This section describes routines for working with modular forms and modular form spaces.

These structures are initialized by the `mfinit`

command; supported
modular form *spaces* with corresponding flags are the following:

***** The full modular form space M_{k}(Γ_{0}(N),χ), where k is an
integer or a half-integer and χ a Dirichlet character modulo N
(flag 4, the default).

***** The cuspidal space S_{k}(Γ_{0}(N),χ) (flag 1).

***** The Eisenstein space ℰ_{k}(Γ_{0}(N),χ) (flag 3), so
that M_{k} = ℰ_{k}⨁ S_{k}.

***** The new space S_{k}^{new}(Γ_{0}(N),χ) (flag 0).

***** The old space S_{k}^{old}(Γ_{0}(N),χ) (flag 2), so that
S_{k} = S_{k}^{new}⨁ S_{k}^{old}.

These resulting `mf`

structure contains a basis of modular forms, which
is accessed by the function `mfbasis`

; the elements of this basis have
Fourier coefficients in the cyclotomic field ℚ(χ). These coefficients
are given algebraically, as rational numbers or `t_POLMOD`

s. The member
function `mf.mod`

recovers the modulus used to define ℚ(χ), which
is a cyclotomic polynomial Φ_{n}(t). When needed, the elements of
ℚ(χ) are considered to be canonically embedded into ℂ via
`Mod`

(t,Φ_{n}(t)) ` ⟼ `

exp(2iπ/n).

The basis of eigenforms for the new space is obtained by the function
`mfeigenbasis`

: the elements of this basis now have Fourier coefficients
in a relative field extension of ℚ(χ). Note that if the space is
larger than the new space (i.e. is the cuspidal or full space) we
nevertheless obtain only the eigenbasis for the new space.

A modular form is represented in a special internal format giving the
possibility to compute an arbitrary number of terms of its Fourier coefficients
at infinity [a(0),a(1),...,a(n)] using the function `mfcoefs`

. These
coefficients are given algebraically, as rational numbers or `t_POLMOD`

s.
The member function `f.mod`

recovers the modulus used in the
coefficients of f, which will be the same as for k = ℚ(χ) (a cyclotomic
polynomial), or define a number field extension K/k.

Modular forms are obtained either directly from other mathematical objects,
e.g., elliptic curves, or by a specific formula, e.g., Eisenstein series or
Ramanujan's Delta function, or by applying standard operators to existing forms
(Hecke operators, Rankin-Cohen brackets,...). A function `mfparams`

is
provided so that one can recover the level, weight, character and field of
definition corresponding to a given modular form.

A number of creation functions and operations are provided. It is however
important to note that strictly speaking some of these operations create
objects which are *not* modular forms: typical examples are
derivation or integration of modular forms, the Eisenstein series E_{2}, eta
quotients, or quotients of modular forms. These objects are nonetheless very
important in the theory, so are not considered as errors; however the user must
be aware that no attempt is made to check that the objects that he handles are
really modular. When the documentation of a function does not state that it
applies to generalized modular forms, then the output is undefined if the
input is not a true modular form.

If F is a modular form in `mf`

, output the L-functions
corresponding to its [ℚ(F):ℚ(χ)] complex embeddings, ready for use with
the `lfun`

package. If F is omitted, output the L-functions attached
to all eigenforms in the new space; the result is a vector whose length is
the number of Galois orbits of newforms. Each entry contains the vector of
L-functions corresponding to the d complex embeddings of an orbit of
dimension d over ℚ(χ).

? mf = mfinit([35,2],0);mffields(mf) %1 = [y, y^2 - y - 4] ? f = mfeigenbasis(mf)[2]; mfparams(f) \\ orbit of dimension two %2 = [35, 2, 1, y^2 - y - 4, t - 1] ? [L1,L2] = lfunmf(mf, f); \\ Two L-functions ? lfun(L1,1) %4 = 0.81018461849460161754947375433874745585 ? lfun(L2,1) %5 = 0.46007635204895314548435893464149369804 ? [ lfun(L,1) | L <- concat(lfunmf(mf)) ] %6 = [0.70291..., 0.81018..., 0.46007...]

The `concat`

instruction concatenates the vectors
corresponding to the various (here two) orbits, so that we obtain the vector
of all the L-functions attached to eigenforms.

The library syntax is `GEN `

.**lfunmf**(GEN mf, GEN F = NULL, long bitprec)

Mf structure corresponding to the Ramanujan Delta function Δ.

? mfcoefs(mfDelta(),4) %1 = [0, 1, -24, 252, -1472]

The library syntax is `GEN `

.**mfDelta**()

k being in 1/2+ℤ_{ ≥ 0}, return the mf structure corresponding to the Cohen-Eisenstein series H_{k} of
weight k on Γ_{0}(4).

? H = mfEH(13/2); mfcoefs(H,4) %1 = [691/32760, -1/252, 0, 0, -2017/252]

The coefficients of H are given by the Cohen-Hurwitz function H(k-1/2,N) and can be obtained for moderately large values of N (the algorithm uses Õ(N) time):

? mfcoef(H,10^5+1) time = 55 ms. %2 = -12514802881532791504208348 ? mfcoef(H,10^7+1) time = 6,044 ms. %3 = -1251433416009877455212672599325104476

The library syntax is `GEN `

.**mfEH**(GEN k)

K being an even nonnegative integer, return the mf structure
corresponding to the standard Eisenstein series E_{k}.

? mfcoefs(mfEk(8), 4) %1 = [1, 480, 61920, 1050240, 7926240]

The library syntax is `GEN `

.**mfEk**(long k)

The unary theta function corresponding to the primitive Dirichlet character ψ. Its level is 4 F(ψ)^2 and its weight is 1 - ψ(-1)/2.

? Ser(mfcoefs(mfTheta(),30)) %1 = 1 + 2*x + 2*x^4 + 2*x^9 + 2*x^16 + 2*x^25 + O(x^31) ? f = mfTheta(8); Ser(mfcoefs(f,30)) %2 = 2*x - 2*x^9 - 2*x^25 + O(x^31) ? mfparams(f) %3 = [256, 1/2, 8, y, t + 1] ? g = mfTheta(-8); Ser(mfcoefs(g,30)) %4 = 2*x + 6*x^9 - 10*x^25 + O(x^31) ? mfparams(g) %5 = [256, 3/2, 8, y, t + 1] ? h = mfTheta(Mod(2,5)); mfparams(h) %6 = [100, 3/2, Mod(7, 20), y, t^2 + 1]

The library syntax is `GEN `

.**mfTheta**(GEN psi = NULL)

Given a `mfatk`

output by `mfatk = mfatkininit(mf,Q)`

and
a modular form f belonging to the pace `mf`

, returns the modular
form g = C x f|W_{Q}, where C = `mfatk[3]`

is a normalizing
constant such that g has the same field of coefficients as f;
`mfatk[3]`

gives the constant C, and `mfatk[1]`

gives
the modular form space to which g belongs (or is set to 0 if
it is `mf`

).

? mf = mfinit([35,2],0); [f] = mfbasis(mf); ? mfcoefs(f, 4) %2 = [0, 3, -1, 0, 3] ? mfatk = mfatkininit(mf,7); ? g = mfatkin(mfatk, f); mfcoefs(g, 4) %4 = [0, 1, -1, -2, 7] ? mfatk = mfatkininit(mf,35); ? g = mfatkin(mfatk, f); mfcoefs(g, 4) %6 = [0, -3, 1, 0, -3]

The library syntax is `GEN `

.**mfatkin**(GEN mfatk, GEN f)

Given a modular form space `mf`

of integral weight k and a primitive
divisor Q of the level N of `mf`

, outputs the Atkin-Lehner
eigenvalues of w_{Q} on the new space, grouped by orbit. If the Nebentypus
χ of `mf`

is a
(trivial or) quadratic character defined modulo N/Q, the result is rounded
and the eigenvalues are ± i^k.

? mf = mfinit([35,2],0); mffields(mf) %1 = [y, y^2 - y - 4] \\ two orbits, dimension 1 and 2 ? mfatkineigenvalues(mf,5) %2 = [[1], [-1, -1]] ? mf = mfinit([12,7,Mod(3,4)],0); ? mfatkineigenvalues(mf,3) %4 = [[I, -I, -I, I, I, -I]] \\ one orbit

To obtain the eigenvalues on a larger space than the new space,
e.g., the full space, you can directly call `[mfB,M,C] = mfatkininit`

and
compute the eigenvalues as the roots of the characteristic polynomial of
M/C, by dividing the roots of `charpoly(M)`

by C. Note that the
characteristic polynomial is computed exactly since M has coefficients in
ℚ(χ), whereas C may be given by a complex number. If the coefficients
of the characteristic polynomial are polmods modulo T they must be embedded
to ℂ first using `subst(lift(), t, exp(2*I*Pi/n))`

, when T is
`poliscyclo(n)`

; note that T = `mf.mod`

.

The library syntax is `GEN `

.**mfatkineigenvalues**(GEN mf, long Q, long prec)

Given a modular form space with parameters N,k,χ and a
primitive divisor Q of the level N, initializes data necessary for
working with the Atkin-Lehner operator W_{Q}, for now only the function
`mfatkin`

. We write χ ~ χ_{Q} χ_{N/Q} where
the two characters are primitive with (coprime) conductors dividing
Q and N/Q respectively. For F ∈ M_{k}(Γ_{0}(N),χ),
the form F | W_{Q} still has level N and weight k but its
Nebentypus may no longer be χ: it becomes χ_{Q} χ_{N/Q})
if k is integral and χ_{Q} χ_{N/Q})(4Q/.) if not.

The result is a technical 4-component vector `[mfB, MC, C, mf]`

, where

***** `mfB`

encodes the modular form space to which
F|W_{Q} belongs when F ∈ M_{k}(Γ_{0}(N), χ): an `mfinit`

corresponding to a new Nebentypus or the integer 0 when the character does
not change. This does not depend on F.

***** `MC`

is the matrix of W_{Q} on the bases of `mf`

and `mfB`

multiplied by a normalizing constant C(k,χ,Q). This matrix has polmod
coefficients in ℚ(χ).

***** `C`

is the complex constant C(k,χ,Q). For k
integral, let A(k,χ, Q) = Q^{ϵ}/g(χ_{Q}), where
ϵ = 0 for k even and 1/2 for k odd and
where g(χ_{Q}) is the Gauss sum attached to χ_{Q}). (A similar, more
complicated, definition holds in half-integral weight depending on the parity
of k - 1/2.) Then if M denotes the matrix of W_{Q} on the bases
of `mf`

and `mfB`

, A.M has coefficients in ℚ(χ).
If A is rational, we let C = 1 and C = A as a floating point complex
number otherwise, and finally `MC`

:= M.C.

? mf=mfinit([32,4],0); [mfB,MC,C]=mfatkininit(mf,32); MC %1 = [5/16 11/2 55/8] [ 1/8 0 -5/4] [1/32 -1/4 11/16] ? C %2 = 1 ? mf=mfinit([32,4,8],0); [mfB,MC,C]=mfatkininit(mf,32); MC %3 = [ 1/8 -7/4] [-1/16 -1/8] ? C %4 = 0.35355339059327376220042218105242451964 ? algdep(C,2) \\ C = 1/sqrt(8) %5 = 8*x^2 - 1

The library syntax is `GEN `

.**mfatkininit**(GEN mf, long Q, long prec)

If NK = [N,k,*CHI*] as in `mfinit`

, gives a basis of the
corresponding subspace of M_{k}(Γ_{0}(N),χ). NK can also be the
output of `mfinit`

, in which case `space`

can be omitted.
To obtain the eigenforms, use `mfeigenbasis`

.

If `space`

is a full space M_{k}, the output is the union of first, a
basis of the space of Eisenstein series, and second, a basis of the cuspidal
space.

? see(L) = apply(f->mfcoefs(f,3), L); ? mf = mfinit([35,2],0); ? see( mfbasis(mf) ) %2 = [[0, 3, -1, 0], [0, -1, 9, -8], [0, 0, -8, 10]] ? see( mfeigenbasis(mf) ) %3 = [[0, 1, 0, 1], [Mod(0, z^2 - z - 4), Mod(1, z^2 - z - 4), \ Mod(-z, z^2 - z - 4), Mod(z - 1, z^2 - z - 4)]] ? mf = mfinit([35,2]); ? see( mfbasis(mf) ) %5 = [[1/6, 1, 3, 4], [1/4, 1, 3, 4], [17/12, 1, 3, 4], \ [0, 3, -1, 0], [0, -1, 9, -8], [0, 0, -8, 10]] ? see( mfbasis([48,4],0) ) %6 = [[0, 3, 0, -3], [0, -3, 0, 27], [0, 2, 0, 30]]

The library syntax is `GEN `

.**mfbasis**(GEN NK, long space)

F being a generalized modular form, return B(d)(F), where B(d) is
the expanding operator τ` ⟼ `

dτ.

? D2=mfbd(mfDelta(),2); mfcoefs(D2, 6) %1 = [0, 0, 1, 0, -24, 0, 252]

The library syntax is `GEN `

.**mfbd**(GEN F, long d)

Compute the m-th Rankin-Cohen bracket of the generalized modular forms F and G.

? E4 = mfEk(4); E6 = mfEk(6); ? D1 = mfbracket(E4,E4,2); mfcoefs(D1,5)/4800 %2 = [0, 1, -24, 252, -1472, 4830] ? D2 = mfbracket(E4,E6,1); mfcoefs(D2,10)/(-3456) %3 = [0, 1, -24, 252, -1472, 4830]

The library syntax is `GEN `

.**mfbracket**(GEN F, GEN G, long m)

Compute the n-th Fourier coefficient a(n) of the generalized modular
form F. Note that this is the n+1-st component of the vector
`mfcoefs(F,n)`

as well as the second component of `mfcoefs(F,1,n)`

.

? mfcoef(mfDelta(),10) %1 = -115920

The library syntax is `GEN `

.**mfcoef**(GEN F, long n)

Compute the vector of Fourier coefficients [a[0],a[d],...,a[nd]] of the generalized modular form F; d must be positive and d = 1 by default.

? D = mfDelta(); ? mfcoefs(D,10) %2 = [0, 1, -24, 252, -1472, 4830, -6048, -16744, 84480, -113643, -115920] ? mfcoefs(D,5,2) %3 = [0, -24, -1472, -6048, 84480, -115920] ? mfcoef(D,10) %4 = -115920

This function also applies when F is a modular form space as output by
`mfinit`

; it then returns the matrix whose columns give the Fourier
expansions of the elements of `mfbasis`

(F):

? mf = mfinit([1,12]); ? mfcoefs(mf,5) %2 = [691/65520 0] [ 1 1] [ 2049 -24] [ 177148 252] [ 4196353 -1472] [ 48828126 4830]

The library syntax is `GEN `

.**mfcoefs**(GEN F, long n, long d)

`mf`

being output by `mfinit`

for the cuspidal space and
F a modular form, gives the smallest level at which F is defined.
In particular, if F is cuspidal and we write F = ∑_{j} B(d_{j}) f_{j}
for new forms f_{j} of level N_{j} (see `mftonew`

), then its conductor
is the least common multiple of the d_{j} N_{j}.

? mf=mfinit([96,6],1); vF = mfbasis(mf); mfdim(mf) %1 = 72 ? vector(10,i, mfconductor(mf, vF[i])) %2 = [3, 6, 12, 24, 48, 96, 4, 8, 12, 16]

The library syntax is `long `

.**mfconductor**(GEN mf, GEN F)

Let N be a positive integer. Return the list of right cosets of
Γ_{0}(N) \Γ, i.e., matrices γ_{j} ∈ Γ such that
Γ = ⨆ _{j} Γ_{0}(N) γ_{j}.
The γ_{j} are chosen in the form [a,b;c,d] with c | N.

? mfcosets(4) %1 = [[0, -1; 1, 0], [1, 0; 1, 1], [0, -1; 1, 2], [0, -1; 1, 3],\ [1, 0; 2, 1], [1, 0; 4, 1]]

We also allow the argument N to be a modular form space, in which case it is replaced by the level of the space:

? M = mfinit([4, 12, 1], 0); mfcosets(M) %2 = [[0, -1; 1, 0], [1, 0; 1, 1], [0, -1; 1, 2], [0, -1; 1, 3],\ [1, 0; 2, 1], [1, 0; 4, 1]]

**Warning.** In the present implementation, the trivial coset is
represented by [1,0;N,1] and is the last in the list.

The library syntax is `GEN `

.**mfcosets**(GEN N)

In the space defined by `NK = [N,k,CHI]`

or `NK = mf`

,
determine if `cusp`

in canonical format (oo or denominator
dividing N) is regular or not.

? mfcuspisregular([4,3,-4],1/2) %1 = 0

The library syntax is `long `

.**mfcuspisregular**(GEN NK, GEN cusp)

Let N be a positive integer. Return the list of cusps of Γ_{0}(N)
in the form a/b with b | N.

? mfcusps(24) %1 = [0, 1/2, 1/3, 1/4, 1/6, 1/8, 1/12, 1/24]

We also allow the argument N to be a modular form space, in which case it is replaced by the level of the space:

? M = mfinit([4, 12, 1], 0); mfcusps(M) %2 = [0, 1/2, 1/4]

The library syntax is `GEN `

.**mfcusps**(GEN N)

Valuation of modular form F in the space `mf`

at
`cusp`

, which can be either oo or any rational number. The
result is either a rational number or oo if F is zero. Let
χ be the Nebentypus of the space `mf`

; if ℚ(F) != ℚ(χ),
return the vector of valuations attached to the [ℚ(F):ℚ(chi)] complex
embeddings of F.

? T=mfTheta(); mf=mfinit([12,1/2]); mfcusps(12) %1 = [0, 1/2, 1/3, 1/4, 1/6, 1/12] ? apply(x->mfcuspval(mf,T,x), %1) %2 = [0, 1/4, 0, 0, 1/4, 0] ? mf=mfinit([12,6,12],1); F=mfbasis(mf)[5]; ? apply(x->mfcuspval(mf,F,x),%1) %4 = [1/12, 1/6, 1/2, 2/3, 1/2, 2] ? mf=mfinit([12,3,-4],1); F=mfbasis(mf)[1]; ? apply(x->mfcuspval(mf,F,x),%1) %6 = [1/12, 1/6, 1/4, 2/3, 1/2, 1] ? mf = mfinit([625,2],0); [F] = mfeigenbasis(mf); mfparams(F) %7 = [625, 2, 1, y^2 - y - 1, t - 1] \\ [Q(F):Q(chi)] = 2 ? mfcuspval(mf, F, 1/25) %8 = [1, 2] \\ one conjugate has valuation 1, and the other is 2 ? mfcuspval(mf, F, 1/5) %9 = [1/25, 1/25]

The library syntax is `GEN `

.**mfcuspval**(GEN mf, GEN F, GEN cusp, long bitprec)

Width of `cusp`

in Γ_{0}(N).

? mfcusps(12) %1 = [0, 1/2, 1/3, 1/4, 1/6, 1/12] ? [mfcuspwidth(12,c) | c <- mfcusps(12)] %2 = [12, 3, 4, 3, 1, 1] ? mfcuspwidth(12, oo) %3 = 1

We also allow the argument N to be a modular form space, in which case it is replaced by the level of the space:

? M = mfinit([4, 12, 1], 0); mfcuspwidth(M, 1/2) %4 = 1

The library syntax is `long `

.**mfcuspwidth**(GEN N, GEN cusp)

m-th formal derivative of the power series corresponding to the generalized modular form F, with respect to the differential operator qd/dq (default m = 1).

? D=mfDelta(); ? mfcoefs(D, 4) %2 = [0, 1, -24, 252, -1472] ? mfcoefs(mfderiv(D), 4) %3 = [0, 1, -48, 756, -5888]

The library syntax is `GEN `

.**mfderiv**(GEN F, long m)

Compute the Serre derivative (q.d/dq)F - kE_2F/12 of the generalized modular form F, which has weight k+2; if F is a true modular form, then its Serre derivative is also modular. If m > 1, compute the m-th iterate, of weight k + 2m.

? mfcoefs(mfderivE2(mfEk(4)),5)*(-3) %1 = [1, -504, -16632, -122976, -532728] ? mfcoefs(mfEk(6),5) %2 = [1, -504, -16632, -122976, -532728]

The library syntax is `GEN `

.**mfderivE2**(GEN F, long m)

Gives a human-readable description of F, which is either a modular
form space or a generalized modular form. If the address of G is given,
puts into G the vector of parameters of the outermost operator defining F;
this vector is empty if F is a leaf (an atomic object such as
`mfDelta()`

, not defined in terms of other forms) or a modular form space.

? E1 = mfeisenstein(4,-3,-4); mfdescribe(E1) %1 = "F_{4}(-3, -4)" ? E2 = mfeisenstein(3,5,-7); mfdescribe(E2) %2 = "F_{3}(5, -7)" ? E3 = mfderivE2(mfmul(E1,E2), 3); mfdescribe(E3,&G) %3 = "DERE2^3(MUL(F_{4}(-3, -4), F_{3}(5, -7)))" ? mfdescribe(G[1][1]) %4 = "MUL(F_{4}(-3, -4), F_{3}(5, -7))" ? G[2] %5 = 3 ? for (i = 0, 4, mf = mfinit([37,4],i); print(mfdescribe(mf))); S_{4}^new(G_{0}(37, 1)) S_{4}(G_{0}(37, 1)) S_{4}^old(G_{0}(37, 1)) E_{4}(G_{0}(37, 1)) M_{4}(G_{0}(37, 1))

The library syntax is `GEN `

.**mfdescribe**(GEN F, GEN *G = NULL)

If NK = [N,k,*CHI*] as in `mfinit`

, gives the dimension of the
corresponding subspace of M_{k}(Γ_{0}(N),χ). NK can also be the
output of `mfinit`

, in which case space must be omitted.

The subspace is described by the small integer `space`

: 0 for the
newspace S_{k}^{new}(Γ_{0}(N),χ), 1 for the cuspidal
space S_{k}, 2 for the oldspace S_{k}^{old}, 3 for the space of
Eisenstein series E_{k} and 4 for the full space M_{k}.

**Wildcards.**
As in `mfinit`

, *CHI* may be the wildcard 0
(all Galois orbits of characters); in this case, the output is a vector of
[*order*, *conrey*, *dim*, *dimdih*] corresponding
to the nontrivial spaces, where

***** *order* is the order of the character,

***** *conrey* its Conrey label from which the character may be recovered
via `znchar`

(*conrey*),

***** *dim* the dimension of the corresponding space,

***** *dimdih* the dimension of the subspace of dihedral forms
corresponding to Hecke characters if k = 1 (this is not implemented for
the old space and set to -1 for the time being) and 0 otherwise.

The spaces are sorted by increasing order of the character; the characters are taken up to Galois conjugation and the Conrey number is the minimal one among Galois conjugates. In weight 1, this is only implemented when the space is 0 (newspace), 1 (cusp space), 2(old space) or 3(Eisenstein series).

**Wildcards for sets of characters.** *CHI* may be a set
of characters, and we return the set of [*dim*,*dimdih*].

**Wildcard for M _{k}(Γ_{1}(N)).**
Additionally, the wildcard

? mfdim([23,2], 0) \\ new space %1 = 2 ? mfdim([96,6], 0) %2 = 10 ? mfdim([10^9,4], 3) \\ Eisenstein space %1 = 40000 ? mfdim([10^9+7,4], 3) %2 = 2 ? mfdim([68,1,-1],0) %3 = 3 ? mfdim([68,1,0],0) %4 = [[2, Mod(67, 68), 1, 1], [4, Mod(47, 68), 1, 1]] ? mfdim([124,1,0],0) %5 = [[6, Mod(67, 124), 2, 0]]

This last example shows that there exists a nondihedral form of weight 1 in level 124.

The library syntax is `GEN `

.**mfdim**(GEN NK, long space)

Given two generalized modular forms F and G, compute F/G assuming
that the quotient will not have poles at infinity. If this is the
case, use `mfshift`

before doing the division.

? D = mfDelta(); \\ Delta ? H = mfpow(mfEk(4), 3); ? J = mfdiv(H, D) *** at top-level: J=mfdiv(H,mfdeltac *** ^ — — — — — — -- *** mfdiv: domain error in mfdiv: ord(G) > ord(F) ? J = mfdiv(H, mfshift(D,1)); ? mfcoefs(J, 4) %4 = [1, 744, 196884, 21493760, 864299970]

The library syntax is `GEN `

.**mfdiv**(GEN F, GEN G)

Vector of the eigenforms for the space `mf`

.
The initial basis of forms computed by `mfinit`

before splitting
is also available via `mfbasis`

.

? mf = mfinit([26,2],0); ? see(L) = for(i=1,#L,print(mfcoefs(L[i],6))); ? see( mfeigenbasis(mf) ) [0, 1, -1, 1, 1, -3, -1] [0, 1, 1, -3, 1, -1, -3] ? see( mfbasis(mf) ) [0, 2, 0, -2, 2, -4, -4] [0, -2, -4, 10, -2, 0, 8]

The eigenforms are internally expressed as (algebraic) linear combinations of
`mfbasis(mf)`

and it is very inefficient to compute many coefficients
of those forms individually: you should rather use `mfcoefs(mf)`

to expand the basis once and for all, then multiply by `mftobasis(mf,f)`

for the forms you're interested in:

? mf = mfinit([96,6],0); B = mfeigenbasis(mf); #B %1 = 8; ? vector(#B, i, mfcoefs(B[i],1000)); \\ expanded individually: slow time = 7,881 ms. ? M = mfcoefs(mf, 1000); \\ initialize once time = 982 ms. ? vector(#B, i, M * mftobasis(mf,B[i])); \\ then expand: much faster time = 623 ms.

When the eigenforms are defined over an extension field of ℚ(χ) for a
nonrational character, their coefficients are hard to read and you may want
to lift them or to express them in an absolute number field. In the
construction below T defines ℚ(f) over ℚ, a is the image of the
generator `Mod`

(t, t^2+t+1) of ℚ(χ) in ℚ(f)
and y - ka is the image of the root y of `f.mod`

:

? mf = mfinit([31, 2, Mod(25,31)], 0); [f] = mfeigenbasis(mf); ? f.mod %2 = Mod(1, t^2 + t + 1)*y^2 + Mod(2*t + 2, t^2 + t + 1) ? v = liftpol(mfcoefs(f,5)) %3 = [0, 1, (-t - 1)*y - 1, t*y + (t + 1), (2*t + 2)*y + 1, t] ? [T,a,k] = rnfequation(mf.mod, f.mod, 1) %4 = [y^4 + 2*y^2 + 4, Mod(-1/2*y^2 - 1, y^4 + 2*y^2 + 4), 0] ? liftpol(substvec(v, [t,y], [a, y-k*a])) %5 = [0, 1, 1/2*y^3 - 1, -1/2*y^3 - 1/2*y^2 - y, -y^3 + 1, -1/2*y^2 - 1]

Beware that the meaning of y has changed in the last line
is different: it now represents of root of T, no longer of `f.mod`

(the notions coincide if k = 0 as here but it will not always be the case).
This can be avoided with an extra variable substitution, for instance

? [T,a,k] = rnfequation(mf.mod, subst(f.mod,'y,'x), 1) %6 = [x^4 + 2*x^2 + 4, Mod(-1/2*x^2 - 1, x^4 + 2*x^2 + 4), 0] ? liftpol(substvec(v, [t,y], [a, x-k*a])) %7 = [0, 1, 1/2*x^3 - 1, -1/2*x^3 - 1/2*x^2 - x, -x^3 + 1, -1/2*x^2 - 1]

The library syntax is `GEN `

.**mfeigenbasis**(GEN mf)

Search for a normalized rational eigen cuspform with quadratic character given restrictions on a few initial coefficients. The meaning of the parameters is as follows:

***** `NK`

governs the limits of the search: it is of the form
[N,k]: search for given level N, weight k and quadratic
character; note that the character (D/.) is uniquely determined by (N,k).
The level N can be replaced by a vector of allowed levels.

***** `AP`

is the search criterion, which can be omitted: a list of
pairs [..., [p,a_{p}],...], where p is a prime number and a_{p} is
either a `t_INT`

(the p-th Fourier coefficient must match a_{p} exactly)
or a `t_INTMOD`

`Mod`

(a,b) (the p-th coefficient must be congruent
to a modulo b).

The result is a vector of newforms f matching the search criteria, sorted by increasing level then increasing |D|.

? #mfeigensearch([[1..80],2], [[2,2],[3,-1]]) %1 = 1 ? #mfeigensearch([[1..80],2], [[2,2],[5,2]]) %2 = 1 ? v = mfeigensearch([[1..20],2], [[3,Mod(2,3)],[7,Mod(5,7)]]); #v %3 = 1 ? F=v[1]; [mfparams(F)[1], mfcoefs(F,15)] %4 = [11, [0, 1, -2, -1, 2, 1, 2, -2, 0, -2, -2, 1, -2, 4, 4, -1]]

The library syntax is `GEN `

.**mfeigensearch**(GEN NK, GEN AP = NULL)

Create the Eisenstein series E_{k}(χ_{1},χ_{2}), where k ≥ 1,
χ_{i} are Dirichlet characters and an omitted character is considered as
trivial. This form belongs to ℰ_{k}(Γ_{0}(N), χ) with χ =
χ_{1}χ_{2} and N is the product of the conductors of χ_{1} and
χ_{2}.

? CHI = Mod(3,4); ? E = mfeisenstein(3, CHI); ? mfcoefs(E, 6) %2 = [-1/4, 1, 1, -8, 1, 26, -8] ? CHI2 = Mod(4,5); ? mfcoefs(mfeisenstein(3,CHI,CHI2), 6) %3 = [0, 1, -1, -10, 1, 25, 10] ? mfcoefs(mfeisenstein(4,CHI,CHI), 6) %4 = [0, 1, 0, -28, 0, 126, 0] ? mfcoefs(mfeisenstein(4), 6) %5 = [1/240, 1, 9, 28, 73, 126, 252]

Note that `mfeisenstein`

(k) is 0 for k odd and
-B_{k}/(2k).E_{k} for k even, where
E_{k}(q) = 1 - (2k/B_{k})∑_{n ≥ 1} σ_{k-1}(n) q^n
is the standard Eisenstein series. In other words it is normalized so that its
linear coefficient is 1.

**Important note.** This function is currently implemented only when
ℚ(χ) is the field of definition of E_{k}(χ_{1},χ_{2}). If it is a
strict subfield, an error is raised:

? mfeisenstein(6, Mod(7,9), Mod(4,9)); *** at top-level: mfeisenstein(6,Mod(7,9),Mod(4,9)) *** ^ — — — — — — — — — — — *** mfeisenstein: sorry, mfeisenstein for these characters is not *** yet implemented.

The reason for this is that each modular form is attached
to a modular form space M_{k}(Γ_{0}(N),χ). This is a ℂ-vector
space but it allows a basis of forms defined over ℚ(χ) and is only
implemented as a ℚ(χ)-vector space: there is
in general no mechanism to take linear combinations of forms in the space
with coefficients belonging to a larger field. (Due to their importance,
eigenforms are the single exception to this restriction; for an eigenform
F, ℚ(F) is built on top of ℚ(χ).) When the property ℚ(χ) =
ℚ(E_{k}(χ_{1},χ_{2}) does not hold, we cannot express E as a
ℚ(χ)-linear combination of the basis forms and many operations will
fail. For this reason, the construction is currently disabled.

The library syntax is `GEN `

.**mfeisenstein**(long k, GEN CHI1 = NULL, GEN CHI2 = NULL)

Let f be a generalized modular form with parameters [N,k,χ,P] (see
`mfparams`

, we denote ℚ(χ) the subfield of ℂ generated by the
values of χ and ℚ(f) the field of definition of f. In this context
ℚ(χ) has a single canonical complex embeding given by
s: `Mod(t, polcyclo(n,t))`

` ⟼ `

exp(2iπ/n) and the number field
ℚ(f) has [ℚ(f):ℚ(χ)] induced embeddings attached to the complex
roots of the polynomial s(P). If ℚ(f) is stricly larger than ℚ(χ)
we only allow an f which is an eigenform, produced by `mfeigenbasis`

.

This function is meant to create embeddings of ℚ(f) and/or apply them
to the object v, typically a vector of Fourier coefficients of f
from `mfcoefs`

.

***** If v is omitted and f is a modular form as above, we return the
embedding of ℚ(χ) if ℚ(χ) = ℚ(f) and a vector containing
[ℚ(f):ℚ(χ)] embeddings of ℚ(f) otherwise.

***** If v is given, it must be a scalar in ℚ(f), or a vector/matrix of
such, we apply the embeddings coefficientwise and return either
a single result if ℚ(f) = ℚ(χ) and a vector of [ℚ(f):ℚ(χ)]
results otherwise.

***** Finally f can be replaced by a single embedding produced by
`mfembed`

(f) (v was omitted) and we apply that particular embedding
to v.

? mf = mfinit([35,2,Mod(11,35)], 0); ? [f] = mfbasis(mf); ? f.mod \\ ℚ(χ) = ℚ(ζ_{3}) %3 = t^2 + t + 1 ? v = mfcoefs(f,5); lift(v) \\ coefficients in ℚ(χ) %4 = [0, 2, -2*t - 2, 2*t, 2*t, -2*t - 2] ? mfembed(f, v) \\ single embedding %5 = [0, 2, -1 - 1.7320...*I, -1 + 1.73205...*I, -1 + 1.7320...*I, ...] ? [F] = mfeigenbasis(mf); ? mffields(mf) %7 = [y^2 + Mod(-2*t, t^2 + t + 1)] \\ [ℚ(f):ℚ(χ)] = 2 ? V = liftpol( mfcoefs(F,5) ); %8 = [0, 1, y + (-t - 1), (t + 1)*y + t, (-2*t - 2)*y + t, -t - 1] ? vall = mfembed(F, V); #vall %9 = 2 \\ 2 embeddings, both applied to V ? vall[1] \\ the first %10 = [0, 1, -1.2071... - 2.0907...*I, 0.2071... - 0.3587...*I, ...] ? vall[2] \\ and the second one %11 = [0, 1, 0.2071... + 0.3587...*I, -1.2071... + 2.0907...*I, ...] ? vE = mfembed(F); #vE \\ same 2 embeddings %12 = 2 ? mfembed(vE[1], V) \\ apply first embedding to V %13 = [0, 1, -1.2071... - 2.0907...*I, 0.2071... - 0.3587...*I, ...]

For convenience, we also allow a modular form space from `mfinit`

instead of f, corresponding to the single embedding of ℚ(χ).

? [mfB,MC,C] = mfatkininit(mf,7); MC \\ coefs in ℚ(χ) %13 = [ Mod(2/7*t, t^2 + t + 1) Mod(-1/7*t - 2/7, t^2 + t + 1)] [Mod(-1/7*t - 2/7, t^2 + t + 1) Mod(2/7*t, t^2 + t + 1)] ? C \\ normalizing constant %14 = 0.33863... - 0.16787*I ? M = mfembed(mf, MC) / C \\ the true matrix for the action of w_{7}[-0.6294... + 0.4186...*I -0.3625... - 0.5450...*I] [-0.3625... - 0.5450...*I -0.6294... + 0.4186...*I] ? exponent(M*conj(M) - 1) \\ M * conj(M) is close to 1 %16 = -126

The library syntax is `GEN `

.**mfembed0**(GEN f, GEN v = NULL, long prec)

Computes the numerical value of the modular form F, belonging
to *mf*, at the complex number `vtau`

or the vector `vtau`

of complex numbers in the completed upper-half plane. The result is given
with absolute error less than 2^{-B}, where B = realbitprecision.

If the field of definition ℚ(F) is larger than ℚ(χ) then F may be embedded into ℂ in d = [ℚ(F):ℚ(χ)] ways, in which case a vector of the d results is returned.

? mf = mfinit([11,2],0); F = mfbasis(mf)[1]; mfparams(F) %1 = [11, 2, 1, y, t-1] \\ Q(F) = Q(chi) = Q ? mfeval(mf,F,I/2) %2 = 0.039405471130100890402470386372028382117 ? mf = mfinit([35,2],0); F = mfeigenbasis(mf)[2]; mfparams(F) %3 = [35, 2, 1, y^2 - y - 4, t - 1] \\ [Q(F) : Q(chi)] = 2 ? mfeval(mf,F,I/2) %4 = [0.045..., 0.0385...] \\ sigma_{1}(F) and sigma_{2}(F) at I/2 ? mf = mfinit([12,4],1); F = mfbasis(mf)[1]; ? mfeval(mf, F, 0.318+10^(-7)*I) %6 = 3.379... E-21 + 6.531... E-21*I \\ instantaneous !

In order to maximize the imaginary part of the argument,
the function computes (f | _{k} γ)(γ^{-1}.τ) for a
suitable γ not necessarily in Γ_{0}(N) (in which case f |
γ is evaluated using `mfslashexpansion`

).

? T = mfTheta(); mf = mfinit(T); mfeval(mf,T,[0,1/2,1,oo]) %1 = [1/2 - 1/2*I, 0, 1/2 - 1/2*I, 1]

The library syntax is `GEN `

.**mfeval**(GEN mf, GEN F, GEN vtau, long bitprec)

Given `mf`

as output by `mfinit`

with parameters
(N,k,χ), returns the vector of polynomials defining each Galois orbit of
newforms over ℚ(χ).

? mf = mfinit([35,2],0); mffields(mf) %1 = [y, y^2 - y - 4]

Here the character is trivial so ℚ(χ) = ℚ) and there are 3 newforms: one is rational (corresponding to y), the other two are conjugate and defined over the quadratic field ℚ[y]/(y^2-y-4).

? [G,chi] = znchar(Mod(3,35)); ? zncharconductor(G,chi) %2 = 35 ? charorder(G,chi) %3 = 12 ? mf = mfinit([35, 2, [G,chi]],0); mffields(mf) %4 = [y, y]

Here the character is primitive of order 12 and the two newforms are
defined over ℚ(χ) = ℚ(ζ_{12}).

? mf = mfinit([35, 2, Mod(13,35)],0); mffields(mf) %3 = [y^2 + Mod(5*t, t^2 + 1)]

This time the character has order 4 and there are two conjugate newforms over ℚ(χ) = Q(i).

The library syntax is `GEN `

.**mffields**(GEN mf)

E being an elliptic curve defined over Q given by an
integral model in `ellinit`

format, computes a 3-component vector
`[mf,F,v]`

, where F is the newform corresponding to E by
modularity, `mf`

is the newspace to which F belongs, and
`v`

gives the coefficients of F on `mfbasis(mf)`

.

? E = ellinit("26a1"); ? [mf,F,co] = mffromell(E); ? co %2 = [3/4, 1/4]~ ? mfcoefs(F, 5) %3 = [0, 1, -1, 1, 1, -3] ? ellan(E, 5) %4 = [1, -1, 1, 1, -3]

The library syntax is `GEN `

.**mffromell**(GEN E)

Modular form corresponding to the eta quotient matrix `eta`

.
If the valuation v at infinity is fractional, return 0. If the eta
quotient is not holomorphic but simply meromorphic, return 0 if
`flag = 0`

; return the eta quotient (divided by q to the power -v if
v < 0, i.e., with valuation 0) if flag is set.

? mffrometaquo(Mat([1,1]),1) %1 = 0 ? mfcoefs(mffrometaquo(Mat([1,24])),6) %2 = [0, 1, -24, 252, -1472, 4830, -6048] ? mfcoefs(mffrometaquo([1,1;23,1]),10) %3 = [0, 1, -1, -1, 0, 0, 1, 0, 1, 0, 0] ? F = mffrometaquo([1,2;2,-1]); mfparams(F) %4 = [16, 1/2, 1, y, t - 1] ? mfcoefs(F,10) %5 = [1, -2, 0, 0, 2, 0, 0, 0, 0, -2, 0] ? mffrometaquo(Mat([1,-24])) %6 = 0 ? f = mffrometaquo(Mat([1,-24]),1); mfcoefs(f,6) %7 = [1, 24, 324, 3200, 25650, 176256, 1073720]

For convenience, a `t_VEC`

is also accepted instead of
a factorization matrix with a single row:

? f = mffrometaquo([1,24]); \\ also valid

The library syntax is `GEN `

.**mffrometaquo**(GEN eta, long flag)

Let L being an L-function in any of the `lfun`

formats representing
a self-dual modular form (for instance an eigenform). Return
`[NK,space,v]`

when `mf = mfinit(NK,space)`

is the modular
form space containing the form and `mftobasis(mf, v)`

will represent it
on the space basis. If L has rational coefficients, this will be enough
to recognize the modular form in *mf*:

? L = lfuncreate(x^2+1); ? lfunan(L,10) %2 = [1, 1, 0, 1, 2, 0, 0, 1, 1, 2] ? [NK,space,v] = mffromlfun(L); NK %4 = [4, 1, -4] ? mf=mfinit(NK,space); w = mftobasis(mf,v) %5 = [1.0000000000000000000000000000000000000]~ ? [f] = mfbasis(mf); mfcoefs(f,10) \\ includes a_{0}! %6 = [1/4, 1, 1, 0, 1, 2, 0, 0, 1, 1, 2]

If L has inexact complex coefficients, one can for instance
compute an eigenbasis for *mf* and check whether one of the attached
L-function is reasonably close to L. In the example, we cheat by
producing the L function from an eigenform in a known space, but the
function does not use this information:

? mf = mfinit([32,6,Mod(5,32)],0); ? [poldegree(K) | K<-mffields(mf)] %2 = [19] \\ one orbit, [Q(F) : Q(chi)] = 19 ? L = lfunmf(mf)[1][1]; \\ one of the 19 L-functions attached to F ? lfunan(L,3) %4 = [1, 5.654... - 0.1812...*I, -7.876... - 19.02...*I] ? [NK,space,v] = mffromlfun(L); NK %5 = [32, 6, Mod(5, 32)] ? vL = concat(lfunmf(mf)); \\ L functions for all cuspidal eigenforms ? an = lfunan(L,10); ? for (i = 1, #vL, if (normlp(lfunan(vL[i],10) - an, oo) < 1e-10, print(i))); 1

The library syntax is `GEN `

.**mffromlfun**(GEN L, long prec)

Q being an even integral positive definite quadratic form
and P a homogeneous spherical polynomial for Q, computes
a 3-component vector [*mf*,F,v], where F is the theta function
corresponding to (Q,P), *mf* is the corresponding space of modular
forms (from `mfinit`

), and v gives the coefficients of F on
`mfbasis(mf)`

.

? [mf,F,v] = mffromqf(2*matid(10)); v %1 = [64/5, 4/5, 32/5]~ ? mfcoefs(F, 5) %2 = [1, 20, 180, 960, 3380, 8424] ? mfcoef(F, 10000) \\ number of ways of writing 10000 as sum of 10 squares %3 = 128205250571893636 ? mfcoefs(F, 10000); \\ fast ! time = 220ms ? [mf,F,v] = mffromqf([2,0;0,2],x^4-6*x^2*y^2+y^4); ? mfcoefs(F,10) %6 = [0, 4, -16, 0, 64, -56, 0, 0, -256, 324, 224] ? mfcoef(F,100000) \\ instantaneous %7 = 41304367104

Odd dimensions are supported, corresponding to forms of half-integral weight:

? [mf,F,v] = mffromqf(2*matid(3)); ? mfisequal(F, mfpow(mfTheta(),3)) %2 = 1 ? mfcoefs(F, 32) \\ illustrate Legendre's 3-square theorem %3 = [ 1, 6, 12, 8, 6, 24, 24, 0, 12, 30, 24, 24, 8, 24, 48, 0, 6, 48, 36, 24,24, 48, 24, 0, 24, 30, 72, 32, 0, 72, 48, 0, 12]

The library syntax is `GEN `

.**mffromqf**(GEN Q, GEN P = NULL)

*mf* being an `mf`

output by `mfinit`

in weight 1,
return a polynomial defining the field fixed by the kernel of the projective
Artin representation attached to *F* (by Deligne-Serre).
Currently only implemented for projective images A_{4}, A_{5} and S_{4}.
The type A_{5} requires the `nflistdata`

package to be installed.

\\ A4 example ? mf = mfinit([4*31,1,Mod(87,124)],0); ? F = mfeigenbasis(mf)[1]; ? mfgaloistype(mf,F) %3 = -12 ? pol = mfgaloisprojrep(mf,F) %4 = x^12 + 68*x^10 + 4808*x^8 + ... + 4096 ? G = galoisinit(pol); galoisidentify(G) %5 = [12,3] \\A4 ? pol4 = polredbest(galoisfixedfield(G,G.gen[3], 1)) %6 = x^4 + 7*x^2 - 2*x + 14 ? polgalois(pol4) %7 = [12, 1, 1, "A4"] ? factor(nfdisc(pol4)) %8 = [ 2 4] [31 2] \\ S4 example ? mf = mfinit([4*37,1,Mod(105,148)],0); ? F = mfeigenbasis(mf)[1]; ? mfgaloistype(mf,F) %11 = -24 ? pol = mfgaloisprojrep(mf,F) %12 = x^24 + 24*x^22 + 256*x^20 + ... + 255488256 ? G = galoisinit(pol); galoisidentify(G) %13 = [24, 12] \\S4 ? pol4 = polredbest(galoisfixedfield(G,G.gen[3..4], 1)) %14 = x^4 - x^3 + 5*x^2 - 7*x + 12 ? polgalois(pol4) %15 = [24, -1, 1, "S4"] ? factor(nfdisc(pol4)) %16 = [ 2 2] [37 3]

The library syntax is `GEN `

.**mfgaloisprojrep**(GEN mf, GEN F, long prec)

`NK`

being either `[N,1,CHI]`

or an `mf`

output by
`mfinit`

in weight 1, gives the vector of types of Galois
representations attached to each cuspidal eigenform,
unless the modular form `F`

is specified, in which case only for `F`

(note that it is not tested whether `F`

belongs to the correct modular
form space, nor whether it is a cuspidal eigenform). Types A_{4}, S_{4},
A_{5} are represented by minus their cardinality -12, -24, or -60,
and type D_{n} is represented by its cardinality, the integer 2n:

? mfgaloistype([124,1, Mod(67,124)]) \\ A4 %1 = [-12] ? mfgaloistype([148,1, Mod(105,148)]) \\ S4 %2 = [-24] ? mfgaloistype([633,1, Mod(71,633)]) \\ D10, A5 %3 = [10, -60] ? mfgaloistype([239,1, -239]) \\ D6, D10, D30 %4 = [6, 10, 30] ? mfgaloistype([71,1, -71]) %5 = [14] ? mf = mfinit([239,1, -239],0); F = mfeigenbasis(mf)[2]; ? mfgaloistype(mf, F) %7 = 10

The function may also return 0 as a type when it failed to determine it; in this case the correct type is either -12 or -60, and most likely -12.

The library syntax is `GEN `

.**mfgaloistype**(GEN NK, GEN F = NULL)

F being a modular form in modular form space *mf*, returns
T(n)F, where T(n) is the n-th Hecke operator.

**Warning.** If F is of level M < N, then T(n)F
is in general not the same in M_{k}(Γ_{0}(M),χ) and in
M_{k}(Γ_{0}(N),χ). We take T(n) at the same level as the one used in
`mf`

.

? mf = mfinit([26,2],0); F = mfbasis(mf)[1]; mftobasis(mf,F) %1 = [1, 0]~ ? G2 = mfhecke(mf,F,2); mftobasis(mf,G2) %2 = [0, 1]~ ? G5 = mfhecke(mf,F,5); mftobasis(mf,G5) %3 = [-2, 1]~

Modular forms of half-integral weight are supported, in
which case n must be a perfect square, else T_{n} will act as 0 (the
operator T_{p} for p | N is not supported yet):

? F = mfpow(mfTheta(),3); mf = mfinit(F); ? mfisequal(mfhecke(mf,F,9), mflinear([F],[4])) %2 = 1

(F is an eigenvector of all T_{p^2}, with eigenvalue p+1 for
odd p.)

**Warning.** When n is a large composite, resp. the square of a large
composite in half-integral weight, it is in general more efficient to use
`mfheckemat`

on the `mftobasis`

coefficients:

? mfcoefs(mfhecke(mf,F,3^10), 10) time = 917 ms. %3 = [324, 1944, 3888, 2592, 1944, 7776, 7776, 0, 3888, 9720, 7776] ? M = mfheckemat(mf,3^10) \\ instantaneous %4 = [324] ? G = mflinear(mf, M*mftobasis(mf,F)); ? mfcoefs(G, 10) \\ instantaneous %6 = [324, 1944, 3888, 2592, 1944, 7776, 7776, 0, 3888, 9720, 7776]

The library syntax is `GEN `

.**mfhecke**(GEN mf, GEN F, long n)

If `vecn`

is an integer, matrix of the Hecke operator T(n) on the
basis formed by `mfbasis(mf)`

. If it is a vector, vector of
such matrices, usually faster than calling each one individually.

? mf=mfinit([32,4],0); mfheckemat(mf,3) %1 = [0 44 0] [1 0 -10] [0 -2 0] ? mfheckemat(mf,[5,7]) %2 = [[0, 0, 220; 0, -10, 0; 1, 0, 12], [0, 88, 0; 2, 0, -20; 0, -4, 0]]

The library syntax is `GEN `

.**mfheckemat**(GEN mf, GEN vecn)

Create the space of modular forms corresponding to the data contained in
`NK`

and `space`

. `NK`

is a vector which can be
either [N,k] (N level, k weight) corresponding to a subspace of
M_{k}(Γ_{0}(N)), or [N,k,*CHI*] (*CHI* a character)
corresponding to a subspace of M_{k}(Γ_{0}(N),χ). Alternatively,
it can be a modular form F or modular form space, in which case we use
`mfparams`

to define the space parameters.

The subspace is described by the small integer `space`

: 0 for the
newspace S_{k}^{new}(Γ_{0}(N),χ), 1 for the cuspidal
space S_{k}, 2 for the oldspace S_{k}^{old}, 3 for the space of
Eisenstein series E_{k} and 4 for the full space M_{k}.

**Wildcards.** For given level and weight, it is advantageous to
compute simultaneously spaces attached to different Galois orbits
of characters, especially in weight 1. The parameter *CHI* may be set
to 0 (wildcard), in which case we return a vector of all `mfinit`

(s) of
non trivial spaces in S_{k}(Γ_{1}(N)), one for each Galois orbit
(see `znchargalois`

). One may also set *CHI* to a vector of
characters and we return a vector of all mfinits of subspaces of
M_{k}(G_{0}(N),χ) for χ in the list, in the same order. In weight 1,
only S_{1}^{new}, S_{1} and E_{1} support wildcards.

The output is a technical structure S, or a vector of structures if
*CHI* was a wildcard, which contains the following information:
[N,k,χ] is given by `mfparams`

(S), the space
dimension is `mfdim`

(S) and a ℂ-basis for the space is
`mfbasis`

(S). The structure is entirely algebraic and does not depend
on the current `realbitprecision`

.

? S = mfinit([36,2], 0); \\ new space ? mfdim(S) %2 = 1 ? mfparams %3 = [36, 2, 1, y] \\ trivial character ? f = mfbasis(S)[1]; mfcoefs(f,10) %4 = [0, 1, 0, 0, 0, 0, 0, -4, 0, 0, 0] ? vS = mfinit([36,2,0],0); \\ with wildcard ? #vS %6 = 4 \\ 4 non trivial spaces (mod Galois action) ? apply(mfdim,vS) %7 = [1, 2, 1, 4] ? mfdim([36,2,0], 0) %8 = [[1, Mod(1, 36), 1, 0], [2, Mod(35, 36), 2, 0], [3, Mod(13, 36), 1, 0], [6, Mod(11, 36), 4, 0]]

The library syntax is `GEN `

.**mfinit**(GEN NK, long space)

Tests whether the eigenform F is a CM form. The answer
is 0 if it is not, and if it is, either the unique negative discriminant
of the CM field, or the pair of two negative discriminants of CM fields,
this latter case occurring only in weight 1 when the projective image is
D_{2} = C_{2} x C_{2}, i.e., coded 4 by `mfgaloistype`

.

? F = mffromell(ellinit([0,1]))[2]; mfisCM(F) %1 = -3 ? mf = mfinit([39,1,-39],0); F=mfeigenbasis(mf)[1]; mfisCM(F) %2 = Vecsmall([-3, -39]) ? mfgaloistype(mf) %3 = [4]

The library syntax is `GEN `

.**mfisCM**(GEN F)

Checks whether the modular forms F and G are equal. If `lim`

is nonzero, only check equality of the first lim+1 Fourier coefficients
and the function then also applies to generalized modular forms.

? D = mfDelta(); F = mfderiv(D); ? G = mfmul(mfEk(2), D); ? mfisequal(F, G) %2 = 1

The library syntax is `long `

.**mfisequal**(GEN F, GEN G, long lim)

If the generalized modular form f is a holomorphic eta quotient,
return the eta quotient matrix, else return 0. If *flag* is set, also accept
meromorphic eta quotients: check whether f = q^{-v(g)} g(q) for some
eta quotient g; if so, return the eta quotient matrix attached to g,
else return 0.
See `mffrometaquo`

.

? mfisetaquo(mfDelta()) %1 = [1 24] ? f = mffrometaquo([1,1;23,1]); ? mfisetaquo(f) %3 = [ 1 1] [23 1] ? f = mffrometaquo([1,-24], 1); ? mfisetaquo(f) \\ nonholomorphic %5 = 0 ? mfisetaquo(f,1) %6 = [1 -24]

The library syntax is `GEN `

.**mfisetaquo**(GEN f, long flag)

`mf`

being a cuspidal space of half-integral weight k ≥ 3/2
with level N and character χ, gives a
basis B of the Kohnen +-space of `mf`

as a matrix whose columns are
the coefficients of B on the basis of `mf`

. The conductor of either
χ or χ.(-4/.) must divide N/4.

? mf = mfinit([36,5/2],1); K = mfkohnenbasis(mf); K~ %1 = [-1 0 0 2 0 0] [ 0 0 0 0 1 0] ? (mfcoefs(mf,20) * K)~ %4 = [0 -1 0 0 2 0 0 0 0 0 0 0 0 -6 0 0 8 0 0 0 0] [0 0 0 0 0 1 0 0 -2 0 0 0 0 0 0 0 0 1 0 0 2] ? mf = mfinit([40,3/2,8],1); mfkohnenbasis(mf) *** at top-level: mfkohnenbasis(mf) *** ^ — — — — — -- *** mfkohnenbasis: incorrect type in mfkohnenbasis [incorrect CHI] (t_VEC).

In the final example both χ = (8/.) and χ.(-4/.) have conductor 8, which does not divide N/4 = 10.

The library syntax is `GEN `

.**mfkohnenbasis**(GEN mf)

Let `mf`

be a cuspidal space of half-integral weight and weight 4N,
with N squarefree and let S_{k}^+(Γ_{0}(4N),χ) be the Kohnen
+-space. Returns `[mf2,M,K,shi]`

, where

***** `mf2`

gives the cuspidal space S_{2k-1}(Γ_{0}(N),χ^2);

***** M is a matrix giving a Hecke-module isomorphism from that space to the
Kohnen +-space S_{k}^+(Γ_{0}(4N),χ);

***** `K`

represents a basis B of the Kohnen +-space as a matrix
whose columns are the coefficients of B on the basis of `mf`

;

***** `shi`

is a vector of pairs (t_{i},n_{i}) gives the linear
combination of Shimura lifts giving M^{-1}: t_{i} is a squarefree positive
integer and n_{i} is a small nonzero integer.

? mf=mfinit([60,5/2],1); [mf2,M,K,shi]=mfkohnenbijection(mf); M %2 = [-3 0 5/2 7/2] [ 1 -1/2 -7 -7] [ 1 1/2 0 -3] [ 0 0 5/2 5/2] ? shi %2 = [[1, 1], [2, 1]]

This last command shows that the map giving the bijection is the sum of the Shimura lift with t = 1 and the one with t = 2.

Since it gives a bijection of Hecke modules, this matrix can be used to transport modular form data from the easily computed space of level N and weight 2k-1 to the more difficult space of level 4N and weight k: matrices of Hecke operators, new space, splitting into eigenspaces and eigenforms. Examples:

? K^(-1)*mfheckemat(mf,121)*K /* matrix of T_11^2 on K. Slowish. */ time = 1,280 ms. %1 = [ 48 24 24 24] [ 0 32 0 -20] [-48 -72 -40 -72] [ 0 0 0 52] ? M*mfheckemat(mf2,11)*M^(-1) /* instantaneous via T_11 on S_{2k-1}*/ time = 0 ms. %2 = [ 48 24 24 24] [ 0 32 0 -20] [-48 -72 -40 -72] [ 0 0 0 52] ? mf20=mfinit(mf2,0); [mftobasis(mf2,b) | b<-mfbasis(mf20)] %3 = [[0, 0, 1, 0]~, [0, 0, 0, 1]~] ? F1=M*[0,0,1,0]~ %4 = [1/2, 1/2, -3/2, -1/2]~ ? F2=M*[0,0,0,1]~ %5 = [3/2, 1/2, -9/2, -1/2] ? K*F1 %6 = [1, 0, 0, 1, 1, 0, 0, 1, -3, 0, 0, -3, 0, 0]~ ? K*F2 %7 = [3, 0, 0, 3, 1, 0, 0, 1, -9, 0, 0, -3, 0, 0]~

This gives a basis of the new space of S_{5/2}^+(Γ_{0}(60)) expressed
on the initial basis of S_{5/2}(Γ_{0}(60)). If we want the eigenforms, we
write instead:

? BE=mfeigenbasis(mf20);[E1,E2]=apply(x->K*M*mftobasis(mf2,x),BE) %1 = [[1, 0, 0, 1, 0, 0, 0, 0, -3, 0, 0, 0, 0, 0]~,\ [0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, -3, 0, 0]~ ? EI1 = mflinear(mf, E1); EI2=mflinear(mf, E2);

These are the two eigenfunctions in the space `mf`

, the first (resp.,
second) will have Shimura image a multiple of BE[1] (resp., BE[2]).
The function `mfkohneneigenbasis`

does this directly.

The library syntax is `GEN `

.**mfkohnenbijection**(GEN mf)

`mf`

being a cuspidal space of half-integral weight k ≥ 3/2 and
`bij`

being the output of `mfkohnenbijection(mf)`

, outputs a
3-component vector `[mf0,BNEW,BEIGEN]`

, where `BNEW`

and
`BEIGEN`

are two matrices whose columns are the coefficients
of a basis of the Kohnen new space and of the eigenforms on the basis of
`mf`

respectively, and `mf0`

is the corresponding new space of
integral weight 2k-1.

? mf=mfinit([44,5/2],1);bij=mfkohnenbijection(mf); ? [mf0,BN,BE]=mfkohneneigenbasis(mf,bij); ? BN~ %2 = [2 0 0 -2 2 0 -8] [2 0 0 4 14 0 -32] ? BE~ %3 = [1 0 0 Mod(y-1, y^2-3) Mod(2*y+1, y^2-3) 0 Mod(-4*y-4, y^2-3)] ? lift(mfcoefs(mf,20)*BE[,1]) %4 = [0, 1, 0, 0, y - 1, 2*y + 1, 0, 0, 0, -4*y - 4, 0, 0,\ -5*y + 3, 0, 0, 0, -6, 0, 0, 0, 7*y + 9]~

The library syntax is `GEN `

.**mfkohneneigenbasis**(GEN mf, GEN bij)

`vF`

being a vector of generalized modular forms and `v`

a vector of coefficients of same length, compute the linear
combination of the entries of `vF`

with coefficients `v`

.
**Note.** Use this in particular to subtract two forms F and G
(with vF = [F,G] and v = [1,-1]), or to multiply an form by
a scalar λ (with vF = [F] and v = [λ]).

? D = mfDelta(); G = mflinear([D],[-3]); ? mfcoefs(G,4) %2 = [0, -3, 72, -756, 4416]

For user convenience, we allow

***** a modular form space `mf`

as a `vF`

argument, which is
understood as `mfbasis(mf)`

;

***** in this case, we also allow a modular form f as v, which
is understood as `mftobasis`

(*mf*, f).

? T = mfpow(mfTheta(),7); F = mfShimura(T,-3); \\ Shimura lift for D=-3 ? mfcoefs(F,8) %2 = [-5/9, 280, 9240, 68320, 295960, 875280, 2254560, 4706240, 9471000] ? mf = mfinit(F); G = mflinear(mf,F); ? mfcoefs(G,8) %4 = [-5/9, 280, 9240, 68320, 295960, 875280, 2254560, 4706240, 9471000]

This last construction allows to replace a general modular form by a simpler linear combination of basis functions, which is often more efficient:

? T10=mfpow(mfTheta(),10); mfcoef(T10, 10^4) \\ direct evaluation time = 399 ms. %5 = 128205250571893636 ? mf=mfinit(T10); F=mflinear(mf,T10); \\ instantaneous ? mfcoef(F, 10^4) \\ after linearization time = 67 ms. %7 = 128205250571893636

The library syntax is `GEN `

.**mflinear**(GEN vF, GEN v)

Given the modular symbol FS associated to an eigenform F by
`mfsymbol(mf,F)`

, computes the even and odd special polynomials as well
as the even and odd periods ω^+ and ω^- as a vector
[[P^+,P^-],[ω^+,ω^-,r]], where
r = Im(ω^+ω^-)/ < F,F > . If F has several embeddings
into ℂ, give the vector of results corresponding to each embedding.

? D=mfDelta(); mf=mfinit(D); DS=mfsymbol(mf,D); ? [pols,oms]=mfmanin(DS); pols %2 = [[4*x^9 - 25*x^7 + 42*x^5 - 25*x^3 + 4*x],\ [-36*x^10 + 691*x^8 - 2073*x^6 + 2073*x^4 - 691*x^2 + 36]] ? oms %3 = [0.018538552324740326472516069364750571812,\ -0.00033105361053212432521308691198949874026*I, 4096/691] ? mf=mfinit([11,2],0); F=mfeigenbasis(mf)[1]; FS=mfsymbol(mf,F); ? [pols,oms]=mfmanin(FS);pols %5 = [[0, 0, 0, 1, 1, 0, 0, -1, -1, 0, 0, 0],\ [2, 0, 10, 5, -5, -10, -10, -5, 5, 10, 0, -2]] ? oms[3] %6 = 24/5

The library syntax is `GEN `

.**mfmanin**(GEN FS, long bitprec)

Multiply the two generalized modular forms F and G.

? E4 = mfEk(4); G = mfmul(mfmul(E4,E4),E4); ? mfcoefs(G, 4) %2 = [1, 720, 179280, 16954560, 396974160] ? mfcoefs(mfpow(E4,3), 4) %3 = [1, 720, 179280, 16954560, 396974160]

The library syntax is `GEN `

.**mfmul**(GEN F, GEN G)

Number of cusps of Γ_{0}(N)

? mfnumcusps(24) %1 = 8 ? mfcusps(24) %1 = [0, 1/2, 1/3, 1/4, 1/6, 1/8, 1/12, 1/24]

The library syntax is `GEN `

.**mfnumcusps**(GEN N)

If F is a modular form space, returns `[N,k,CHI,space,Φ]`

,
level, weight, character χ, and space code; where Φ is the
cyclotomic polynomial
defining the field of values of `CHI`

. If F is a generalized modular
form, returns `[N,k,CHI,P,Φ]`

, where P is the (polynomial giving
the) field of definition of F as a relative extension of the cyclotomic field
ℚ(χ) = ℚ[t]/(Φ): in that case the level N may be a multiple of the
level of F and the polynomial P may define a larger field than ℚ(F).
If you want the true level of F from this result, use
`mfconductor(mfinit(F),F)`

. The polynomial P defines an extension of
ℚ(χ) = ℚ[t]/(Φ(t)); it has coefficients in that number field
(polmods in t).

In contrast with `mfparams(F)[4]`

which always gives the polynomial
P defining the relative extension ℚ(F)/ℚ(χ), the member function
`F.mod`

returns the polynomial used to define ℚ(F) over ℚ
(either a cyclotomic polynomial or a polynomial with cyclotomic
coefficients).

? E1 = mfeisenstein(4,-3,-4); E2 = mfeisenstein(3,5,-7); E3 = mfmul(E1,E2); ? apply(mfparams, [E1,E2,E3]) %2 = [[12, 4, 12, y, t-1], [35, 3, -35, y, t-1], [420, 7, -420, y, t-1]] ? mf = mfinit([36,2,Mod(13,36)],0); [f] = mfeigenbasis(mf); mfparams(mf) %3 = [36, 2, Mod(13, 36), 0, t^2 + t + 1] ? mfparams(f) %4 = [36, 2, Mod(13, 36), y, t^2 + t + 1] ? f.mod %5 = t^2 + t + 1 ? mf = mfinit([36,4,Mod(13,36)],0); [f] = mfeigenbasis(mf); ? lift(mfparams(f)) %7 = [36, 4, 13, y^3 + (2*t-2)*y^2 + (-4*t+6)*y + (10*t-1), t^2+t+1]

The library syntax is `GEN `

.**mfparams**(GEN F)

Period polynomial of the cuspidal part of the form f, in other words
∫_{0}^{i oo }(X-τ)^{k-2}f(τ)dτ. If `flag`

is 0, ordinary
period polynomial. If it is 1 or -1, even or odd part of that polynomial.
f can also be the modular symbol output by `mfsymbol`

(mf,f).

? D = mfDelta(); mf = mfinit(D,0); ? PP = mfperiodpol(mf, D, -1); PP/=polcoef(PP, 1); bestappr(PP) %1 = x^9 - 25/4*x^7 + 21/2*x^5 - 25/4*x^3 + x ? PM = mfperiodpol(mf, D, 1); PM/=polcoef(PM, 0); bestappr(PM) %2 = -x^10 + 691/36*x^8 - 691/12*x^6 + 691/12*x^4 - 691/36*x^2 + 1

The library syntax is `GEN `

.**mfperiodpol**(GEN mf, GEN f, long flag, long bitprec)

Basis of period polynomials for weight k. If flag = 1 or -1, basis of odd or even period polynomials.

? mfperiodpolbasis(12,1) %1 = [x^8 - 3*x^6 + 3*x^4 - x^2, x^10 - 1] ? mfperiodpolbasis(12,-1) %2 = [4*x^9 - 25*x^7 + 42*x^5 - 25*x^3 + 4*x]

The library syntax is `GEN `

.**mfperiodpolbasis**(long k, long flag)

Petersson scalar product of the modular forms f and g belonging to
the same modular form space `mf`

, given by the corresponding
"modular symbols" `fs`

and `gs`

output by `mfsymbol`

(also in weight 1 and half-integral weight, where symbols do not exist).
If `gs`

is omitted it is understood to be equal to `fs`

.
The scalar product is normalized by the factor 1/[Γ:Γ_{0}(N)].
Note that f and g can both be noncuspidal, in which case the program
returns an error if the product is divergent.
If the fields of definition ℚ(f) and ℚ(g) are equal to ℚ(χ)
the result is a scalar. If [ℚ(f):ℚ(χ)] = d > 1 and
[ℚ(g):ℚ(χ)] = e > 1 the result is a d x e matrix corresponding
to all the embeddings of f and g. In the intermediate cases d = 1 or
e = 1 the result is a row or column vector.

? D=mfDelta(); mf=mfinit(D); DS=mfsymbol(mf,D); mfpetersson(DS) %1 = 1.0353620568043209223478168122251645932 E-6 ? mf=mfinit([11,6],0);B=mfeigenbasis(mf);BS=vector(#B,i,mfsymbol(mf,B[i])); ? mfpetersson(BS[1]) %3 = 1.6190120685220988139111708455305245466 E-5 ? mfpetersson(BS[1],BS[2]) %4 = [-3.826479006582967148 E-42 - 2.801547395385577002 E-41*I,\ 1.6661127341163336125 E-41 + 1.1734725972345985061 E-41*I,\ 0.E-42 - 6.352626992842664490 E-41*I]~ ? mfpetersson(BS[2]) %5 = [ 2.7576133733... E-5 2.0... E-42 6.3... E-43 ] [ -4.1... E-42 6.77837030070... E-5 3.3...E-42 ] [ -6.32...E-43 3.6... E-42 2.27268958069... E-5] ? mf=mfinit([23,2],0); F=mfeigenbasis(mf)[1]; FS=mfsymbol(mf,F); ? mfpetersson(FS) %5 = [0.0039488965740025031688548076498662860143 -3.56 ... E-40] [ -3.5... E-40 0.0056442542987647835101583821368582485396]

Noncuspidal example:

? E1=mfeisenstein(5,1,-3);E2=mfeisenstein(5,-3,1); ? mf=mfinit([12,5,-3]); cusps=mfcusps(12); ? apply(x->mfcuspval(mf,E1,x),cusps) %3 = [0, 0, 1, 0, 1, 1] ? apply(x->mfcuspval(mf,E2,x),cusps) %4 = [1/3, 1/3, 0, 1/3, 0, 0] ? E1S=mfsymbol(mf,E1);E2S=mfsymbol(mf,E2); ? mfpetersson(E1S,E2S) %6 = -1.884821671646... E-5 - 1.9... E-43*I

Weight 1 and 1/2-integral weight example:

? mf=mfinit([23,1,-23],1);F=mfbasis(mf)[1];FS=mfsymbol(mf,F); ? mfpetersson(mf,FS) %2 = 0.035149946790370230814006345508484787443 ? mf=mfinit([4,9/2],1);F=mfbasis(mf)[1];FS=mfsymbol(mf,F); ? mfpetersson(FS) %4 = 0.00015577084407139192774373662467908966030

The library syntax is `GEN `

.**mfpetersson**(GEN fs, GEN gs = NULL)

Compute F^n, where n is an integer and F is a generalized modular form:

? G = mfpow(mfEk(4), 3); \\ E4^3 ? mfcoefs(G, 4) %2 = [1, 720, 179280, 16954560, 396974160]

The library syntax is `GEN `

.**mfpow**(GEN F, long n)

`NK`

being of the form `[N,k]`

with k possibly half-integral,
search for a modular form with rational coefficients, of weight k and
level N, whose initial coefficients a(0),... are equal to V;
`space`

specifies the modular form spaces in which to search, in
`mfinit`

or `mfdim`

notation. The output is a list of matching forms
with that given level and weight. Note that the character is of the form
(D/.), where D is a (positive or negative) fundamental discriminant
dividing N. The forms are sorted by increasing |D|.

The parameter N can be replaced by a vector of allowed levels, in which case the list of forms is sorted by increasing level, then increasing |D|. If a form is found at level N, any multiple of N with the same D is not considered. Some useful possibilities are

***** `[N`

: all levels between N_{1}..N_{2}]_{1} and N_{2},
endpoints included;

***** `F * [N`

: same but levels divisible by F;_{1}..N_{2}]

***** `divisors`

(N_{0}): all levels dividing N_{0}.

Note that this is different from `mfeigensearch`

, which only searches
for rational eigenforms.

? F = mfsearch([[1..40], 2], [0,1,2,3,4], 1); #F %1 = 3 ? [ mfparams(f)[1..3] | f <- F ] %2 = [[38, 2, 1], [40, 2, 8], [40, 2, 40]] ? mfcoefs(F[1],10) %3 = [0, 1, 2, 3, 4, -5, -8, 1, -7, -5, 7]

The library syntax is `GEN `

.**mfsearch**(GEN NK, GEN V, long space)

Divide the generalized modular form F by q^s, omitting the remainder if there is one. One can have s < 0.

? D=mfDelta(); mfcoefs(mfshift(D,1), 4) %1 = [1, -24, 252, -1472, 4830] ? mfcoefs(mfshift(D,2), 4) %2 = [-24, 252, -1472, 4830, -6048] ? mfcoefs(mfshift(D,-1), 4) %3 = [0, 0, 1, -24, 252]

The library syntax is `GEN `

.**mfshift**(GEN F, long s)

F being a modular form of half-integral weight k ≥ 3/2 and t a
positive squarefree integer, returns the Shimura lift G of weight 2k-1
corresponding to D. This function returns [*mf2*,G,v]
where *mf2* is a modular form space containing G and v expresses G
in terms of `mfbasis`

(*mf2*); so that G is
`mflinear`

(*mf2*,v).

? F = mfpow(mfTheta(), 7); mf = mfinit(F); ? [mf2, G, v] = mfshimura(mf, F, 3); mfcoefs(G,5) %2 = [-5/9, 280, 9240, 68320, 295960, 875280] ? mfparams(G) \\ the level may be lower than expected %3 = [1, 6, 1, y, t - 1] ? mfparams(mf2) %4 = [2, 6, 1, 4, t - 1] ? v %5 = [280, 0]~ ? mfcoefs(mf2, 5) %6 = [-1/504 -1/504] [ 1 0] [ 33 1] [ 244 0] [ 1057 33] [ 3126 0] ? mf = mfinit([60,5/2],1); F = mflinear(mf,mfkohnenbasis(mf)[,1]); ? mfparams(mfshimura(mf,F)[2]) %8 = [15, 4, 1, y, t - 1] ? mfparams(mfshimura(mf,F,6)[2]) %9 = [15, 4, 1, y, t - 1]

The library syntax is `GEN `

.**mfshimura**(GEN mf, GEN F, long D)

Let *mf* be a modular form space in level N, f a modular form
belonging to *mf* and let g be in M_{2}^+(Q). This function
computes the Fourier expansion of f|_{k} g to n terms. We first describe
the behaviour when `flrat`

is 0: the result is a
vector v of floating point complex numbers such that
f|_{k} g(τ) = q^α ∑_{m ≥ 0} v[m+1] q^{m/w},
where q = e(τ), w is the width of the cusp g(i oo )
(namely (N/(c^2,N) if g is integral) and α is a rational number.
If `params`

is given, it is set to the parameters [α,w,
`matid`

(2)].

If `flrat`

is 1, the program tries to rationalize the expression, i.e.,
to express the coefficients as rational numbers or polmods. We
write g = λ.M.A where λ ∈ ℚ^{*},
M ∈ SL_{2}(ℤ) and A = [a,b;0,d] is upper triangular,
integral and primitive with a > 0, d > 0 and 0 ≤ b < d. Let
α and w by the parameters attached to the expansion of
F := f |_{k} M as above, i.e.
F(τ) = q^α ∑_{m ≥ 0} v[m+1] q^{m/w}.
The function returns the expansion v of F = f |_{k} M and sets
the parameters to [α, w, A]. Finally, the desired expansion is
(a/d)^{k/2} F(τ + b/d). The latter is identical to the returned
expansion when A is the identity, i.e. when g ∈ PSL_{2}(ℤ).
If this is not the case, the expansion differs from v by the multiplicative
constant (a/d)^{k/2} e(α b/(dw)) and a twist by a root of unity
q^{1/w} → e(b/(dw)) q^{1/w}. The complications introduced by this extra
matrix A allow to recognize the coefficients in a much smaller cyclotomic
field, hence to obtain a simpler description overall. (Note that this
rationalization step may result in an error if the program cannot perform it.)

? mf = mfinit([32,4],0); f = mfbasis(mf)[1]; ? mfcoefs(f, 10) %2 = [0, 3, 0, 0, 0, 2, 0, 0, 0, 47, 0] ? mfatk = mfatkininit(mf,32); mfcoefs(mfatkin(mfatk,f),10) / mfatk[3] %3 = [0, 1, 0, 16, 0, 22, 0, 32, 0, -27, 0] ? mfatk[3] \\ here normalizing constant C = 1, but need in general %4 = 1 ? mfslashexpansion(mf,f,[0,-1;1,0],10,1,¶ms) * 32^(4/2) %5 = [0, 1, 0, 16, 0, 22, 0, 32, 0, -27, 0] ? params %6 = [0, 32, [1, 0; 0, 1]] ? mf = mfinit([12,8],0); f = mfbasis(mf)[1]; ? mfslashexpansion(mf,f,[1,0;2,1],7,0) %7 = [0, 0, 0, 0.6666666... + 0.E-38*I, 0, -3.999999... + 6.92820...*I, 0,\ -11.99999999... - 20.78460969...*I] ? mfslashexpansion(mf,f,[1,0;2,1],7,1, ¶ms) %8 = [0, 0, 0, 2/3, 0, Mod(8*t, t^2+t+1), 0, Mod(-24*t-24, t^2+t+1)] ? params %9 = [0, 3, [1, 0; 0, 1]]

If [ℚ(f):ℚ(χ)] > 1, the coefficients may be polynomials in y,
where y is any root of the polynomial giving the field of definition of
f (`f.mod`

or `mfparams(f)[4]`

).

? mf=mfinit([23,2],0);f=mfeigenbasis(mf)[1]; ? mfcoefs(f,5) %1 = [Mod(0, y^2 - y - 1), Mod(1, y^2 - y - 1), Mod(-y, y^2 - y - 1),\ Mod(2*y - 1, y^2 - y - 1), Mod(y - 1, y^2 - y - 1), Mod(-2*y, y^2 - y - 1)] ? mfslashexpansion(mf,f,[1,0;0,1],5,1) %2 = [0, 1, -y, 2*y - 1, y - 1, -2*y] ? mfslashexpansion(mf,f,[0,-1;1,0],5,1) %3 = [0, -1/23, 1/23*y, -2/23*y + 1/23, -1/23*y + 1/23, 2/23*y]

**Caveat.** In half-integral weight, we *define* the "slash"
operation as
(f |_{k} g)(τ) := ((c τ + d)^{-1/2})^{2k} f( g.τ),
with the principal determination of the square root. In particular,
the standard cocycle condition is no longer satisfied and we only
have f | (gg') = ± (f | g) | g'.

The library syntax is `GEN `

.**mfslashexpansion**(GEN mf, GEN f, GEN g, long n, long flrat, GEN *params = NULL, long prec)

Identify the modular space *mf*, resp. the modular form f in
*mf* if present, as the flag given to `mfinit`

.
Returns 0 (newspace), 1 (cuspidal space), 2 (old space),
3 (Eisenstein space) or 4 (full space).

? mf = mfinit([1,12],1); mfspace(mf) %1 = 1 ? mfspace(mf, mfDelta()) %2 = 0 \\ new space

This function returns -1 when the form f is modular but does not belong to the space.

? mf = mfinit([1,2]; mfspace(mf, mfEk(2)) %3 = -1

When f is not modular and is for instance only quasi-modular, the function returns nonsense:

? M6 = mfinit([1,6]); ? dE4 = mfderiv(mfEk(4)); \\ not modular ! ? mfspace(M6,dE4) \\ asserts (wrongly) that E4' belongs to new space %3 = 0

The library syntax is `long `

.**mfspace**(GEN mf, GEN f = NULL)

`mf`

from `mfinit`

with integral weight containing the new space
(either the new space itself or the cuspidal space or the full space), and
preferably the newspace itself for efficiency, split the space into Galois
orbits of eigenforms of the newspace, satisfying various restrictions.

The functions returns [vF, vK], where vF gives (Galois orbit of)
eigenforms and vK is a list of polynomials defining each Galois orbit.
The eigenforms are given in `mftobasis`

format, i.e. as a matrix
whose columns give the forms with respect to `mfbasis(mf)`

.

If `dimlim`

is set, only the Galois orbits of dimension ≤ `dimlim`

are computed (i.e. the rational eigenforms if `dimlim`

= 1 and the
character is real). This can considerably speed up the function when a Galois
orbit is defined over a large field.

`flag`

speeds up computations when the dimension is large: if flag = d > 0,
when the dimension of the eigenspace is > d, only the Galois polynomial is
computed.

Note that the function `mfeigenbasis`

returns all eigenforms in an
easier to use format (as modular forms which can be input as is in other
functions); `mfsplit`

is only useful when you can restrict
to orbits of small dimensions, e.g. rational eigenforms.

? mf=mfinit([11,2],0); f=mfeigenbasis(mf)[1]; mfcoefs(f,16) %1 = [0, 1, -2, -1, ...] ? mf=mfinit([23,2],0); f=mfeigenbasis(mf)[1]; mfcoefs(f,16) %2 = [Mod(0, z^2 - z - 1), Mod(1, z^2 - z - 1), Mod(-z, z^2 - z - 1), ...] ? mf=mfinit([179,2],0); apply(poldegree, mffields(mf)) %3 = [1, 3, 11] ? mf=mfinit([719,2],0); ? [vF,vK] = mfsplit(mf, 5); \\ fast when restricting to small orbits time = 192 ms. ? #vF \\ a single orbit %5 = 1 ? poldegree(vK[1]) \\ of dimension 5 %6 = 5 ? [vF,vK] = mfsplit(mf); \\ general case is slow time = 2,104 ms. ? apply(poldegree,vK) %8 = [5, 10, 45] \\ because degree 45 is large...

The library syntax is `GEN `

.**mfsplit**(GEN mf, long dimlim, long flag)

Gives the Sturm bound for modular forms on Γ_{0}(N) and
weight k, i.e., an upper bound for the order of the zero at infinity of
a nonzero form. `NK`

is either

***** a pair [N,k], in which case the bound is the floor of (kN/12).∏_{p | N} (1+1/p);

***** or the output of `mfinit`

in which case the exact upper bound is returned.

? NK = [96,6]; mfsturm(NK) %1 = 97 ? mf=mfinit(NK,1); mfsturm(mf) %2 = 76 ? mfdim(NK,0) \\ new space %3 = 72

The library syntax is `long `

.**mfsturm**(GEN NK)

Initialize data for working with all period polynomials of the modular
form f: this is essential for efficiency for functions such as
`mfsymboleval`

, `mfmanin`

, and `mfpetersson`

. An `mfsymbol`

contains an `mf`

structure and can always be used whenever an `mf`

would be needed.

? mf=mfinit([23,2],0);F=mfeigenbasis(mf)[1]; ? FS=mfsymbol(mf,F); ? mfsymboleval(FS,[0,oo]) %3 = [8.762565143790690142 E-39 + 0.0877907874...*I, -5.617375463602574564 E-39 + 0.0716801031...*I] ? mfpetersson(FS) %4 = [0.0039488965740025031688548076498662860143 1.2789721111175127425 E-40] [1.2630501762985554269 E-40 0.0056442542987647835101583821368582485396]

By abuse of language, initialize data for working with `mfpetersson`

in
weight 1 and half-integral weight (where no symbol exist); the `mf`

argument may be an `mfsymbol`

attached to a form on the space,
which avoids recomputing data independent of the form.

? mf=mfinit([12,9/2],1); F=mfbasis(mf); ? fs=mfsymbol(mf,F[1]); time = 476 ms ? mfpetersson(fs) %2 = 1.9722437519492014682047692073275406145 E-5 ? f2s = mfsymbol(mf,F[2]); time = 484 ms. ? mfpetersson(f2s) %4 = 1.2142222531326333658647877864573002476 E-5 ? gs = mfsymbol(fs,F[2]); \\ re-use existing symbol, a little faster time = 430 ms. ? mfpetersson(gs) == %4 \\ same value %6 = 1

For simplicity, we also allow `mfsymbol(f)`

instead of
`mfsymbol(mfinit(f), f)`

:

The library syntax is `GEN `

.**mfsymbol**(GEN mf, GEN f = NULL, long bitprec)

Evaluation of the modular symbol fs (corresponding to the modular
form f) output by `mfsymbol`

on the given path `path`

, where
`path`

is either a vector [s_{1},s_{2}] or an integral matrix [a,b;c,d]
representing the path [a/c,b/d]. In both cases s_{1} or s_{2} (or a/c or
b/d) can also be elements of the upper half-plane.
To avoid possibly lengthy `mfsymbol`

computations, the program also
accepts fs of the form `[mf,F]`

, but in that case s_{1} and s_{2}
are limited to `oo`

and elements of the upper half-plane.
The result is the polynomial equal to
∫_{s1}^{s2}(X-τ)^{k-2}F(τ)dτ, the integral being
computed along a geodesic joining s_{1} and s_{2}. If `ga`

in GL_{2}^+(ℚ)
is given, replace F by F|_{k}γ. Note that if the integral diverges,
the result will be a rational function.
If the field of definition ℚ(f) is larger than ℚ(χ) then f can be
embedded into ℂ in d = [ℚ(f):ℚ(χ)] ways, in which case a vector of
the d results is returned.

? mf=mfinit([35,2],1);f=mfbasis(mf)[1];fs=mfsymbol(mf,f); ? mfsymboleval(fs,[0,oo]) %1 = 0.31404011074188471664161704390256378537*I ? mfsymboleval(fs,[1,3;2,5]) %2 = -0.1429696291... - 0.2619975641...*I ? mfsymboleval(fs,[I,2*I]) %3 = 0.00088969563028739893631700037491116258378*I ? E2=mfEk(2);E22=mflinear([E2,mfbd(E2,2)],[1,-2]);mf=mfinit(E22); ? E2S = mfsymbol(mf,E22); ? mfsymboleval(E2S,[0,1]) %6 = (-1.00000...*x^2 + 1.00000...*x - 0.50000...)/(x^2 - x)

The rational function which is given in case the integral diverges is easy to interpret. For instance:

? E4=mfEk(4);mf=mfinit(E4);ES=mfsymbol(mf,E4); ? mfsymboleval(ES,[I,oo]) %2 = 1/3*x^3 - 0.928067...*I*x^2 - 0.833333...*x + 0.234978...*I ? mfsymboleval(ES,[0,I]) %3 = (-0.234978...*I*x^3 - 0.833333...*x^2 + 0.928067...*I*x + 0.333333...)/x

`mfsymboleval(ES,[a,oo])`

is the limit as T → oo of
∫_{a}^{iT}(X-τ)^{k-2}F(τ)dτ + a(0)(X-iT)^{k-1}/(k-1) ,
where a(0) is the 0th coefficient of F at infinity. Similarly,
`mfsymboleval(ES,[0,a])`

is the limit as T → oo of
∫_{i/T}^a(X-τ)^{k-2}F(τ)dτ+b(0)(1+iTX)^{k-1}/(k-1) ,
where b(0) is the 0th coefficient of F|_{k} S at infinity.

The library syntax is `GEN `

.**mfsymboleval**(GEN fs, GEN path, GEN ga = NULL, long bitprec)

F being a form in M_{k}(SL_{2}(ℤ)), computes the first n+1
canonical Taylor expansion of F around τ = I. If `flreal = 0`

,
computes only an algebraic equivalence class. If `flreal`

is set,
compute p_{n} such that for τ close enough to I we have
f(τ) = (2I/(τ+I))^k∑_{n >= 0}p_{n}((τ-I)/(τ+I))^n .

? D=mfDelta(); ? mftaylor(D,8) %2 = [1/1728, 0, -1/20736, 0, 1/165888, 0, 1/497664, 0, -11/3981312]

The library syntax is `GEN `

.**mftaylor**(GEN F, long n, long flreal, long prec)

Coefficients of the form F on the basis given by `mfbasis(mf)`

.
A q-expansion or vector of coefficients
can also be given instead of F, but in this case an error message may occur
if the expansion is too short. An error message is also given if F does not
belong to the modular form space. If `flag`

is set, instead of
error messages the output is an affine space of solutions if a q-expansion
or vector of coefficients is given, or the empty column otherwise.

? mf = mfinit([26,2],0); mfdim(mf) %1 = 2 ? F = mflinear(mf,[a,b]); mftobasis(mf,F) %2 = [a, b]~

A q-expansion or vector of coefficients can also be given instead of F.

? Th = 1 + 2*sum(n=1, 8, q^(n^2), O(q^80)); ? mf = mfinit([4,5,Mod(3,4)]); ? mftobasis(mf, Th^10) %3 = [64/5, 4/5, 32/5]~

If F does not belong to the corresponding space, the result is incorrect
and simply matches the coefficients of F up to some bound, and
the function may either return an empty column or an error message.
If `flag`

is set, there are no error messages, and the result is
an empty column if F is a modular form; if F is supplied via a series
or vector of coefficients which does not contain enough information to force
a unique (potential) solution, the function returns [v,K] where v is a
solution and K is a matrix of maximal rank describing the affine space of
potential solutions v + K.x.

? mf = mfinit([4,12],1); ? mftobasis(mf, q-24*q^2+O(q^3), 1) %2 = [[43/64, -63/8, 800, 21/64]~, [1, 0; 24, 0; 2048, 768; -1, 0]] ? mftobasis(mf, [0,1,-24,252], 1) %3 = [[1, 0, 1472, 0]~, [0; 0; 768; 0]] ? mftobasis(mf, [0,1,-24,252,-1472], 1) %4 = [1, 0, 0, 0]~ \\ now uniquely determined ? mftobasis(mf, [0,1,-24,252,-1472,0], 1) %5 = [1, 0, 0, 0]~ \\ wrong result: no such form exists ? mfcoefs(mflinear(mf,%), 5) \\ double check %6 = [0, 1, -24, 252, -1472, 4830] ? mftobasis(mf, [0,1,-24,252,-1472,0]) *** at top-level: mftobasis(mf,[0,1, *** ^ — — — — — — -- *** mftobasis: domain error in mftobasis: form does not belong to space ? mftobasis(mf, mfEk(10)) *** at top-level: mftobasis(mf,mfEk( *** ^ — — — — — — -- *** mftobasis: domain error in mftobasis: form does not belong to space ? mftobasis(mf, mfEk(10), 1) %7 = []~

The library syntax is `GEN `

.**mftobasis**(GEN mf, GEN F, long flag)

M being a matrix in SL_{2}(Z) and `Lcosets`

being
`mfcosets(N)`

, a list of right cosets of Γ_{0}(N),
find the coset to which M belongs. The output is a pair
[γ,i] such that M = γ `Lcosets`

[i], γ ∈ Γ_{0}(N).

? N = 4; L = mfcosets(N); ? mftocoset(N, [1,1;2,3], L) %2 = [[-1, 1; -4, 3], 5]

The library syntax is `GEN `

.**mftocoset**(long N, GEN M, GEN Lcosets)

`mf`

being being a full or cuspidal space with parameters [N,k,χ]
and F a cusp form in that space, returns a vector of 3-component vectors
[M,d,G], where f(χ) | M | N, d | N/M, and G is a form
in S_{k}^{new}(Γ_{0}(M),χ) such that F is equal to the sum of
the B(d)(G) over all these 3-component vectors.

? mf = mfinit([96,6],1); F = mfbasis(mf)[60]; s = mftonew(mf,F); #s %1 = 1 ? [M,d,G] = s[1]; [M,d] %2 = [48, 2] ? mfcoefs(F,10) %3 = [0, 0, -160, 0, 0, 0, 0, 0, 0, 0, -14400] ? mfcoefs(G,10) %4 = [0, 0, -160, 0, 0, 0, 0, 0, 0, 0, -14400]

The library syntax is `GEN `

.**mftonew**(GEN mf, GEN F)

If NK = [N,k,CHI,.] as in `mfinit`

with k integral, gives the
trace form in the corresponding subspace of S_{k}(Γ_{0}(N),χ).
The supported values for `space`

are 0: the newspace (default),
1: the full cuspidal space.

? F = mftraceform([23,2]); mfcoefs(F,16) %1 = [0, 2, -1, 0, -1, -2, -5, 2, 0, 4, 6, -6, 5, 6, 4, -10, -3] ? F = mftraceform([23,1,-23]); mfcoefs(F,16) %2 = [0, 1, -1, -1, 0, 0, 1, 0, 1, 0, 0, 0, 0, -1, 0, 0, -1]

The library syntax is `GEN `

.**mftraceform**(GEN NK, long space)

F being a generalized modular form, returns the twist of F by the
integer D, i.e., the form G such that
`mfcoef(G,n) = `

(D/n)`mfcoef(F,n)`

, where (D/n) is the Kronecker
symbol.

? mf = mfinit([11,2],0); F = mfbasis(mf)[1]; mfcoefs(F, 5) %1 = [0, 1, -2, -1, 2, 1] ? G = mftwist(F,-3); mfcoefs(G, 5) %2 = [0, 1, 2, 0, 2, -1] ? mf2 = mfinit([99,2],0); mftobasis(mf2, G) %3 = [1/3, 0, 1/3, 0]~

Note that twisting multiplies the level by D^2. In particular it is not an involution:

? H = mftwist(G,-3); mfcoefs(H, 5) %4 = [0, 1, -2, 0, 2, 1] ? mfparams(G) %5 = [99, 2, 1, y, t - 1]

The library syntax is `GEN `

.**mftwist**(GEN F, GEN D)