# Congruence Subgroup $$\Gamma_H(N)$$¶

AUTHORS:

• Jordi Quer
• David Loeffler
class sage.modular.arithgroup.congroup_gammaH.GammaH_class(level, H, Hlist=None)

The congruence subgroup $$\Gamma_H(N)$$ for some subgroup $$H \trianglelefteq (\ZZ / N\ZZ)^\times$$, which is the subgroup of $${\rm SL}_2(\ZZ)$$ consisting of matrices of the form $$\begin{pmatrix} a & b \\ c & d \end{pmatrix}$$ with $$N \mid c$$ and $$a, b \in H$$.

TESTS:

We test calculation of various invariants of the group:

sage: GammaH(33,[2]).projective_index()
96
sage: GammaH(33,[2]).genus()
5
sage: GammaH(7,[2]).genus()
0
sage: GammaH(23, [1..22]).genus()
2
sage: Gamma0(23).genus()
2
sage: GammaH(23, [1]).genus()
12
sage: Gamma1(23).genus()
12


We calculate the dimensions of some modular forms spaces:

sage: GammaH(33,[2]).dimension_cusp_forms(2)
5
sage: GammaH(33,[2]).dimension_cusp_forms(3)
0
sage: GammaH(33,[2,5]).dimension_cusp_forms(2)
3
sage: GammaH(32079, [21676]).dimension_cusp_forms(20)
180266112


We can sometimes show that there are no weight 1 cusp forms:

sage: GammaH(20, [9]).dimension_cusp_forms(1)
0

coset_reps()

Return a set of coset representatives for self \ SL2Z.

EXAMPLES:

sage: list(Gamma1(3).coset_reps())
[
[1 0]  [-1 -2]  [ 0 -1]  [-2  1]  [1 0]  [-3 -2]  [ 0 -1]  [-2 -3]
[0 1], [ 3  5], [ 1  0], [ 5 -3], [1 1], [ 8  5], [ 1  2], [ 5  7]
]
sage: len(list(Gamma1(31).coset_reps())) == 31**2 - 1
True

dimension_new_cusp_forms(k=2, p=0)

Return the dimension of the space of new (or $$p$$-new) weight $$k$$ cusp forms for this congruence subgroup.

INPUT:

• k - an integer (default: 2), the weight. Not fully implemented for k = 1.
• p - integer (default: 0); if nonzero, compute the $$p$$-new subspace.

OUTPUT: Integer

EXAMPLES:

sage: GammaH(33,[2]).dimension_new_cusp_forms()
3
sage: Gamma1(4*25).dimension_new_cusp_forms(2, p=5)
225
sage: Gamma1(33).dimension_new_cusp_forms(2)
19
sage: Gamma1(33).dimension_new_cusp_forms(2,p=11)
21

divisor_subgroups()

Given this congruence subgroup $$\Gamma_H(N)$$, return all subgroups $$\Gamma_G(M)$$ for $$M$$ a divisor of $$N$$ and such that $$G$$ is equal to the image of $$H$$ modulo $$M$$.

EXAMPLES:

sage: G = GammaH(33,[2]); G
Congruence Subgroup Gamma_H(33) with H generated by [2]
sage: G._list_of_elements_in_H()
[1, 2, 4, 8, 16, 17, 25, 29, 31, 32]
sage: G.divisor_subgroups()
[Modular Group SL(2,Z),
Congruence Subgroup Gamma0(3),
Congruence Subgroup Gamma0(11),
Congruence Subgroup Gamma_H(33) with H generated by [2]]

extend(M)

Return the subgroup of $$\Gamma_0(M)$$, for $$M$$ a multiple of $$N$$, obtained by taking the preimage of this group under the reduction map; in other words, the intersection of this group with $$\Gamma_0(M)$$.

EXAMPLES:

sage: G = GammaH(33, [2])
sage: G.extend(99)
Congruence Subgroup Gamma_H(99) with H generated by [2, 35, 68]
sage: G.extend(11)
Traceback (most recent call last):
...
ValueError: M (=11) must be a multiple of the level (33) of self

gamma0_coset_reps()

Return a set of coset representatives for self \ Gamma0(N), where N is the level of self.

EXAMPLE:

sage: GammaH(108, [1,-1]).gamma0_coset_reps()
[
[1 0]  [-43 -45]  [ 31  33]  [-49 -54]  [ 25  28]  [-19 -22]
[0 1], [108 113], [108 115], [108 119], [108 121], [108 125],

[-17 -20]  [ 47  57]  [ 13  16]  [ 41  52]  [  7   9]  [-37 -49]
[108 127], [108 131], [108 133], [108 137], [108 139], [108 143],

[-35 -47]  [ 29  40]  [ -5  -7]  [ 23  33]  [-11 -16]  [ 53  79]
[108 145], [108 149], [108 151], [108 155], [108 157], [108 161]
]

generators(algorithm='farey')

Return generators for this congruence subgroup. The result is cached.

INPUT:

• algorithm (string): either farey (default) or todd-coxeter.

If algorithm is set to "farey", then the generators will be calculated using Farey symbols, which will always return a minimal generating set. See farey_symbol for more information.

If algorithm is set to "todd-coxeter", a simpler algorithm based on Todd-Coxeter enumeration will be used. This tends to return far larger sets of generators.

EXAMPLE:

sage: GammaH(7, [2]).generators()
[
[1 1]  [ 2 -1]  [ 4 -3]
[0 1], [ 7 -3], [ 7 -5]
]
sage: GammaH(7, [2]).generators(algorithm="todd-coxeter")
[
[1 1]  [-90  29]  [ 15   4]  [-10  -3]  [ 1 -1]  [1 0]  [1 1]  [-3 -1]
[0 1], [301 -97], [-49 -13], [  7   2], [ 0  1], [7 1], [0 1], [ 7  2],

[-13   4]  [-5 -1]  [-5 -2]  [-10   3]  [ 1  0]  [ 9 -1]  [-20   7]
[ 42 -13], [21  4], [28 11], [ 63 -19], [-7  1], [28 -3], [-63  22],

[1 0]  [-3 -1]  [ 15  -4]  [ 2 -1]  [ 22  -7]  [-5  1]  [  8  -3]
[7 1], [ 7  2], [ 49 -13], [ 7 -3], [ 63 -20], [14 -3], [-21   8],

[11  5]  [-13  -4]
[35 16], [-42 -13]
]

image_mod_n()

Return the image of this group in $$SL(2, \ZZ / N\ZZ)$$.

EXAMPLE:

sage: Gamma0(3).image_mod_n()
Matrix group over Ring of integers modulo 3 with 2 generators (
[2 0]  [1 1]
[0 2], [0 1]
)


TEST:

sage: for n in [2..20]:
...     for g in Gamma0(n).gamma_h_subgroups():
...       G = g.image_mod_n()
...       assert G.order() == Gamma(n).index() / g.index()

index()

Return the index of self in SL2Z.

EXAMPLE:

sage: [G.index() for G in Gamma0(40).gamma_h_subgroups()]
[72, 144, 144, 144, 144, 288, 288, 288, 288, 144, 288, 288, 576, 576, 144, 288, 288, 576, 576, 144, 288, 288, 576, 576, 288, 576, 1152]

is_even()

Return True precisely if this subgroup contains the matrix -1.

EXAMPLES:

sage: GammaH(10, [3]).is_even()
True
sage: GammaH(14, [1]).is_even()
False

is_subgroup(other)

Return True if self is a subgroup of right, and False otherwise.

EXAMPLES:

sage: GammaH(24,[7]).is_subgroup(SL2Z)
True
sage: GammaH(24,[7]).is_subgroup(Gamma0(8))
True
sage: GammaH(24, []).is_subgroup(GammaH(24, [7]))
True
sage: GammaH(24, []).is_subgroup(Gamma1(24))
True
sage: GammaH(24, [17]).is_subgroup(GammaH(24, [7]))
False
sage: GammaH(1371, [169]).is_subgroup(GammaH(457, [169]))
True

ncusps()

Return the number of orbits of cusps (regular or otherwise) for this subgroup.

EXAMPLE:

sage: GammaH(33,[2]).ncusps()
8
sage: GammaH(32079, [21676]).ncusps()
28800


AUTHORS:

• Jordi Quer
nirregcusps()

Return the number of irregular cusps for this subgroup.

EXAMPLES:

sage: GammaH(3212, [2045, 2773]).nirregcusps()
720

nregcusps()

Return the number of orbits of regular cusps for this subgroup. A cusp is regular if we may find a parabolic element generating the stabiliser of that cusp whose eigenvalues are both +1 rather than -1. If G contains -1, all cusps are regular.

EXAMPLES:

sage: GammaH(20, [17]).nregcusps()
4
sage: GammaH(20, [17]).nirregcusps()
2
sage: GammaH(3212, [2045, 2773]).nregcusps()
1440
sage: GammaH(3212, [2045, 2773]).nirregcusps()
720


AUTHOR:

• Jordi Quer
nu2()

Return the number of orbits of elliptic points of order 2 for this group.

EXAMPLE:

sage: [H.nu2() for n in [1..10] for H in Gamma0(n).gamma_h_subgroups()]
[1, 1, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0]
sage: GammaH(33,[2]).nu2()
0
sage: GammaH(5,[2]).nu2()
2


AUTHORS:

• Jordi Quer
nu3()

Return the number of orbits of elliptic points of order 3 for this group.

EXAMPLE:

sage: [H.nu3() for n in [1..10] for H in Gamma0(n).gamma_h_subgroups()]
[1, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 2, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
sage: GammaH(33,[2]).nu3()
0
sage: GammaH(7,[2]).nu3()
2


AUTHORS:

• Jordi Quer
reduce_cusp(c)

Compute a minimal representative for the given cusp c. Returns a cusp c’ which is equivalent to the given cusp, and is in lowest terms with minimal positive denominator, and minimal positive numerator for that denominator.

Two cusps $$u_1/v_1$$ and $$u_2/v_2$$ are equivalent modulo $$\Gamma_H(N)$$ if and only if

$v_1 = h v_2 \bmod N\quad \text{and}\quad u_1 = h^{-1} u_2 \bmod {\rm gcd}(v_1,N)$

or

$v_1 = -h v_2 \bmod N\quad \text{and}\quad u_1 = -h^{-1} u_2 \bmod {\rm gcd}(v_1,N)$

for some $$h \in H$$.

EXAMPLES:

sage: GammaH(6,[5]).reduce_cusp(5/3)
1/3
sage: GammaH(12,[5]).reduce_cusp(Cusp(8,9))
1/3
sage: GammaH(12,[5]).reduce_cusp(5/12)
Infinity
sage: GammaH(12,[]).reduce_cusp(Cusp(5,12))
5/12
sage: GammaH(21,[5]).reduce_cusp(Cusp(-9/14))
1/7
sage: Gamma1(5).reduce_cusp(oo)
Infinity
sage: Gamma1(5).reduce_cusp(0)
0

restrict(M)

Return the subgroup of $$\Gamma_0(M)$$, for $$M$$ a divisor of $$N$$, obtained by taking the image of this group under reduction modulo $$N$$.

EXAMPLES:

sage: G = GammaH(33,[2])
sage: G.restrict(11)
Congruence Subgroup Gamma0(11)
sage: G.restrict(1)
Modular Group SL(2,Z)
sage: G.restrict(15)
Traceback (most recent call last):
...
ValueError: M (=15) must be a divisor of the level (33) of self

to_even_subgroup()

Return the smallest even subgroup of $$SL(2, \ZZ)$$ containing self.

EXAMPLE:

sage: GammaH(11, [4]).to_even_subgroup()
Congruence Subgroup Gamma0(11)
sage: Gamma1(11).to_even_subgroup()
Congruence Subgroup Gamma_H(11) with H generated by [10]

sage.modular.arithgroup.congroup_gammaH.GammaH_constructor(level, H)

Return the congruence subgroup $$\Gamma_H(N)$$, which is the subgroup of $$SL_2(\ZZ)$$ consisting of matrices of the form $$\begin{pmatrix} a & b \\ c & d \end{pmatrix}$$ with $$N | c$$ and $$a, b \in H$$, for $$H$$ a specified subgroup of $$(\ZZ/N\ZZ)^\times$$.

INPUT:

• level – an integer

• H – either 0, 1, or a list
• If H is a list, return $$\Gamma_H(N)$$, where $$H$$ is the subgroup of $$(\ZZ/N\ZZ)^*$$ generated by the elements of the list.
• If H = 0, returns $$\Gamma_0(N)$$.
• If H = 1, returns $$\Gamma_1(N)$$.

EXAMPLES:

sage: GammaH(11,0) # indirect doctest
Congruence Subgroup Gamma0(11)
sage: GammaH(11,1)
Congruence Subgroup Gamma1(11)
sage: GammaH(11,[10])
Congruence Subgroup Gamma_H(11) with H generated by [10]
sage: GammaH(11,[10,1])
Congruence Subgroup Gamma_H(11) with H generated by [10]
sage: GammaH(14,[10])
Traceback (most recent call last):
...
ArithmeticError: The generators [10] must be units modulo 14

sage.modular.arithgroup.congroup_gammaH.is_GammaH(x)

Return True if x is a congruence subgroup of type GammaH.

EXAMPLES:

sage: from sage.modular.arithgroup.all import is_GammaH
sage: is_GammaH(GammaH(13, [2]))
True
sage: is_GammaH(Gamma0(6))
True
sage: is_GammaH(Gamma1(6))
True
sage: is_GammaH(sage.modular.arithgroup.congroup_generic.CongruenceSubgroup(5))
False

sage.modular.arithgroup.congroup_gammaH.mumu(N)

Return 0 if any cube divides $$N$$. Otherwise return $$(-2)^v$$ where $$v$$ is the number of primes that exactly divide $$N$$.

This is similar to the Moebius function.

INPUT:

• N - an integer at least 1

OUTPUT: Integer

EXAMPLES:

sage: from sage.modular.arithgroup.congroup_gammaH import mumu
sage: mumu(27)
0
sage: mumu(6*25)
4
sage: mumu(7*9*25)
-2
sage: mumu(9*25)
1


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Congruence Subgroup $$\Gamma_1(N)$$