# Subspace of ambient spaces of modular symbols¶

class sage.modular.modsym.subspace.ModularSymbolsSubspace(ambient_hecke_module, submodule, dual_free_module=None, check=False)

Subspace of ambient space of modular symbols

boundary_map()

The boundary map to the corresponding space of boundary modular symbols. (This is the restriction of the map on the ambient space.)

EXAMPLES:

sage: M = ModularSymbols(1, 24, sign=1) ; M
Modular Symbols space of dimension 3 for Gamma_0(1) of weight 24 with sign 1 over Rational Field
sage: M.basis()
([X^18*Y^4,(0,0)], [X^20*Y^2,(0,0)], [X^22,(0,0)])
sage: M.cuspidal_submodule().basis()
([X^18*Y^4,(0,0)], [X^20*Y^2,(0,0)])
sage: M.eisenstein_submodule().basis()
([X^18*Y^4,(0,0)] + 166747/324330*[X^20*Y^2,(0,0)] + 236364091/6742820700*[X^22,(0,0)],)
sage: M.boundary_map()
Hecke module morphism boundary map defined by the matrix
[ 0]
[ 0]
[-1]
Domain: Modular Symbols space of dimension 3 for Gamma_0(1) of weight ...
Codomain: Space of Boundary Modular Symbols for Modular Group SL(2,Z) ...
sage: M.cuspidal_subspace().boundary_map()
Hecke module morphism defined by the matrix


Domain: Modular Symbols subspace of dimension 2 of Modular Symbols space ...
Codomain: Space of Boundary Modular Symbols for Modular Group SL(2,Z) ...
sage: M.eisenstein_submodule().boundary_map()
Hecke module morphism defined by the matrix
[-236364091/6742820700]
Domain: Modular Symbols subspace of dimension 1 of Modular Symbols space ...
Codomain: Space of Boundary Modular Symbols for Modular Group SL(2,Z) ...

cuspidal_submodule()

Return the cuspidal subspace of this subspace of modular symbols.

EXAMPLES:

sage: S = ModularSymbols(42,4).cuspidal_submodule() ; S
Modular Symbols subspace of dimension 40 of Modular Symbols space of dimension 48 for Gamma_0(42) of weight 4 with sign 0 over Rational Field
sage: S.is_cuspidal()
True
sage: S.cuspidal_submodule()
Modular Symbols subspace of dimension 40 of Modular Symbols space of dimension 48 for Gamma_0(42) of weight 4 with sign 0 over Rational Field


The cuspidal submodule of the cuspidal submodule is just itself:

sage: S.cuspidal_submodule() is S
True
sage: S.cuspidal_submodule() == S
True


An example where we abuse the _set_is_cuspidal function:

sage: M = ModularSymbols(389)
sage: S = M.eisenstein_submodule()
sage: S._set_is_cuspidal(True)
sage: S.cuspidal_submodule()
Modular Symbols subspace of dimension 1 of Modular Symbols space of dimension 65 for Gamma_0(389) of weight 2 with sign 0 over Rational Field

dual_star_involution_matrix()

Return the matrix of the dual star involution, which is induced by complex conjugation on the linear dual of modular symbols.

EXAMPLES:

sage: S = ModularSymbols(6,4) ; S.dual_star_involution_matrix()
[ 1  0  0  0  0  0]
[ 0  1  0  0  0  0]
[ 0 -2  1  2  0  0]
[ 0  2  0 -1  0  0]
[ 0 -2  0  2  1  0]
[ 0  2  0 -2  0  1]
sage: S.star_involution().matrix().transpose() == S.dual_star_involution_matrix()
True

eisenstein_subspace()

Return the Eisenstein subspace of this space of modular symbols.

EXAMPLES:

sage: ModularSymbols(24,4).eisenstein_subspace()
Modular Symbols subspace of dimension 8 of Modular Symbols space of dimension 24 for Gamma_0(24) of weight 4 with sign 0 over Rational Field
sage: ModularSymbols(20,2).cuspidal_subspace().eisenstein_subspace()
Modular Symbols subspace of dimension 0 of Modular Symbols space of dimension 7 for Gamma_0(20) of weight 2 with sign 0 over Rational Field

factorization()

Returns a list of pairs $$(S,e)$$ where $$S$$ is simple spaces of modular symbols and self is isomorphic to the direct sum of the $$S^e$$ as a module over the anemic Hecke algebra adjoin the star involution.

The cuspidal $$S$$ are all simple, but the Eisenstein factors need not be simple.

The factors are sorted by dimension - don’t depend on much more for now.

ASSUMPTION: self is a module over the anemic Hecke algebra.

EXAMPLES: Note that if the sign is 1 then the cuspidal factors occur twice, one with each star eigenvalue.

sage: M = ModularSymbols(11)
sage: D = M.factorization(); D
(Modular Symbols subspace of dimension 1 of Modular Symbols space of dimension 3 for Gamma_0(11) of weight 2 with sign 0 over Rational Field) *
(Modular Symbols subspace of dimension 1 of Modular Symbols space of dimension 3 for Gamma_0(11) of weight 2 with sign 0 over Rational Field) *
(Modular Symbols subspace of dimension 1 of Modular Symbols space of dimension 3 for Gamma_0(11) of weight 2 with sign 0 over Rational Field)
sage: [A.T(2).matrix() for A, _ in D]
[[-2], , [-2]]
sage: [A.star_eigenvalues() for A, _ in D]
[[-1], , ]


In this example there is one old factor squared.

sage: M = ModularSymbols(22,sign=1)
sage: S = M.cuspidal_submodule()
sage: S.factorization()
(Modular Symbols subspace of dimension 1 of Modular Symbols space of dimension 2 for Gamma_0(11) of weight 2 with sign 1 over Rational Field)^2

sage: M = ModularSymbols(Gamma0(22), 2, sign=1)
sage: M1 = M.decomposition()
sage: M1.factorization()
Modular Symbols subspace of dimension 3 of Modular Symbols space of dimension 5 for Gamma_0(22) of weight 2 with sign 1 over Rational Field

is_cuspidal()

Return True if self is cuspidal.

EXAMPLES:

sage: ModularSymbols(42,4).cuspidal_submodule().is_cuspidal()
True
sage: ModularSymbols(12,6).eisenstein_submodule().is_cuspidal()
False

is_eisenstein()

Return True if self is an Eisenstein subspace.

EXAMPLES:

sage: ModularSymbols(22,6).cuspidal_submodule().is_eisenstein()
False
sage: ModularSymbols(22,6).eisenstein_submodule().is_eisenstein()
True

star_involution()

Return the star involution on self, which is induced by complex conjugation on modular symbols.

EXAMPLES:

sage: M = ModularSymbols(1,24)
sage: M.star_involution()
Hecke module morphism Star involution on Modular Symbols space of dimension 5 for Gamma_0(1) of weight 24 with sign 0 over Rational Field defined by the matrix
[ 1  0  0  0  0]
[ 0 -1  0  0  0]
[ 0  0  1  0  0]
[ 0  0  0 -1  0]
[ 0  0  0  0  1]
Domain: Modular Symbols space of dimension 5 for Gamma_0(1) of weight ...
Codomain: Modular Symbols space of dimension 5 for Gamma_0(1) of weight ...
sage: M.cuspidal_subspace().star_involution()
Hecke module morphism defined by the matrix
[ 1  0  0  0]
[ 0 -1  0  0]
[ 0  0  1  0]
[ 0  0  0 -1]
Domain: Modular Symbols subspace of dimension 4 of Modular Symbols space ...
Codomain: Modular Symbols subspace of dimension 4 of Modular Symbols space ...
sage: M.plus_submodule().star_involution()
Hecke module morphism defined by the matrix
[1 0 0]
[0 1 0]
[0 0 1]
Domain: Modular Symbols subspace of dimension 3 of Modular Symbols space ...
Codomain: Modular Symbols subspace of dimension 3 of Modular Symbols space ...
sage: M.minus_submodule().star_involution()
Hecke module morphism defined by the matrix
[-1  0]
[ 0 -1]
Domain: Modular Symbols subspace of dimension 2 of Modular Symbols space ...
Codomain: Modular Symbols subspace of dimension 2 of Modular Symbols space ...