Segfault in braid group centralizer calculation

[Tracy Hall]

Attempting to calculate the centralizer of a particular element of the braid group on 5 strands causes a segmentation fault.

sage: BG = BraidGroup(5); BG
Braid group on 5 strands

sage: b = BG([3, 3, 4, 3, 3, 2, 1, 4, 3, 2]); b
s2^2*s3*s2^2*s1*s0*s3*s2*s1

sage: b.left_normal_form()
(1, s2*s3*s2, s2*s1*s0*s3*s2*s1*s0)

sage: b.ultra_summit_set()
[[s0*s1*s0*s2*s1*s0*s3*s2*s1*s0]]

sage: b.is_pseudoanosov()
False

sage: b.centralizer()
---------------------------------------------------------------------------
SignalError                               Traceback (most recent call last)
Cell In [7], line 1
----> 1 b.centralizer()
File /ext/sage/9.8/src/sage/groups/braid.py:1546, in Braid.centralizer(self)
   1534 def centralizer(self):
   1535     """
   1536     Return a list of generators of the centralizer of the braid.
   1537
   (...)
   1544
   1545     """
-> 1546     l = centralizer(self)
   1547     B = self.parent()
   1548     return [B._element_from_libbraiding(b) for b in l]
File /ext/sage/9.8/src/sage/libs/braiding.pyx:233, in sage.libs.braiding.centralizer()
    231         return [[[0], [i+1, nstrands - i -1]] for i in range(nstrands//2-1)] + [[[0], [nstrands//2]]]
    232 l = braid.Tietze()
--> 233 sig_on()
    234 cdef list[list[list[int]]] rop = CentralizerGenerators(nstrands, l)
    235 sig_off()
SignalError: Segmentation fault

[Tracy Hall]

(error present at least from SageMath 8.2 until SageMath 9.8)

[Dima Pasechnik]

On Mon, Apr 17, 2023 at 7:40 AM Tracy Hall <h.t...@gmail.com> wrote:

(error present at least from SageMath 8.2 until SageMath 9.8)

in 10.0.beta9 one gets

sage: BG = BraidGroup(5)

sage: b = BG([3, 3, 4, 3, 3, 2, 1, 4, 3, 2]); b
s2^2*s3*s2^2*s1*s0*s3*s2*s1

sage: b.centralizer()
double free or corruption (out)

(and then it hangs)

 

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[Dima Pasechnik]

filed a bug in https://github.com/sagemath/sage/issues/35529