#11875: Correct general brokenness of Farey symbols
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   Reporter:  davidloeffler  |          Owner:  craigcitro      
       Type:  defect         |         Status:  new             
   Priority:  blocker        |      Milestone:  sage-4.7.2      
  Component:  modular forms  |       Keywords:  modular subgroup
Work_issues:                 |       Upstream:  N/A             
   Reviewer:                 |         Author:                  
     Merged:                 |   Dependencies:                  
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 The new Farey symbols code from #11709 needs some serious work before it
 can be allowed into a production release of Sage.

 See [http://groups.google.com/group/sage-
 devel/browse_thread/thread/e6b65915cde174b this sage-devel thread].

 As an absolute minimum, we should:

 * remove the file "sage.modular.arithgroup.noncongruence_example" (which
 is makes outrageously false mathematical statements in its docstrings --
 the "example noncongruence subgroup" is actually the congruence subgroup
 {{{GammaH(8, [5])}}} -- and is implemented in a stupidly broken way with
 no doctests and no usable methods whatsoever)

 * bring doctest coverage for the Python and Cython files in
 sage/modular/arithgroup back up to 100%, and make sure there are
 loads/dumps tests for all the classes;

 * make sure that the Farey symbol code works with all of the subclasses of
 ArithmeticSubgroup and add doctests to prove it;

 * add a warning in the documentation that commands like "index" and
 "generators" in Farey symbol code return the index, generators etc of the
 image of the group in PSL2Z, whereas Sage's general design is to work with
 subgroups of SL2Z.

-- 
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/11875>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica, 
and MATLAB

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