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Fix RegularSequence.subsequence for zero sequence #37283

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Merged
merged 2 commits into from
Feb 25, 2024
Merged

Fix RegularSequence.subsequence for zero sequence #37283

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87218b4
37282: Fix RegularSequence.subsequence for zero sequence
cheuberg Feb 10, 2024
6b3480e
37282: Fix typo
cheuberg Feb 12, 2024
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12 changes: 10 additions & 2 deletions src/sage/combinat/regular_sequence.py
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Original file line number Diff line number Diff line change
Expand Up @@ -762,6 +762,12 @@ def subsequence(self, a, b):

sage: S.regenerated().subsequence(1, -4)
2-regular sequence 0, 0, 0, 0, 1, 3, 6, 9, 12, 18, ...

Check that the zero sequence is handled correctly (issue:`37282`)
::

sage: Seq2.zero().subsequence(1, 1)
2-regular sequence 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, ...
"""
from itertools import chain
from sage.rings.integer_ring import ZZ
Expand Down Expand Up @@ -835,14 +841,16 @@ def matrix_row(r, c):
d, f = rule[r, c]
return [self.mu[f] if d == j else zero_M for j in kernel]

# We explicitly set the ring when creating vectors in order to avoid
# problems with the zero sequence, see issue:`37282`.
result = P.element_class(
P,
{r: Matrix.block([matrix_row(r, c) for c in kernel])
for r in A},
vector(chain.from_iterable(
vector(P.coefficient_ring(), chain.from_iterable(
b.get(c, 0) * self.left
for c in kernel)),
vector(chain.from_iterable(
vector(P.coefficient_ring(), chain.from_iterable(
(self.coefficient_of_n(c, multiply_left=False) if c >= 0 else zero_R)
for c in kernel)))

Expand Down