Opened 6 years ago

Last modified 5 years ago

#21798 new enhancement

Implement degeneracy maps to make JH(22, [-1]).decomposition() work

Reported by: pbruin Owned by:
Priority: major Milestone: sage-7.5
Component: modular forms Keywords:
Cc: mderickx Merged in:
Authors: Reviewers:
Report Upstream: N/A Work issues:
Branch: Commit:
Dependencies: #21927 Stopgaps:

Status badges

Description (last modified by pbruin)

In SageMath 7.5.rc0:

sage: JH(22, [-1]).decomposition()
Traceback (most recent call last):
...
NotImplementedError:

The thing that is not implemented is a degeneracy map between spaces of modular symbols.

See also #21799.

Change History (7)

comment:1 Changed 6 years ago by pbruin

  • Cc mderickx added
  • Description modified (diff)

comment:2 Changed 6 years ago by jdemeyer

The funny                  spacing in the error message should also be fixed.

comment:3 Changed 5 years ago by pbruin

It seems the following is a more immediate example:

sage: ModularSymbols(GammaH(11, [-1]), 2).modular_symbols_of_level(22)
Traceback (most recent call last):
...
ValueError: N (=22) should be a factor of the level                of this space (=11)

This does work when replacing GammaH(11, [-1]) by Gamma1(11) or Gamma0(11).

comment:4 Changed 5 years ago by pbruin

The above error is apparently deliberate, according to the following comment in the method sage.modular.modsym.ambient.ModularSymbolsAmbient_wtk_gamma_h.modular_symbols_of_level:

# We deliberately don't allow N to be a multiple of the level here,
# because there are many possibilities for what H could be at the
# higher level (and we don't implement the degeneracy raising maps
# anyway)

A canonical choice for the H at higher level N is to take the inverse image of H in (Z/NZ)×. However, we then still have to implement the degeneracy maps. Doing this will require implementing the function sage.modular.arithgroup.congroup.degeneracy_coset_representatives_gamma_h and the method sage.modular.modsym.ambient.ModularSymbolsAmbient_wtk_gamma_h._degeneracy_raising_matrix_1.

comment:5 Changed 5 years ago by pbruin

  • Dependencies set to #21927

After #21927, the error occurs slightly later on, and a NotImplementedError is raised instead of a ValueError.

comment:6 Changed 5 years ago by pbruin

  • Description modified (diff)
  • Summary changed from JH(22, [-1]).decomposition() raises ValueError to Implement degeneracy maps to make JH(22, [-1]).decomposition() work
  • Type changed from defect to enhancement

comment:7 Changed 5 years ago by pbruin

A more immediate example:

sage: M = ModularSymbols(GammaH(22, [-1]), 2)
sage: N = ModularSymbols(GammaH(11, [-1]), 2)
sage: N.degeneracy_map(M, 1)
Traceback (most recent call last):
...
NotImplementedError:
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