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Fix kernel basis computation for polynomial matrix in corner case #37208

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merged 5 commits into from
Feb 2, 2024
Merged

Fix kernel basis computation for polynomial matrix in corner case #37208

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3bbf8aa
fixing the pex case, but in fact others don't return the correct erro…
vneiger Jan 30, 2024
fedfcdc
fix in other similar places
vneiger Jan 30, 2024
29f8555
fix d <= 0 -> d < 0
vneiger Jan 30, 2024
9f15f69
fix error message in related reverse method for univariate polynomial…
vneiger Jan 31, 2024
f503b52
add test
vneiger Jan 31, 2024
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15 changes: 11 additions & 4 deletions src/sage/matrix/matrix_polynomial_dense.pyx
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Original file line number Diff line number Diff line change
Expand Up @@ -3980,6 +3980,13 @@ cdef class Matrix_polynomial_dense(Matrix_generic_dense):
sage: Matrix(pR, 3, 2, [[x,0],[1,0],[x+1,0]]).minimal_kernel_basis()
[6 x 0]
[6 6 1]

TESTS:

We check that PR #37208 is fixed::

sage: Matrix(pR, 2, 0).minimal_kernel_basis().is_sparse()
False
"""
from sage.matrix.constructor import matrix

Expand All @@ -4001,11 +4008,11 @@ cdef class Matrix_polynomial_dense(Matrix_generic_dense):
return matrix(self.base_ring(), 0, m)

if n == 0: # early exit: kernel is identity
return matrix.identity(self.base_ring(), m, m)
return matrix.identity(self.base_ring(), m)
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d = self.degree() # well defined since m > 0 and n > 0
if d == -1: # matrix is zero: kernel is identity
return matrix.identity(self.base_ring(), m, m)
return matrix.identity(self.base_ring(), m)

# degree bounds on the kernel basis
degree_bound = min(m,n)*d+max(shifts)
Expand Down Expand Up @@ -4040,11 +4047,11 @@ cdef class Matrix_polynomial_dense(Matrix_generic_dense):
return matrix(self.base_ring(), n, 0)

if m == 0: # early exit: kernel is identity
return matrix.identity(self.base_ring(), n, n)
return matrix.identity(self.base_ring(), n)

d = self.degree() # well defined since m > 0 and n > 0
if d == -1: # matrix is zero
return matrix.identity(self.base_ring(), n, n)
return matrix.identity(self.base_ring(), n)

# degree bounds on the kernel basis
degree_bound = min(m,n)*d+max(shifts)
Expand Down
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