Modular forms

One of SageMath’s computational specialities is (the very technical field of) modular forms and can do a lot more than is even suggested in this very brief introduction.

Cusp forms

How do you compute the dimension of a space of cusp forms using Sage?

To compute the dimension of the space of cusp forms for Gamma use the command dimension_cusp_forms. Here is an example from section “Modular forms” in the Tutorial:

sage: from sage.modular.dims import dimension_cusp_forms
sage: dimension_cusp_forms(Gamma0(11),2)
1
sage: dimension_cusp_forms(Gamma0(1),12)
1
sage: dimension_cusp_forms(Gamma1(389),2)
6112

Related commands: dimension_new__cusp_forms_gamma0 (for dimensions of newforms), dimension_modular_forms (for modular forms), and dimension_eis (for Eisenstein series). The syntax is similar - see the Reference Manual for examples.

Coset representatives

The explicit representation of fundamental domains of arithmetic quotients \(H/\Gamma\) can be determined from the cosets of \(\Gamma\) in \(SL_2(\ZZ)\). How are these cosets computed in Sage?

Here is an example of computing the coset representatives of \(SL_2(\ZZ)/\Gamma_0(11)\):

sage: G = Gamma0(11); G
Congruence Subgroup Gamma0(11)
sage: list(G.coset_reps())
[
[1 0]  [ 0 -1]  [1 0]  [ 0 -1]  [ 0 -1]  [ 0 -1]  [ 0 -1]  [ 0 -1]
[0 1], [ 1  0], [1 1], [ 1  2], [ 1  3], [ 1  4], [ 1  5], [ 1  6],

[ 0 -1]  [ 0 -1]  [ 0 -1]  [ 0 -1]
[ 1  7], [ 1  8], [ 1  9], [ 1 10]
]

Modular symbols and Hecke operators

Next we illustrate computation of Hecke operators on a space of modular symbols of level 1 and weight 12.

sage: M = ModularSymbols(1,12)
sage: M.basis()
([X^8*Y^2,(0,0)], [X^9*Y,(0,0)], [X^10,(0,0)])
sage: t2 = M.T(2)
sage: f = t2.charpoly('x'); f
x^3 - 2001*x^2 - 97776*x - 1180224
sage: factor(f)
(x - 2049) * (x + 24)^2
sage: M.T(11).charpoly('x').factor()
(x - 285311670612) * (x - 534612)^2

Here t2 represents the Hecke operator \(T_2\) on the space of Full Modular Symbols for \(\Gamma_0(1)\) of weight \(12\) with sign \(0\) and dimension \(3\) over \(\QQ\).

sage: M = ModularSymbols(Gamma1(6),3,sign=0)
sage: M
Modular Symbols space of dimension 4 for Gamma_1(6) of weight 3 with sign 0
over Rational Field
sage: M.basis()
([X,(0,5)], [X,(3,5)], [X,(4,5)], [X,(5,5)])
sage: M._compute_hecke_matrix_prime(2).charpoly()
x^4 - 17*x^2 + 16
sage: M.integral_structure()
Free module of degree 4 and rank 4 over Integer Ring
Echelon basis matrix:
[1 0 0 0]
[0 1 0 0]
[0 0 1 0]
[0 0 0 1]

See the section on modular forms in the Tutorial or the Reference Manual for more examples.

Genus formulas

Sage can compute the genus of \(X_0(N)\), \(X_1(N)\), and related curves. Here are some examples of the syntax:

sage: from sage.modular.dims import dimension_cusp_forms
sage: dimension_cusp_forms(Gamma0(22))
2
sage: dimension_cusp_forms(Gamma0(30))
3
sage: dimension_cusp_forms(Gamma1(30))
9

See the code for computing dimensions of spaces of modular forms (in sage/modular/dims.py) or the paper by Oesterlé and Cohen {CO} for some details.