Opened 7 years ago

Closed 4 years ago

Last modified 4 years ago

#13158 closed defect (fixed)

Local components bug finding minimal twist

Reported by: davidloeffler Owned by: craigcitro
Priority: minor Milestone: sage-6.4
Component: modular forms Keywords: local components
Cc: Merged in:
Authors: David Loeffler Reviewers: Frédéric Chapoton
Report Upstream: N/A Work issues:
Branch: 3577e02 (Commits) Commit:
Dependencies: Stopgaps:

Description

The following is from an email of François Brunault:

I got the following error when trying to compute minimal twists in Sage. Here f is the newform attached to the elliptic curve 256a1, and the minimal twist of f has level 64.

Best, François

sage: f=Newforms(256,names='a')[0]
sage: T=TypeSpace(f,2)
sage: T.is_minimal()
False
sage: T.minimal_twist()
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)

/Users/francoisbrunault/<ipython console> in <module>()

/Users/francoisbrunault/Documents/SAGE/sage-5.0.beta13/local/lib/python2.7/site-packages/sage/modular/local_comp/type_space.pyc in minimal_twist(self)
   383             D._set_sign(D.star_eigenvalues()[0])
   384         M = ModularForms(D.group(), D.weight())
--> 385         ff = Newform(M, D, names='a')
   386         return ff
   387

/Users/francoisbrunault/Documents/SAGE/sage-5.0.beta13/local/lib/python2.7/site-packages/sage/modular/modform/element.pyc in __init__(self, parent, component, names, check)
   636                 raise ValueError, "component must be cuspidal"
   637             if not component.is_simple():
--> 638                 raise ValueError, "component must be simple"
   639         extension_field = component.eigenvalue(1,name=names).parent()
   640         if extension_field != parent.base_ring(): # .degree() != 1 and rings.is_NumberField(extension_field):

ValueError: component must be simple

Change History (10)

comment:1 Changed 6 years ago by jdemeyer

  • Milestone changed from sage-5.11 to sage-5.12

comment:2 Changed 6 years ago by vbraun_spam

  • Milestone changed from sage-6.1 to sage-6.2

comment:3 Changed 6 years ago by vbraun_spam

  • Milestone changed from sage-6.2 to sage-6.3

comment:4 Changed 5 years ago by vbraun_spam

  • Milestone changed from sage-6.3 to sage-6.4

comment:5 Changed 4 years ago by davidloeffler

  • Branch set to u/davidloeffler/local_components_bug_finding_minimal_twist

comment:6 Changed 4 years ago by davidloeffler

  • Authors set to David Loeffer
  • Commit set to 1994692c566e53f79774947647a03ab3c2042b98
  • Status changed from new to needs_review

Somehow, this sat idle for 3 years despite the fact that the fix is about 2 lines! It turns out that the code relies on the assumption that the trace of the type space down to the minimal level is simple, but if f is of CM type it can be the direct sum of two simple modules of opposite sign.


New commits:

1994692Trac 13158: local components bug finding minimal twist

comment:7 Changed 4 years ago by chapoton

  • Branch changed from u/davidloeffler/local_components_bug_finding_minimal_twist to public/ticket/13158
  • Commit changed from 1994692c566e53f79774947647a03ab3c2042b98 to 3577e02d18bf8e0a449c82d88880ea6e62de3a88
  • Reviewers set to Frédéric Chapoton
  • Status changed from needs_review to positive_review

This looks good to me. I have only made minor changes, and allow myself to set to positive review.


New commits:

9ce7fbcMerge branch 'u/davidloeffler/local_components_bug_finding_minimal_twist' into 6.8.b6
3577e02trac #13518 review commit

comment:8 Changed 4 years ago by davidloeffler

Thanks for the review! The patchbot seems a bit confused, but I'm sure it'll catch up shortly :-)

comment:9 Changed 4 years ago by vbraun

  • Branch changed from public/ticket/13158 to 3577e02d18bf8e0a449c82d88880ea6e62de3a88
  • Resolution set to fixed
  • Status changed from positive_review to closed

comment:10 Changed 4 years ago by jdemeyer

  • Authors changed from David Loeffer to David Loeffler
  • Commit 3577e02d18bf8e0a449c82d88880ea6e62de3a88 deleted
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