The functions Decomposition and NewformDecomposition express a space of modular symbols as a direct sum of Hecke-stable subspaces.
In the intrinsics below, the Proof parameter affects the internal characteristic polynomial computations. If Proof is set to false and this causes a characteristic polynomial computation to fail, then the sum of the dimensions of the spaces returned by Decomposition will be less than the dimension of M. Thus setting Proof equal to false is usually safe.
Proof: BoolElt Default: true
The decomposition of M with respect to the Hecke operators T_p with p coprime to the level of M and p <= bound. If bound is too small, the constituents of the decomposition are not guaranteed to be "irreducible", in the sense that they can not be decomposed further into kernels and images of Hecke operators T_p with p prime to the level of M. When Decomposition is called, the result is cached, so each successive call results in a possibly more refined decomposition.
Important Note: In some cases NewformDecomposition is significantly faster than Decomposition.
Proof: BoolElt Default: true
Unsorted decomposition of M into factors corresponding to the Galois conjugacy classes of newforms of level some divisor of the level of M. We require that IsCuspidal(M) is true.
The space of modular symbols corresponding to the Galois-conjugacy class of newforms associated to M. The level of the newforms is allowed to be a proper divisor of the level of M. The space M must have been created using NewformDecomposition.
Sort the sequence D of spaces of modular symbols with respect to the lt comparison operator.
True if and only if Decomposition(M) has cardinality 1.
The ordering determined as follows:
Rule (3) is included so that our ordering extends the one used in (most of!) [Cre97].
- This rule applies only if NewformDecomposition was used to construct both of M_1 and M_2: If Level(AssociatedNewSpace(M1)) and Level(AssociatedNewSpace(M2)) are not equal then the M_i with larger associated level is first.
- The smaller dimension is first.
- The following applies when the weight is 2 and the character is trivial: Order by W_q eigenvalues, starting with the smallest p | N, with the eigenvalue +1 being less than the eigenvalue -1.
- Order by abs(( trace)(a_p)), with p not dividing the level, and with positive trace being smaller in the event that the two absolute values are equal.
> M := ModularSymbols(37,2); M;
Full Modular symbols space of level 37, weight 2, and dimension 5
> D := Decomposition(M,2); D;
[
Modular symbols space of level 37, weight 2, and dimension 1,
Modular symbols space of level 37, weight 2, and dimension 2,
Modular symbols space of level 37, weight 2, and dimension 2
]
> IsIrreducible(D[2]);
true
> C := CuspidalSubspace(M); C;
Modular symbols space of level 37, weight 2, and dimension 4
> N := NewformDecomposition(C); N;
[
Modular symbols space of level 37, weight 2, and dimension 2,
Modular symbols space of level 37, weight 2, and dimension 2
]
Next, we use NewformDecomposition to decompose a space having plentiful old subspaces.
> M := ModularSymbols(90,2); M;
Full Modular symbols space of level 90, weight 2, and dimension 37
> D := Decomposition(M,11); D;
[
Modular symbols space of level 90, weight 2, and dimension 11,
Modular symbols space of level 90, weight 2, and dimension 4,
Modular symbols space of level 90, weight 2, and dimension 2,
Modular symbols space of level 90, weight 2, and dimension 2,
Modular symbols space of level 90, weight 2, and dimension 4,
Modular symbols space of level 90, weight 2, and dimension 8,
Modular symbols space of level 90, weight 2, and dimension 6
]
> C := CuspidalSubspace(M); C;
Modular symbols space of level 90, weight 2, and dimension 22
> N := NewformDecomposition(C); N;
[
Modular symbols space of level 90, weight 2, and dimension 2,
Modular symbols space of level 90, weight 2, and dimension 2,
Modular symbols space of level 90, weight 2, and dimension 2,
Modular symbols space of level 90, weight 2, and dimension 4,
Modular symbols space of level 90, weight 2, and dimension 4,
Modular symbols space of level 90, weight 2, and dimension 8
]
The above decomposition uses all of the Hecke operator; it suggests that the decomposition D is not as fine as possible. Indeed, D[7] breaks up further:
> Decomposition(D[7],11);
[
Modular symbols space of level 90, weight 2, and dimension 6
]
> Decomposition(D[7],19);
[
Modular symbols space of level 90, weight 2, and dimension 4,
Modular symbols space of level 90, weight 2, and dimension 2
]
The function AssociatedNewSpace allows us to see where each of these subspace comes from. By definition they each arise by taking images under the degeneracy maps from a single Galois-conjugacy class of newforms of some level dividing 90.
> [Level(AssociatedNewSpace(A)) : A in N];
[ 90, 90, 90, 45, 30, 15 ]
> A := N[4];
> qEigenform(AssociatedNewSpace(A),7);
q + q^2 - q^4 - q^5 + O(q^7)
> qExpansionBasis(A,7);
[
q - 2*q^4 - q^5 + O(q^7),
q^2 + q^4 + O(q^7)
]