Opened 3 years ago

Closed 2 years ago

#21497 closed defect (fixed)

modular symbols - is_cuspidal sometimes incorrectly returns False

Reported by: dgulotta Owned by:
Priority: major Milestone: sage-7.6
Component: modular forms Keywords:
Cc: Merged in:
Authors: Daniel Gulotta Reviewers: Vincent Delecroix, Frédéric Chapoton
Report Upstream: N/A Work issues:
Branch: 7076bd6 (Commits) Commit: 7076bd6abc5f86b436af2a5d276d5b8ea1ed2f47
Dependencies: Stopgaps:

Description

In the following example, the modular symbols are cuspidal but Sage says that they are not:

sage: M = ModularSymbols(Gamma0(3),weight=22,sign=1)
sage: N = next(S for S in M.decomposition(anemic=False) if S.hecke_matrix(3).trace()==-128844)
sage: N.is_cuspidal()
True
sage: [g.is_cuspidal() for g in N.gens()]
[False, False]

Change History (16)

comment:1 Changed 3 years ago by dgulotta

  • Type changed from PLEASE CHANGE to defect

It seems that the underlying issue is that the code checks whether g is cuspidal by checking whether g==M.cuspidal_submodule()(g). This test returns false because Sage can't find a common parent of N and M.cuspidal_submodule().

comment:2 Changed 3 years ago by dgulotta

  • Branch set to /u/dgulotta/modsym_coerce

comment:3 Changed 3 years ago by dgulotta

  • Branch /u/dgulotta/modsym_coerce deleted

comment:4 Changed 3 years ago by dgulotta

  • Branch set to u/dgulotta/modular_symbols___is_cuspidal_sometimes_incorrectly_returns_false

comment:5 Changed 3 years ago by dgulotta

  • Commit set to ed46e306b4e4fb806a0c006da1a87352a64b9113
  • Status changed from new to needs_review

New commits:

ed46e30make coercion work for Hecke submodules

comment:6 Changed 3 years ago by git

  • Commit changed from ed46e306b4e4fb806a0c006da1a87352a64b9113 to 916247c5f3104270b0b9c7f873495d869a9c2d8c

Branch pushed to git repo; I updated commit sha1. New commits:

916247cadd documentation to fix for trac 21497

comment:7 Changed 3 years ago by git

  • Commit changed from 916247c5f3104270b0b9c7f873495d869a9c2d8c to 3e39a86b8bf499c8b5af57c3a2bec7f9a71fa85c

Branch pushed to git repo; I updated commit sha1. New commits:

3e39a86fix typo

comment:8 Changed 2 years ago by vdelecroix

Could you add your full name in the "Author" field of the ticket?

comment:9 Changed 2 years ago by vdelecroix

The following syntax is wrong

TESTS::

Make sure that the element is created correctly when the codomain is
not the full module (related to :trac:`21497`):

It should be

TESTS:    <---- only one colon

Make sure that the element is created correctly when the codomain is
not the full module (related to :trac:`21497`)::   <----- but two colons here

comment:10 Changed 2 years ago by vdelecroix

For direct sums you can use +

sage: V = QQ**4
sage: U1 = V.subspace([(1,0,0,-1)])
sage: U2 = V.subspace([(2,1,0,0), (1,0,1,0)])
sage: U1+U2
Vector space of degree 4 and dimension 3 over Rational Field
Basis matrix:
[ 1  0  0 -1]
[ 0  1  0  2]
[ 0  0  1  1]
sage: M = ModularSymbols(Gamma0(3),weight=22,sign=1)
sage: U1 = M.cuspidal_submodule()
sage: U2 = M.eisenstein_submodule()
sage: U1+U2
Modular Symbols subspace of dimension 8 of Modular Symbols space of dimension 8 for Gamma_0(3) of weight 22 with sign 1 over Rational Field

comment:11 Changed 2 years ago by vdelecroix

  • Reviewers set to Vincent Delecroix
  • Status changed from needs_review to needs_work

comment:12 Changed 2 years ago by git

  • Commit changed from 3e39a86b8bf499c8b5af57c3a2bec7f9a71fa85c to 7076bd6abc5f86b436af2a5d276d5b8ea1ed2f47

Branch pushed to git repo; I updated commit sha1. New commits:

7076bd6fix docstring, remove Hecke module pushout as it is not needed

comment:13 Changed 2 years ago by dgulotta

  • Authors set to Daniel Gulotta
  • Status changed from needs_work to needs_review

It seems that implementing _pushout_ was not necessary; just fixing the homspaces is enough to make is_cuspidal work.

comment:14 Changed 2 years ago by chapoton

  • Reviewers changed from Vincent Delecroix to Vincent Delecroix, Frédéric Chapoton
  • Status changed from needs_review to positive_review

ok, let it be

comment:15 Changed 2 years ago by chapoton

  • Milestone changed from sage-7.4 to sage-7.6

comment:16 Changed 2 years ago by vbraun

  • Branch changed from u/dgulotta/modular_symbols___is_cuspidal_sometimes_incorrectly_returns_false to 7076bd6abc5f86b436af2a5d276d5b8ea1ed2f47
  • Resolution set to fixed
  • Status changed from positive_review to closed
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