently, the SymPy's default method of zero testing _iszero is only guaranteed to be accurate in some limited domain of numerics and symbols, and any complicated expressions beyond its decidability are treated as None, which behaves similarly to logical False.

The list of methods using zero testing procedures are as followings.

echelon_form , is_echelon , rank , rref , nullspace , eigenvects , inverse_ADJ , inverse_GE , inverse_LU , LUdecomposition , LUdecomposition_Simple , LUsolve

They have property iszerofunc opened up for user to specify zero testing method, which can accept any function with single input and boolean output, while being defaulted with _iszero.

Here is an example of solving an issue caused by undertested zero. [#zerotestexampleidea-fn]_ [#zerotestexamplediscovery-fn]_

    >>> from sympy import *
    >>> q = Symbol("q", positive = True)
    >>> m = Matrix([
    ... [-2*cosh(q/3),      exp(-q),            1],
    ... [      exp(q), -2*cosh(q/3),            1],
    ... [           1,            1, -2*cosh(q/3)]])
    >>> m.nullspace()
    []

In Julia:

julia> q = sympy.Symbol("q", positive = true)
q

julia> m = Sym[-2*cosh(q/3) exp(-q) 1; exp(q) -2*cosh(q/3) 1; 1 1 -2*cosh(q/3)]
3×3 Array{Sym,2}:
 -2*cosh(q/3)       exp(-q)             1
       exp(q)  -2*cosh(q/3)             1
            1             1  -2*cosh(q/3) 

julia> m.nullspace()
0-element Array{Any,1}

You can trace down which expression is being underevaluated, by injecting a custom zero test with warnings enabled.

    >>> import warnings
    >>>
    >>> def my_iszero(x):
    ...     try:
    ...         result = x.is_zero
    ...     except AttributeError:
    ...         result = None
    ...
    ...     # Warnings if evaluated into None
    ...     if result == None:
    ...         warnings.warn("Zero testing of {} evaluated into {}".format(x, result))
    ...     return result
    ...
    >>> m.nullspace(iszerofunc=my_iszero) # doctest: +SKIP
    __main__:9: UserWarning: Zero testing of 4*cosh(q/3)**2 - 1 evaluated into None
    __main__:9: UserWarning: Zero testing of (-exp(q) - 2*cosh(q/3))*(-2*cosh(q/3) - exp(-q)) - (4*cosh(q/3)**2 - 1)**2 evaluated into None
    __main__:9: UserWarning: Zero testing of 2*exp(q)*cosh(q/3) - 16*cosh(q/3)**4 + 12*cosh(q/3)**2 + 2*exp(-q)*cosh(q/3) evaluated into None
    __main__:9: UserWarning: Zero testing of -(4*cosh(q/3)**2 - 1)*exp(-q) - 2*cosh(q/3) - exp(-q) evaluated into None
    []

In Julia:

Is this available??

In this case, (-exp(q) - 2*cosh(q/3))*(-2*cosh(q/3) - exp(-q)) - (4*cosh(q/3)**2 - 1)**2 should yield zero, but the zero testing had failed to catch. possibly meaning that a stronger zero test should be introduced. For this specific example, rewriting to exponentials and applying simplify would make zero test stronger for hyperbolics, while being harmless to other polynomials or transcendental functions.

    >>> def my_iszero(x):
    ...     try:
    ...         result = x.rewrite(exp).simplify().is_zero
    ...     except AttributeError:
    ...         result = None
    ...
    ...     # Warnings if evaluated into None
    ...     if result == None:
    ...         warnings.warn("Zero testing of {} evaluated into {}".format(x, result))
    ...     return result
    ...
    >>> m.nullspace(iszerofunc=my_iszero) # doctest: +SKIP
    __main__:9: UserWarning: Zero testing of -2*cosh(q/3) - exp(-q) evaluated into None
    ⎡⎡  ⎛   q         ⎛q⎞⎞  -q         2⎛q⎞    ⎤⎤
    ⎢⎢- ⎜- ℯ  - 2⋅cosh⎜─⎟⎟⋅ℯ   + 4⋅cosh ⎜─⎟ - 1⎥⎥
    ⎢⎢  ⎝             ⎝3⎠⎠              ⎝3⎠    ⎥⎥
    ⎢⎢─────────────────────────────────────────⎥⎥
    ⎢⎢          ⎛      2⎛q⎞    ⎞     ⎛q⎞       ⎥⎥
    ⎢⎢        2⋅⎜4⋅cosh ⎜─⎟ - 1⎟⋅cosh⎜─⎟       ⎥⎥
    ⎢⎢          ⎝       ⎝3⎠    ⎠     ⎝3⎠       ⎥⎥
    ⎢⎢                                         ⎥⎥
    ⎢⎢           ⎛   q         ⎛q⎞⎞            ⎥⎥
    ⎢⎢          -⎜- ℯ  - 2⋅cosh⎜─⎟⎟            ⎥⎥
    ⎢⎢           ⎝             ⎝3⎠⎠            ⎥⎥
    ⎢⎢          ────────────────────           ⎥⎥
    ⎢⎢                   2⎛q⎞                  ⎥⎥
    ⎢⎢             4⋅cosh ⎜─⎟ - 1              ⎥⎥
    ⎢⎢                    ⎝3⎠                  ⎥⎥
    ⎢⎢                                         ⎥⎥
    ⎣⎣                    1                    ⎦⎦

In Julia:

Is this available?

You can clearly see nullspace returning proper result, after injecting an alternative zero test.

Note that this approach is only valid for some limited cases of matrices containing only numerics, hyperbolics, and exponentials. For other matrices, you should use different method opted for their domains.

Possible suggestions would be either taking advantage of rewriting and simplifying, with tradeoff of speed [#zerotestsimplifysolution-fn]_ , or using random numeric testing, with tradeoff of accuracy [#zerotestnumerictestsolution-fn]_ .

If you wonder why there is no generic algorithm for zero testing that can work with any symbolic entities, it's because of the constant problem stating that zero testing is undecidable [#constantproblemwikilink-fn]_ , and not only the SymPy, but also other computer algebra systems [#mathematicazero-fn]_ [#matlabzero-fn]_ would face the same fundamental issue.

However, discovery of any zero test failings can provide some good examples to improve SymPy, so if you have encountered one, you can report the issue to SymPy issue tracker [#sympyissues-fn]_ to get detailed help from the community.

* [#sympyissues-fn] https://github.com/sympy/sympy/issues