# Sparse matrices over $$\ZZ/n\ZZ$$ for $$n$$ small¶

This is a compiled implementation of sparse matrices over $$\ZZ/n\ZZ$$ for $$n$$ small.

Todo

move vectors into a Cython vector class - add _add_ and _mul_ methods.

EXAMPLES:

sage: a = matrix(Integers(37),3,3,range(9),sparse=True); a
[0 1 2]
[3 4 5]
[6 7 8]
sage: type(a)
<type 'sage.matrix.matrix_modn_sparse.Matrix_modn_sparse'>
sage: parent(a)
Full MatrixSpace of 3 by 3 sparse matrices over Ring of integers modulo 37
sage: a^2
[15 18 21]
[ 5 17 29]
[32 16  0]
sage: a+a
[ 0  2  4]
[ 6  8 10]
[12 14 16]
sage: b = a.new_matrix(2,3,range(6)); b
[0 1 2]
[3 4 5]
sage: a*b
Traceback (most recent call last):
...
TypeError: unsupported operand parent(s) for *: 'Full MatrixSpace of 3 by 3 sparse matrices over Ring of integers modulo 37' and 'Full MatrixSpace of 2 by 3 sparse matrices over Ring of integers modulo 37'
sage: b*a
[15 18 21]
[ 5 17 29]

sage: TestSuite(a).run()
sage: TestSuite(b).run()

sage: a.echelonize(); a
[ 1  0 36]
[ 0  1  2]
[ 0  0  0]
sage: b.echelonize(); b
[ 1  0 36]
[ 0  1  2]
sage: a.pivots()
(0, 1)
sage: b.pivots()
(0, 1)
sage: a.rank()
2
sage: b.rank()
2
sage: a[2,2] = 5
sage: a.rank()
3

class sage.matrix.matrix_modn_sparse.Matrix_modn_sparse

Create a sparse matrix over the integers modulo n.

INPUT:

• parent – a matrix space over the integers modulo n
• entries – see matrix()
• copy – ignored (for backwards compatibility)
• coerce – if False, assume without checking that the entries lie in the base ring
density()

Return the density of self, i.e., the ratio of the number of nonzero entries of self to the total size of self.

EXAMPLES:

sage: A = matrix(QQ,3,3,[0,1,2,3,0,0,6,7,8],sparse=True)
sage: A.density()
2/3


Notice that the density parameter does not ensure the density of a matrix; it is only an upper bound.

sage: A = random_matrix(GF(127),200,200,density=0.3, sparse=True)
sage: A.density()
2073/8000

determinant(algorithm=None)

Return the determinant of this matrix.

INPUT:

• algorithm - either "linbox" (default) or "generic".

EXAMPLES:

sage: A = matrix(GF(3), 4, range(16), sparse=True)
sage: B = identity_matrix(GF(3), 4, sparse=True)
sage: (A + B).det()
2
sage: (A + B).det(algorithm="linbox")
2
sage: (A + B).det(algorithm="generic")
2
sage: (A + B).det(algorithm="hey")
Traceback (most recent call last):
...
ValueError: no algorithm 'hey'

sage: matrix(GF(11), 1, 2, sparse=True).det()
Traceback (most recent call last):
...
ValueError: self must be a square matrix

matrix_from_columns(cols)

Return the matrix constructed from self using columns with indices in the columns list.

EXAMPLES:

sage: M = MatrixSpace(GF(127),3,3,sparse=True)
sage: A = M(range(9)); A
[0 1 2]
[3 4 5]
[6 7 8]
sage: A.matrix_from_columns([2,1])
[2 1]
[5 4]
[8 7]

matrix_from_rows(rows)

Return the matrix constructed from self using rows with indices in the rows list.

INPUT:

• rows - list or tuple of row indices

EXAMPLES:

sage: M = MatrixSpace(GF(127),3,3,sparse=True)
sage: A = M(range(9)); A
[0 1 2]
[3 4 5]
[6 7 8]
sage: A.matrix_from_rows([2,1])
[6 7 8]
[3 4 5]

p
rank(algorithm=None)

Return the rank of this matrix.

INPUT:

• algorithm - either "linbox" (only available for matrices over prime fields) or "generic"

EXAMPLES:

sage: A = matrix(GF(127), 2, 2, sparse=True)
sage: A[0,0] = 34
sage: A[0,1] = 102
sage: A[1,0] = 55
sage: A[1,1] = 74
sage: A.rank()
2

sage: A._clear_cache()
sage: A.rank(algorithm="generic")
2
sage: A._clear_cache()
sage: A.rank(algorithm="hey")
Traceback (most recent call last):
...
ValueError: no algorithm 'hey'


REFERENCES:

Note

For very sparse matrices Gaussian elimination is faster because it barely has anything to do. If the fill in needs to be considered, ‘Symbolic Reordering’ is usually much faster.

swap_rows(r1, r2)
transpose()

Return the transpose of self.

EXAMPLES:

sage: A = matrix(GF(127),3,3,[0,1,0,2,0,0,3,0,0],sparse=True)
sage: A
[0 1 0]
[2 0 0]
[3 0 0]
sage: A.transpose()
[0 2 3]
[1 0 0]
[0 0 0]


.T is a convenient shortcut for the transpose:

sage: A.T
[0 2 3]
[1 0 0]
[0 0 0]