Arithmetic subgroups, finite index subgroups of \(\SL_2(\ZZ)\)¶
- class sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup[source]¶
Bases:
Group
Base class for arithmetic subgroups of \(\SL_2(\ZZ)\). Not intended to be used directly, but still includes quite a few general-purpose routines which compute data about an arithmetic subgroup assuming that it has a working element testing routine.
- Element[source]¶
alias of
ArithmeticSubgroupElement
- are_equivalent(x, y, trans=False)[source]¶
Test whether or not cusps \(x\) and \(y\) are equivalent modulo
self
.If
self
has areduce_cusp()
method, use that; otherwise do a slow explicit test.If
trans == False
, returnsTrue
orFalse
. Iftrans == True
, then return eitherFalse
or an element ofself
mapping \(x\) onto \(y\).EXAMPLES:
sage: Gamma0(7).are_equivalent(Cusp(1/3), Cusp(0), trans=True) [ 3 -1] [-14 5] sage: Gamma0(7).are_equivalent(Cusp(1/3), Cusp(1/7)) False
>>> from sage.all import * >>> Gamma0(Integer(7)).are_equivalent(Cusp(Integer(1)/Integer(3)), Cusp(Integer(0)), trans=True) [ 3 -1] [-14 5] >>> Gamma0(Integer(7)).are_equivalent(Cusp(Integer(1)/Integer(3)), Cusp(Integer(1)/Integer(7))) False
- as_permutation_group()[source]¶
Return
self
as an arithmetic subgroup defined in terms of the permutation action of \(SL(2,\ZZ)\) on its right cosets.This method uses Todd-Coxeter enumeration (via the method
todd_coxeter()
) which can be extremely slow for arithmetic subgroups with relatively large index in \(SL(2,\ZZ)\).EXAMPLES:
sage: # needs sage.groups sage: G = Gamma(3) sage: P = G.as_permutation_group(); P Arithmetic subgroup of index 24 sage: G.ncusps() == P.ncusps() True sage: G.nu2() == P.nu2() True sage: G.nu3() == P.nu3() True sage: G.an_element() in P True sage: P.an_element() in G True
>>> from sage.all import * >>> # needs sage.groups >>> G = Gamma(Integer(3)) >>> P = G.as_permutation_group(); P Arithmetic subgroup of index 24 >>> G.ncusps() == P.ncusps() True >>> G.nu2() == P.nu2() True >>> G.nu3() == P.nu3() True >>> G.an_element() in P True >>> P.an_element() in G True
- coset_reps(G=None)[source]¶
Return right coset representatives for
self \\ G
, where \(G\) is another arithmetic subgroup that containsself
. IfG == None
, default toG = SL2Z
.For generic arithmetic subgroups \(G\) this is carried out by Todd-Coxeter enumeration; here \(G\) is treated as a black box, implementing nothing but membership testing.
EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().coset_reps() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.coset_reps(Gamma0(3)) [ [1 0] [ 0 -1] [ 0 -1] [ 0 -1] [0 1], [ 1 0], [ 1 1], [ 1 2] ]
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().coset_reps() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.coset_reps(Gamma0(Integer(3))) [ [1 0] [ 0 -1] [ 0 -1] [ 0 -1] [0 1], [ 1 0], [ 1 1], [ 1 2] ]
- cusp_data(c)[source]¶
Return a triple \((g, w, t)\) where \(g\) is an element of
self
generating the stabiliser of the given cusp, \(w\) is the width of the cusp, and \(t\) is 1 if the cusp is regular and -1 if not.EXAMPLES:
sage: Gamma1(4).cusp_data(Cusps(1/2)) ( [ 1 -1] [ 4 -3], 1, -1 )
>>> from sage.all import * >>> Gamma1(Integer(4)).cusp_data(Cusps(Integer(1)/Integer(2))) ( [ 1 -1] [ 4 -3], 1, -1 )
- cusp_width(c)[source]¶
Return the width of the orbit of cusps represented by c.
EXAMPLES:
sage: Gamma0(11).cusp_width(Cusps(oo)) 1 sage: Gamma0(11).cusp_width(0) 11 sage: [Gamma0(100).cusp_width(c) for c in Gamma0(100).cusps()] [100, 1, 4, 1, 1, 1, 4, 25, 1, 1, 4, 1, 25, 4, 1, 4, 1, 1]
>>> from sage.all import * >>> Gamma0(Integer(11)).cusp_width(Cusps(oo)) 1 >>> Gamma0(Integer(11)).cusp_width(Integer(0)) 11 >>> [Gamma0(Integer(100)).cusp_width(c) for c in Gamma0(Integer(100)).cusps()] [100, 1, 4, 1, 1, 1, 4, 25, 1, 1, 4, 1, 25, 4, 1, 4, 1, 1]
- cusps(algorithm='default')[source]¶
Return a sorted list of inequivalent cusps for
self
, i.e. a set of representatives for the orbits ofself
on \(\mathbb{P}^1(\QQ)\).These should be returned in a reduced form where this makes sense.
INPUT:
algorithm
– which algorithm to use to compute the cusps ofself
.'default'
finds representatives for a known complete set of cusps.'modsym'
computes the boundary map on the space of weight two modular symbols associated toself
, which finds the cusps forself
in the process.
EXAMPLES:
sage: Gamma0(36).cusps() [0, 1/18, 1/12, 1/9, 1/6, 1/4, 1/3, 5/12, 1/2, 2/3, 5/6, Infinity] sage: Gamma0(36).cusps(algorithm='modsym') == Gamma0(36).cusps() # needs sage.libs.flint True sage: GammaH(36, [19,29]).cusps() == Gamma0(36).cusps() True sage: Gamma0(1).cusps() [Infinity]
>>> from sage.all import * >>> Gamma0(Integer(36)).cusps() [0, 1/18, 1/12, 1/9, 1/6, 1/4, 1/3, 5/12, 1/2, 2/3, 5/6, Infinity] >>> Gamma0(Integer(36)).cusps(algorithm='modsym') == Gamma0(Integer(36)).cusps() # needs sage.libs.flint True >>> GammaH(Integer(36), [Integer(19),Integer(29)]).cusps() == Gamma0(Integer(36)).cusps() True >>> Gamma0(Integer(1)).cusps() [Infinity]
- dimension_cusp_forms(k=2)[source]¶
Return the dimension of the space of weight k cusp forms for this group.
For \(k \ge 2\), this is given by a standard formula in terms of k and various invariants of the group; see Diamond + Shurman, “A First Course in Modular Forms”, section 3.5 and 3.6. If k is not given, default to k = 2.
For dimensions of spaces of cusp forms with character for Gamma1, use the dimension_cusp_forms method of the Gamma1 class, or the standalone function dimension_cusp_forms().
For weight 1 cusp forms this generic implementation only works in cases where one can prove solely via Riemann-Roch theory that there aren’t any cusp forms (i.e. when the number of regular cusps is strictly greater than the degree of the canonical divisor). Otherwise a
NotImplementedError
is raised.EXAMPLES:
sage: Gamma1(31).dimension_cusp_forms(2) 26 sage: Gamma(5).dimension_cusp_forms(1) 0 sage: Gamma1(4).dimension_cusp_forms(1) # irregular cusp 0 sage: Gamma(13).dimension_cusp_forms(1) Traceback (most recent call last): ... NotImplementedError: Computation of dimensions of weight 1 cusp forms spaces not implemented in general
>>> from sage.all import * >>> Gamma1(Integer(31)).dimension_cusp_forms(Integer(2)) 26 >>> Gamma(Integer(5)).dimension_cusp_forms(Integer(1)) 0 >>> Gamma1(Integer(4)).dimension_cusp_forms(Integer(1)) # irregular cusp 0 >>> Gamma(Integer(13)).dimension_cusp_forms(Integer(1)) Traceback (most recent call last): ... NotImplementedError: Computation of dimensions of weight 1 cusp forms spaces not implemented in general
- dimension_eis(k=2)[source]¶
Return the dimension of the space of weight k Eisenstein series for this group, which is a subspace of the space of modular forms complementary to the space of cusp forms.
INPUT:
k
– integer (default: 2)
EXAMPLES:
sage: GammaH(33,[2]).dimension_eis() 7 sage: GammaH(33,[2]).dimension_eis(3) 0 sage: GammaH(33, [2,5]).dimension_eis(2) 3 sage: GammaH(33, [4]).dimension_eis(1) 4
>>> from sage.all import * >>> GammaH(Integer(33),[Integer(2)]).dimension_eis() 7 >>> GammaH(Integer(33),[Integer(2)]).dimension_eis(Integer(3)) 0 >>> GammaH(Integer(33), [Integer(2),Integer(5)]).dimension_eis(Integer(2)) 3 >>> GammaH(Integer(33), [Integer(4)]).dimension_eis(Integer(1)) 4
- dimension_modular_forms(k=2)[source]¶
Return the dimension of the space of weight k modular forms for this group.
This is given by a standard formula in terms of k and various invariants of the group; see Diamond + Shurman, “A First Course in Modular Forms”, section 3.5 and 3.6. If k is not given, defaults to k = 2.
For dimensions of spaces of modular forms with character for Gamma1, use the dimension_modular_forms method of the Gamma1 class, or the standalone function dimension_modular_forms().
For weight 1 modular forms this generic implementation only works in cases where one can prove solely via Riemann-Roch theory that there aren’t any cusp forms (i.e. when the number of regular cusps is strictly greater than the degree of the canonical divisor). Otherwise a
NotImplementedError
is raised.EXAMPLES:
sage: Gamma1(31).dimension_modular_forms(2) 55 sage: Gamma1(3).dimension_modular_forms(1) 1 sage: Gamma1(4).dimension_modular_forms(1) # irregular cusp 1 sage: Gamma(13).dimension_modular_forms(1) Traceback (most recent call last): ... NotImplementedError: Computation of dimensions of weight 1 cusp forms spaces not implemented in general
>>> from sage.all import * >>> Gamma1(Integer(31)).dimension_modular_forms(Integer(2)) 55 >>> Gamma1(Integer(3)).dimension_modular_forms(Integer(1)) 1 >>> Gamma1(Integer(4)).dimension_modular_forms(Integer(1)) # irregular cusp 1 >>> Gamma(Integer(13)).dimension_modular_forms(Integer(1)) Traceback (most recent call last): ... NotImplementedError: Computation of dimensions of weight 1 cusp forms spaces not implemented in general
- farey_symbol()[source]¶
Return the Farey symbol associated to this subgroup.
See the
farey_symbol
module for more information.EXAMPLES:
sage: Gamma1(4).farey_symbol() FareySymbol(Congruence Subgroup Gamma1(4))
>>> from sage.all import * >>> Gamma1(Integer(4)).farey_symbol() FareySymbol(Congruence Subgroup Gamma1(4))
- gen(i)[source]¶
Return the \(i\)-th generator of self, i.e. the \(i\)-th element of the tuple
self.gens()
.EXAMPLES:
sage: SL2Z.gen(1) [1 1] [0 1]
>>> from sage.all import * >>> SL2Z.gen(Integer(1)) [1 1] [0 1]
- generalised_level()[source]¶
Return the generalised level of
self
, i.e., the least common multiple of the widths of all cusps.If
self
is even, Wohlfart’s theorem tells us that this is equal to the (conventional) level ofself
whenself
is a congruence subgroup. This can fail ifself
is odd, but the actual level is at most twice the generalised level. See the paper by Kiming, Schuett and Verrill for more examples.EXAMPLES:
sage: Gamma0(18).generalised_level() 18 sage: from sage.modular.arithgroup.arithgroup_perm import HsuExample18 # needs sage.groups sage: HsuExample18().generalised_level() # needs sage.groups 24
>>> from sage.all import * >>> Gamma0(Integer(18)).generalised_level() 18 >>> from sage.modular.arithgroup.arithgroup_perm import HsuExample18 # needs sage.groups >>> HsuExample18().generalised_level() # needs sage.groups 24
In the following example, the actual level is twice the generalised level. This is the group \(G_2\) from Example 17 of K-S-V.
sage: G = CongruenceSubgroup(8, [ [1,1,0,1], [3,-1,4,-1] ]) sage: G.level() 8 sage: G.generalised_level() 4
>>> from sage.all import * >>> G = CongruenceSubgroup(Integer(8), [ [Integer(1),Integer(1),Integer(0),Integer(1)], [Integer(3),-Integer(1),Integer(4),-Integer(1)] ]) >>> G.level() 8 >>> G.generalised_level() 4
- generators(algorithm='farey')[source]¶
Return a list of generators for this congruence subgroup. The result is cached.
INPUT:
algorithm
– string; eitherfarey
ortodd-coxeter
If
algorithm
is set to'farey'
, then the generators will be calculated using Farey symbols, which will always return a minimal generating set. Seefarey_symbol
for more information.If
algorithm
is set to'todd-coxeter'
, a simpler algorithm based on Todd-Coxeter enumeration will be used. This is exceedingly slow for general subgroups, and the list of generators will be far from minimal (indeed it may contain repetitions).EXAMPLES:
sage: Gamma(2).generators() [ [1 2] [ 3 -2] [-1 0] [0 1], [ 2 -1], [ 0 -1] ] sage: Gamma(2).generators(algorithm='todd-coxeter') [ [1 2] [-1 0] [ 1 0] [-1 0] [-1 2] [-1 0] [1 0] [0 1], [ 0 -1], [-2 1], [ 0 -1], [-2 3], [ 2 -1], [2 1] ]
>>> from sage.all import * >>> Gamma(Integer(2)).generators() [ [1 2] [ 3 -2] [-1 0] [0 1], [ 2 -1], [ 0 -1] ] >>> Gamma(Integer(2)).generators(algorithm='todd-coxeter') [ [1 2] [-1 0] [ 1 0] [-1 0] [-1 2] [-1 0] [1 0] [0 1], [ 0 -1], [-2 1], [ 0 -1], [-2 3], [ 2 -1], [2 1] ]
- gens(*args, **kwds)[source]¶
Return a tuple of generators for this congruence subgroup.
The generators need not be minimal. For arguments, see
generators()
.EXAMPLES:
sage: SL2Z.gens() ( [ 0 -1] [1 1] [ 1 0], [0 1] )
>>> from sage.all import * >>> SL2Z.gens() ( [ 0 -1] [1 1] [ 1 0], [0 1] )
- genus()[source]¶
Return the genus of the modular curve of
self
.EXAMPLES:
sage: Gamma1(5).genus() 0 sage: Gamma1(31).genus() 26 sage: from sage.modular.dims import dimension_cusp_forms sage: Gamma1(157).genus() == dimension_cusp_forms(Gamma1(157), 2) True sage: GammaH(7, [2]).genus() 0 sage: [Gamma0(n).genus() for n in [1..23]] [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 2, 2] sage: [n for n in [1..200] if Gamma0(n).genus() == 1] [11, 14, 15, 17, 19, 20, 21, 24, 27, 32, 36, 49]
>>> from sage.all import * >>> Gamma1(Integer(5)).genus() 0 >>> Gamma1(Integer(31)).genus() 26 >>> from sage.modular.dims import dimension_cusp_forms >>> Gamma1(Integer(157)).genus() == dimension_cusp_forms(Gamma1(Integer(157)), Integer(2)) True >>> GammaH(Integer(7), [Integer(2)]).genus() 0 >>> [Gamma0(n).genus() for n in (ellipsis_range(Integer(1),Ellipsis,Integer(23)))] [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 2, 2] >>> [n for n in (ellipsis_range(Integer(1),Ellipsis,Integer(200))) if Gamma0(n).genus() == Integer(1)] [11, 14, 15, 17, 19, 20, 21, 24, 27, 32, 36, 49]
- index()[source]¶
Return the index of
self
in the full modular group.EXAMPLES:
sage: Gamma0(17).index() 18 sage: sage.modular.arithgroup.congroup_generic.CongruenceSubgroup(5).index() Traceback (most recent call last): ... NotImplementedError
>>> from sage.all import * >>> Gamma0(Integer(17)).index() 18 >>> sage.modular.arithgroup.congroup_generic.CongruenceSubgroup(Integer(5)).index() Traceback (most recent call last): ... NotImplementedError
- is_abelian()[source]¶
Return
True
if this arithmetic subgroup is abelian.Since arithmetic subgroups are always nonabelian, this always returns
False
.EXAMPLES:
sage: SL2Z.is_abelian() False sage: Gamma0(3).is_abelian() False sage: Gamma1(12).is_abelian() False sage: GammaH(4, [3]).is_abelian() False
>>> from sage.all import * >>> SL2Z.is_abelian() False >>> Gamma0(Integer(3)).is_abelian() False >>> Gamma1(Integer(12)).is_abelian() False >>> GammaH(Integer(4), [Integer(3)]).is_abelian() False
- is_congruence()[source]¶
Return
True
ifself
is a congruence subgroup.EXAMPLES:
sage: Gamma0(5).is_congruence() True sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().is_congruence() Traceback (most recent call last): ... NotImplementedError
>>> from sage.all import * >>> Gamma0(Integer(5)).is_congruence() True >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().is_congruence() Traceback (most recent call last): ... NotImplementedError
- is_even()[source]¶
Return
True
precisely if this subgroup contains the matrix -1.EXAMPLES:
sage: SL2Z.is_even() True sage: Gamma0(20).is_even() True sage: Gamma1(5).is_even() False sage: GammaH(11, [3]).is_even() False
>>> from sage.all import * >>> SL2Z.is_even() True >>> Gamma0(Integer(20)).is_even() True >>> Gamma1(Integer(5)).is_even() False >>> GammaH(Integer(11), [Integer(3)]).is_even() False
- is_finite()[source]¶
Return
True
if this arithmetic subgroup is finite.Since arithmetic subgroups are always infinite, this always returns
False
.EXAMPLES:
sage: SL2Z.is_finite() False sage: Gamma0(3).is_finite() False sage: Gamma1(12).is_finite() False sage: GammaH(4, [3]).is_finite() False
>>> from sage.all import * >>> SL2Z.is_finite() False >>> Gamma0(Integer(3)).is_finite() False >>> Gamma1(Integer(12)).is_finite() False >>> GammaH(Integer(4), [Integer(3)]).is_finite() False
- is_normal()[source]¶
Return
True
precisely if this subgroup is a normal subgroup ofSL2Z
.EXAMPLES:
sage: Gamma(3).is_normal() True sage: Gamma1(3).is_normal() False
>>> from sage.all import * >>> Gamma(Integer(3)).is_normal() True >>> Gamma1(Integer(3)).is_normal() False
- is_odd()[source]¶
Return
True
precisely if this subgroup does not contain the matrix -1.EXAMPLES:
sage: SL2Z.is_odd() False sage: Gamma0(20).is_odd() False sage: Gamma1(5).is_odd() True sage: GammaH(11, [3]).is_odd() True
>>> from sage.all import * >>> SL2Z.is_odd() False >>> Gamma0(Integer(20)).is_odd() False >>> Gamma1(Integer(5)).is_odd() True >>> GammaH(Integer(11), [Integer(3)]).is_odd() True
- is_parent_of(x)[source]¶
Check whether this group is a valid parent for the element x.
Required by Sage’s testing framework.
EXAMPLES:
sage: Gamma(3).is_parent_of(ZZ(1)) False sage: Gamma(3).is_parent_of([1,0,0,1]) False sage: Gamma(3).is_parent_of(SL2Z([1,1,0,1])) False sage: Gamma(3).is_parent_of(SL2Z(1)) True
>>> from sage.all import * >>> Gamma(Integer(3)).is_parent_of(ZZ(Integer(1))) False >>> Gamma(Integer(3)).is_parent_of([Integer(1),Integer(0),Integer(0),Integer(1)]) False >>> Gamma(Integer(3)).is_parent_of(SL2Z([Integer(1),Integer(1),Integer(0),Integer(1)])) False >>> Gamma(Integer(3)).is_parent_of(SL2Z(Integer(1))) True
- is_regular_cusp(c)[source]¶
Return
True
if the orbit of the given cusp is a regular cusp forself
, otherwiseFalse
.This is automatically true if -1 is in
self
.EXAMPLES:
sage: Gamma1(4).is_regular_cusp(Cusps(1/2)) False sage: Gamma1(4).is_regular_cusp(Cusps(oo)) True
>>> from sage.all import * >>> Gamma1(Integer(4)).is_regular_cusp(Cusps(Integer(1)/Integer(2))) False >>> Gamma1(Integer(4)).is_regular_cusp(Cusps(oo)) True
- is_subgroup(right)[source]¶
Return
True
ifself
is a subgroup of right, andFalse
otherwise.For generic arithmetic subgroups this is done by the absurdly slow algorithm of checking all of the generators of
self
to see if they are inright
.EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().is_subgroup(SL2Z) Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.is_subgroup(Gamma1(18), Gamma0(6)) True
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().is_subgroup(SL2Z) Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.is_subgroup(Gamma1(Integer(18)), Gamma0(Integer(6))) True
- matrix_space()[source]¶
Return the parent space of the matrices, which is always
MatrixSpace(ZZ, 2)
.EXAMPLES:
sage: Gamma(3).matrix_space() Full MatrixSpace of 2 by 2 dense matrices over Integer Ring
>>> from sage.all import * >>> Gamma(Integer(3)).matrix_space() Full MatrixSpace of 2 by 2 dense matrices over Integer Ring
- ncusps()[source]¶
Return the number of cusps of this arithmetic subgroup.
This is provided as a separate function since for dimension formulae in even weight all we need to know is the number of cusps, and this can be calculated very quickly, while enumerating all cusps is much slower.
EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.ncusps(Gamma0(7)) 2
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.ncusps(Gamma0(Integer(7))) 2
- ngens()[source]¶
Return the size of the minimal generating set of
self
returned bygenerators()
.EXAMPLES:
sage: Gamma0(22).ngens() 8 sage: Gamma1(14).ngens() 13 sage: GammaH(11, [3]).ngens() 3 sage: SL2Z.ngens() 2
>>> from sage.all import * >>> Gamma0(Integer(22)).ngens() 8 >>> Gamma1(Integer(14)).ngens() 13 >>> GammaH(Integer(11), [Integer(3)]).ngens() 3 >>> SL2Z.ngens() 2
- nirregcusps()[source]¶
Return the number of cusps of
self
that are “irregular”, i.e. their stabiliser can only be generated by elements with both eigenvalues -1 rather than +1.If the group contains -1, every cusp is clearly regular.
EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nirregcusps(Gamma1(4)) 1
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nirregcusps(Gamma1(Integer(4))) 1
- nregcusps()[source]¶
Return the number of cusps of
self
that are “regular”, i.e. their stabiliser has a generator with both eigenvalues +1 rather than -1.If the group contains -1, every cusp is clearly regular.
EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nregcusps(Gamma1(4)) 2
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nregcusps(Gamma1(Integer(4))) 2
- nu2()[source]¶
Return the number of orbits of elliptic points of order 2 for this arithmetic subgroup.
EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().nu2() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nu2(Gamma0(1105)) == 8 True
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().nu2() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nu2(Gamma0(Integer(1105))) == Integer(8) True
- nu3()[source]¶
Return the number of orbits of elliptic points of order 3 for this arithmetic subgroup.
EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().nu3() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nu3(Gamma0(1729)) == 8 True
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().nu3() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'> >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nu3(Gamma0(Integer(1729))) == Integer(8) True
We test that a bug in handling of subgroups not containing -1 is fixed:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nu3(GammaH(7, [2])) 2
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup.nu3(GammaH(Integer(7), [Integer(2)])) 2
- order()[source]¶
Return the number of elements in this arithmetic subgroup.
Since arithmetic subgroups are always infinite, this always returns infinity.
EXAMPLES:
sage: SL2Z.order() +Infinity sage: Gamma0(5).order() +Infinity sage: Gamma1(2).order() +Infinity sage: GammaH(12, [5]).order() +Infinity
>>> from sage.all import * >>> SL2Z.order() +Infinity >>> Gamma0(Integer(5)).order() +Infinity >>> Gamma1(Integer(2)).order() +Infinity >>> GammaH(Integer(12), [Integer(5)]).order() +Infinity
- projective_index()[source]¶
Return the index of the image of
self
in \(\PSL_2(\ZZ)\). This is equal to the index ofself
ifself
contains -1, and half of this otherwise.This is equal to the degree of the natural map from the modular curve of
self
to the \(j\)-line.EXAMPLES:
sage: Gamma0(5).projective_index() 6 sage: Gamma1(5).projective_index() 12
>>> from sage.all import * >>> Gamma0(Integer(5)).projective_index() 6 >>> Gamma1(Integer(5)).projective_index() 12
- reduce_cusp(c)[source]¶
Given a cusp \(c \in \mathbb{P}^1(\QQ)\), return the unique reduced cusp equivalent to c under the action of self, where a reduced cusp is an element \(\tfrac{r}{s}\) with r,s coprime nonnegative integers, s as small as possible, and r as small as possible for that s.
Note
This function should be overridden by all subclasses.
EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().reduce_cusp(1/4) Traceback (most recent call last): ... NotImplementedError
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().reduce_cusp(Integer(1)/Integer(4)) Traceback (most recent call last): ... NotImplementedError
- sturm_bound(weight=2)[source]¶
Return the Sturm bound for modular forms of the given weight and level this subgroup.
INPUT:
weight
– integer \(\geq 2\) (default: 2)
EXAMPLES:
sage: Gamma0(11).sturm_bound(2) 2 sage: Gamma0(389).sturm_bound(2) 65 sage: Gamma0(1).sturm_bound(12) 1 sage: Gamma0(100).sturm_bound(2) 30 sage: Gamma0(1).sturm_bound(36) 3 sage: Gamma0(11).sturm_bound() 2 sage: Gamma0(13).sturm_bound() 3 sage: Gamma0(16).sturm_bound() 4 sage: GammaH(16,[13]).sturm_bound() 8 sage: GammaH(16,[15]).sturm_bound() 16 sage: Gamma1(16).sturm_bound() 32 sage: Gamma1(13).sturm_bound() 28 sage: Gamma1(13).sturm_bound(5) 70
>>> from sage.all import * >>> Gamma0(Integer(11)).sturm_bound(Integer(2)) 2 >>> Gamma0(Integer(389)).sturm_bound(Integer(2)) 65 >>> Gamma0(Integer(1)).sturm_bound(Integer(12)) 1 >>> Gamma0(Integer(100)).sturm_bound(Integer(2)) 30 >>> Gamma0(Integer(1)).sturm_bound(Integer(36)) 3 >>> Gamma0(Integer(11)).sturm_bound() 2 >>> Gamma0(Integer(13)).sturm_bound() 3 >>> Gamma0(Integer(16)).sturm_bound() 4 >>> GammaH(Integer(16),[Integer(13)]).sturm_bound() 8 >>> GammaH(Integer(16),[Integer(15)]).sturm_bound() 16 >>> Gamma1(Integer(16)).sturm_bound() 32 >>> Gamma1(Integer(13)).sturm_bound() 28 >>> Gamma1(Integer(13)).sturm_bound(Integer(5)) 70
FURTHER DETAILS: This function returns a positive integer \(n\) such that the Hecke operators \(T_1,\ldots, T_n\) acting on cusp forms generate the Hecke algebra as a \(\ZZ\)-module when the character is trivial or quadratic. Otherwise, \(T_1,\ldots,T_n\) generate the Hecke algebra at least as a \(\ZZ[\varepsilon]\)-module, where \(\ZZ[\varepsilon]\) is the ring generated by the values of the Dirichlet character \(\varepsilon\). Alternatively, this is a bound such that if two cusp forms associated to this space of modular symbols are congruent modulo \((\lambda, q^n)\), then they are congruent modulo \(\lambda\).
REFERENCES:
See the Agashe-Stein appendix to Lario and Schoof, Some computations with Hecke rings and deformation rings, Experimental Math., 11 (2002), no. 2, 303-311.
This result originated in the paper Sturm, On the congruence of modular forms, Springer LNM 1240, 275-280, 1987.
REMARK: Kevin Buzzard pointed out to me (William Stein) in Fall 2002 that the above bound is fine for \(\Gamma_1(N)\) with character, as one sees by taking a power of \(f\). More precisely, if \(f \cong 0 \pmod{p}\) for first \(s\) coefficients, then \(f^r \cong 0 \pmod{p}\) for first \(sr\) coefficients. Since the weight of \(f^r\) is \(r\cdot k(f)\), it follows that if \(s \geq b\), where \(b\) is the Sturm bound for \(\Gamma_0(N)\) at weight \(k(f)\), then \(f^r\) has valuation large enough to be forced to be \(0\) at \(r*k(f)\) by Sturm bound (which is valid if we choose \(r\) correctly). Thus \(f \cong 0 \pmod{p}\). Conclusion: For \(\Gamma_1(N)\) with fixed character, the Sturm bound is exactly the same as for \(\Gamma_0(N)\).
A key point is that we are finding \(\ZZ[\varepsilon]\) generators for the Hecke algebra here, not \(\ZZ\)-generators. So if one wants generators for the Hecke algebra over \(\ZZ\), this bound must be suitably modified (and I’m not sure what the modification is).
AUTHORS:
William Stein
- to_even_subgroup()[source]¶
Return the smallest even subgroup of \(SL(2, \ZZ)\) containing
self
.EXAMPLES:
sage: sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().to_even_subgroup() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'>
>>> from sage.all import * >>> sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup().to_even_subgroup() Traceback (most recent call last): ... NotImplementedError: Please implement _contains_sl2 for <class 'sage.modular.arithgroup.arithgroup_generic.ArithmeticSubgroup_with_category'>
- todd_coxeter(G=None, on_right=True)[source]¶
Compute coset representatives for
self \\ G
and action of standard generators on them via Todd-Coxeter enumeration.If
G
isNone
, default toSL2Z
. The method also computes generators of the subgroup at same time.INPUT:
G
– intermediate subgroup (currently not implemented if different from SL(2,Z))on_right
– boolean (default:True
); ifTrue
return right coset enumeration, ifFalse
return left one
This is extremely slow in general.
OUTPUT: list of coset representatives
a list of generators for the group
l
– list of integers that correspond to the action of the standard parabolic element [[1,1],[0,1]] of \(SL(2,\ZZ)\) on the cosets ofself
.s
– list of integers that correspond to the action of the standard element of order \(2\) [[0,-1],[1,0]] on the cosets ofself
EXAMPLES:
sage: L = SL2Z([1,1,0,1]) sage: S = SL2Z([0,-1,1,0]) sage: G = Gamma(2) sage: reps, gens, l, s = G.todd_coxeter() sage: len(reps) == G.index() True sage: all(reps[i] * L * ~reps[l[i]] in G for i in range(6)) True sage: all(reps[i] * S * ~reps[s[i]] in G for i in range(6)) True sage: G = Gamma0(7) sage: reps, gens, l, s = G.todd_coxeter() sage: len(reps) == G.index() True sage: all(reps[i] * L * ~reps[l[i]] in G for i in range(8)) True sage: all(reps[i] * S * ~reps[s[i]] in G for i in range(8)) True sage: G = Gamma1(3) sage: reps, gens, l, s = G.todd_coxeter(on_right=False) sage: len(reps) == G.index() True sage: all(~reps[l[i]] * L * reps[i] in G for i in range(8)) True sage: all(~reps[s[i]] * S * reps[i] in G for i in range(8)) True sage: G = Gamma0(5) sage: reps, gens, l, s = G.todd_coxeter(on_right=False) sage: len(reps) == G.index() True sage: all(~reps[l[i]] * L * reps[i] in G for i in range(6)) True sage: all(~reps[s[i]] * S * reps[i] in G for i in range(6)) True
>>> from sage.all import * >>> L = SL2Z([Integer(1),Integer(1),Integer(0),Integer(1)]) >>> S = SL2Z([Integer(0),-Integer(1),Integer(1),Integer(0)]) >>> G = Gamma(Integer(2)) >>> reps, gens, l, s = G.todd_coxeter() >>> len(reps) == G.index() True >>> all(reps[i] * L * ~reps[l[i]] in G for i in range(Integer(6))) True >>> all(reps[i] * S * ~reps[s[i]] in G for i in range(Integer(6))) True >>> G = Gamma0(Integer(7)) >>> reps, gens, l, s = G.todd_coxeter() >>> len(reps) == G.index() True >>> all(reps[i] * L * ~reps[l[i]] in G for i in range(Integer(8))) True >>> all(reps[i] * S * ~reps[s[i]] in G for i in range(Integer(8))) True >>> G = Gamma1(Integer(3)) >>> reps, gens, l, s = G.todd_coxeter(on_right=False) >>> len(reps) == G.index() True >>> all(~reps[l[i]] * L * reps[i] in G for i in range(Integer(8))) True >>> all(~reps[s[i]] * S * reps[i] in G for i in range(Integer(8))) True >>> G = Gamma0(Integer(5)) >>> reps, gens, l, s = G.todd_coxeter(on_right=False) >>> len(reps) == G.index() True >>> all(~reps[l[i]] * L * reps[i] in G for i in range(Integer(6))) True >>> all(~reps[s[i]] * S * reps[i] in G for i in range(Integer(6))) True
- sage.modular.arithgroup.arithgroup_generic.is_ArithmeticSubgroup(x)[source]¶
Return
True
ifx
is of typeArithmeticSubgroup
.EXAMPLES:
sage: from sage.modular.arithgroup.all import is_ArithmeticSubgroup sage: is_ArithmeticSubgroup(GL(2, GF(7))) doctest:warning... DeprecationWarning: The function is_ArithmeticSubgroup is deprecated; use 'isinstance(..., ArithmeticSubgroup)' instead. See https://github.com/sagemath/sage/issues/38035 for details. False sage: is_ArithmeticSubgroup(Gamma0(4)) True
>>> from sage.all import * >>> from sage.modular.arithgroup.all import is_ArithmeticSubgroup >>> is_ArithmeticSubgroup(GL(Integer(2), GF(Integer(7)))) doctest:warning... DeprecationWarning: The function is_ArithmeticSubgroup is deprecated; use 'isinstance(..., ArithmeticSubgroup)' instead. See https://github.com/sagemath/sage/issues/38035 for details. False >>> is_ArithmeticSubgroup(Gamma0(Integer(4))) True