#11598: Change to is_congruence method of modular subgroups
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Reporter: davidloeffler | Owner: craigcitro
Type: defect | Status: needs_work
Priority: major | Milestone: sage-4.7.2
Component: modular forms | Keywords: modular congruence subgroup
Work_issues: | Upstream: N/A
Reviewer: Vincent Delecroix | Author: David Loeffler
Merged: | Dependencies: #11422
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Changes (by vdelecroix):
* status: needs_review => needs_work
* reviewer: => Vincent Delecroix
Comment:
Replying to [comment:2 davidloeffler]:
I was asking myself what should be the definition of congruence subgroup.
In order to use Wohlfart's theorem it is necessary to use the projective
definition and that's why it was implemented that way. For all standard
congruence groups it makes no difference. Could you explain me a concrete
case where the difference matter ?
In any case, you should add an example of an odd arithmetic subgroup which
is not of congruence but which becomes one when we add the element -id.
Do you know a method, given an even subgroup, to build all odd subgroups
it may come from ?
> Just realised that, since the new algorithm in the odd case is
*extremely* slow, it is better to not call it from the generic test
routine! Here's a new patch.
For that particular question, it should be possible, following Hsu method
(who produces uniform presentation for PSL(Z,Z/pZ)), to get a congruence
test method for odd subgroups.
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Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11598#comment:3>
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