#4762: Odd error message for congruence subgroups
---------------------------+------------------------------------------------
Reporter: ljpk | Owner: craigcitro
Type: defect | Status: new
Priority: minor | Milestone: sage-3.2.2
Component: modular forms | Keywords:
---------------------------+------------------------------------------------
I was looking at generators of various different congruence subgroups, and
got the following error:
{{{
sage: Gamma0(5).generators()[0]
[1 1]
[0 1]
sage: Gamma0(5).generators()[0] in Gamma0(7)
---------------------------------------------------------------------------
ValueError Traceback (most recent call
last)
/home/ljpk/<ipython console> in <module>()
/home/was/s/local/lib/python2.5/site-packages/sage/groups/group.so in
sage.groups.group.Group.__contains__ (sage/groups/group.c:1034)()
/home/was/s/local/lib/python2.5/site-packages/sage/modular/congroup.pyc in
__call__(self, x, check)
1654 if isinstance(x, CongruenceSubgroupElement) and x.parent()
== self:
1655 return x
-> 1656 x = CongruenceSubgroupElement(self, x, check=check)
1657 if not check:
1658 return x
/home/was/s/local/lib/python2.5/site-
packages/sage/modular/congroup_element.pyc in __init__(self, parent, x,
check)
46 if not congroup.is_CongruenceSubgroup(parent):
47 raise TypeError, "parent (= %s) must be a
congruence subgroup"%parent
---> 48 x = M2Z(x)
49 if x.determinant() != 1:
50 raise ValueError, "matrix must have determinant 1"
/home/was/s/local/lib/python2.5/site-packages/sage/matrix/matrix_space.pyc
in __call__(self, entries, coerce, copy, rows)
306 entries = 0
307
--> 308 if entries == 0 and hasattr(self, '__zero_matrix'):
309 return self.zero_matrix()
310
/home/was/s/local/lib/python2.5/site-packages/sage/structure/element.so in
sage.structure.element.Element.__richcmp__
(sage/structure/element.c:5247)()
/home/was/s/local/lib/python2.5/site-packages/sage/structure/element.so in
sage.structure.element.Element._richcmp (sage/structure/element.c:4954)()
/home/was/s/local/lib/python2.5/site-packages/sage/modular/congroup.pyc in
__call__(self, x, check)
1654 if isinstance(x, CongruenceSubgroupElement) and x.parent()
== self:
1655 return x
-> 1656 x = CongruenceSubgroupElement(self, x, check=check)
1657 if not check:
1658 return x
/home/was/s/local/lib/python2.5/site-
packages/sage/modular/congroup_element.pyc in __init__(self, parent, x,
check)
48 x = M2Z(x)
49 if x.determinant() != 1:
---> 50 raise ValueError, "matrix must have determinant 1"
51 x.set_immutable()
52
ValueError: matrix must have determinant 1
}}}
It might be correct not to allow coercions from one congruence subgroup to
another, but the matrix *does* have determinant 1, so the error message
should be changed to one that is more suitable.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/4762>
Sage <http://sagemath.org/>
Sage - Open Source Mathematical Software: Building the Car Instead of
Reinventing the Wheel
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