#11598: Change to is_congruence method of modular subgroups
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Reporter: davidloeffler | Owner: craigcitro
Type: defect | Status: new
Priority: major | Milestone: sage-4.7.2
Component: modular forms | Keywords: modular congruence subgroup
Work_issues: | Upstream: N/A
Reviewer: | Author: David Loeffler
Merged: | Dependencies: 11422
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The new functionality added in ticket #11422 makes it possible to
manipulate arbitrary finite index (not necessarily congruence) subgroups
of SL2Z. This massively extends my old code which only worked for
subgroups containing -1.
The "is_congruence" method in the new code, though, *defines* a subgroup
to be congruence if its image modulo -1 is a congruence subgroup of PSL2Z.
This is not the same as the conventional definition of a congruence
subgroup of SL2Z. The patch below modifies the algorithm so it uses the
conventional notion of "congruence".
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Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11598>
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