
// Make newform 3900.2.cd.a in Magma, downloaded from the LMFDB on 06 June 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_3900_cd();"
// To make the character of type GrpDrchElt with Codomain the HeckeField, type "MakeCharacter_3900_cd_Hecke();"
// To make the coeffs of the qexp of the newform in the Hecke field type "qexpCoeffs();"
// To make the newform (type ModFrm), type "MakeNewformModFrm_3900_2_cd_a();".
// This may take a long time!  To see verbose output, uncomment the SetVerbose lines below.
// The precision argument determines an initial guess on how many Fourier coefficients to use.
// This guess is increased enough to uniquely determine the newform.
// To make the Hecke irreducible modular symbols subspace (type ModSym)
// containing the newform, type "MakeNewformModSym_3900_2_cd_a();".
// This may take a long time!  To see verbose output, uncomment the SetVerbose line below.
// The default sign is -1.  You can change this with the optional parameter "sign".
function ConvertToHeckeField(input: pass_field := false, Kf := [])
    if not pass_field then
        poly := [1, -1, 1];
        Kf := NumberField(Polynomial([elt : elt in poly]));
        AssignNames(~Kf, ["nu"]);
    end if;
    Rfbasis := [Kf.1^i : i in [0..Degree(Kf)-1]];
    inp_vec := Vector(Rfbasis)*ChangeRing(Transpose(Matrix([[elt : elt in row] : row in input])),Kf);
    return Eltseq(inp_vec);
end function;


// To make the character of type GrpDrchElt, type "MakeCharacter_3900_cd();"
function MakeCharacter_3900_cd()
    N := 3900;
    order := 6;
    char_gens := [1951, 1301, 3277, 301];
    v := [6, 6, 6, 1];
    // chi(gens[i]) = zeta^v[i]
    assert UnitGenerators(DirichletGroup(N)) eq char_gens;
    F := CyclotomicField(order);
    chi := DirichletCharacterFromValuesOnUnitGenerators(DirichletGroup(N,F),[F|F.1^e:e in v]);
    return MinimalBaseRingCharacter(chi);
end function;

// To make the character of type GrpDrchElt with Codomain the HeckeField, type "MakeCharacter_3900_cd_Hecke();"
function MakeCharacter_3900_cd_Hecke(Kf)
    N := 3900;
    order := 6;
    char_gens := [1951, 1301, 3277, 301];
    char_values := [[1, 0], [1, 0], [1, 0], [0, 1]];
    assert UnitGenerators(DirichletGroup(N)) eq char_gens;
    values := ConvertToHeckeField(char_values : pass_field := true, Kf := Kf); // the value of chi on the gens as elements in the Hecke field
    F := Universe(values);// the Hecke field
    chi := DirichletCharacterFromValuesOnUnitGenerators(DirichletGroup(N,F),values);
    return chi;
end function;


function ExtendMultiplicatively(weight, aps, character)
    prec := NextPrime(NthPrime(#aps)) - 1; // we will able to figure out a_0 ... a_prec
    primes := PrimesUpTo(prec);
    prime_powers := primes;
    assert #primes eq #aps;
    log_prec := Floor(Log(prec)/Log(2)); // prec < 2^(log_prec+1)
    F := Universe(aps);
    FXY<X, Y> := PolynomialRing(F, 2);
    // 1/(1 - a_p T + p^(weight - 1) * char(p) T^2) = 1 + a_p T + a_{p^2} T^2 + ...
    R<T> := PowerSeriesRing(FXY : Precision := log_prec + 1);
    recursion := Coefficients(1/(1 - X*T + Y*T^2));
    coeffs := [F!0: i in [1..prec]];
    coeffs[1] := 1; //a_1
    for i := 1 to #primes do
        p := primes[i];
        coeffs[p] := aps[i];
        b := p^(weight - 1) * F!character(p);
        r := 2;
        p_power := p * p;
        //deals with powers of p
        while p_power le prec do
            Append(~prime_powers, p_power);
            coeffs[p_power] := Evaluate(recursion[r + 1], [aps[i], b]);
            p_power *:= p;
            r +:= 1;
        end while;    
    end for;
    Sort(~prime_powers);
    for pp in prime_powers do
        for k := 1 to Floor(prec/pp) do
            if GCD(k, pp) eq 1 then
                coeffs[pp*k] := coeffs[pp]*coeffs[k];
            end if;
        end for;
    end for;
    return coeffs;
end function;


function qexpCoeffs()
    // To make the coeffs of the qexp of the newform in the Hecke field type "qexpCoeffs();"
    weight := 2;
    raw_aps := [[0, 0], [-1, 1], [0, 0], [-4, 2], [2, 2], [1, 3], [0, 3], [-4, 2], [6, -6], [-9, 9], [0, 0], [3, 3], [-5, -5], [0, 2], [-2, 4], [9, 0], [16, -8], [0, 11], [-6, -6], [12, -6], [-3, 6], [-8, 0], [2, -4], [-4, -4], [-8, 4], [-3, 3], [-14, 0], [-18, 18], [-8, 16], [0, 3], [16, -16], [-12, 0], [-18, 9], [0, 4], [-10, 5], [10, -20], [-23, 0], [16, -8], [-4, -4], [0, 18], [-18, 18], [-13, 0], [0, 0], [-1, -1], [-4, -4], [0, -10], [8, -8], [-8, -8], [-12, 6], [4, -8], [-18, 0], [6, -12], [30, -15], [0, 24], [-3, 3], [6, -6], [0, -30], [4, 4], [0, -19], [-17, 34], [-14, 14], [-2, 1], [-10, 20], [-18, 0], [14, 0], [-1, 2], [16, -8], [23, 0], [0, 18], [-8, -8], [15, 15], [-2, 4], [2, -2], [0, 31], [8, 8], [-16, 8], [21, 0], [32, -16], [21, 21], [-14, 7], [0, 0], [7, -14], [-18, -18], [0, -29], [10, -10], [-24, 0], [-8, 4], [-5, -5], [6, -3], [2, -4], [6, 0], [4, 4], [20, -10], [-30, 30], [0, 0], [0, 6], [1, 1], [-9, 0], [-10, 10], [-1, 2], [34, 0], [-1, -1], [0, 24], [-6, 6], [-26, 0], [5, -10], [-12, -12], [7, -14], [12, 0], [-37, 37], [0, 8], [3, 3], [30, -15], [-8, 16], [32, -16], [0, 21], [8, -4], [0, 0], [6, -6], [0, -12], [-7, -7], [5, -5], [30, 0], [16, -8], [-6, -6], [6, 0], [18, -9], [0, 36], [-10, 0], [-21, 42], [16, 16], [-24, -24], [-26, 26], [-2, 2], [24, -12], [-20, -20], [0, 0], [48, -24], [0, 18], [21, -21], [-4, 8], [16, 16], [0, -40], [-16, 32], [11, -11], [48, -24], [-25, 50], [3, 0], [-14, 0], [22, -44], [42, -21], [-9, 9], [-16, 0], [0, 0], [28, -28], [0, 0], [0, 32], [-66, 33], [41, 0], [0, 0], [8, 8], [0, 54], [26, -52], [0, 24], [5, 5], [-12, 24], [-2, 2], [0, -55], [-4, 8], [45, 0], [26, -52], [-6, 3], [0, 48], [-26, 13], [-4, 0], [0, 27], [-6, -6], [33, -66], [20, -20], [-34, 34], [-30, 60], [12, 0], [23, 0], [-32, 64], [-10, -10], [0, -9], [-2, 0], [36, -72], [35, 35], [20, -10], [0, 5], [-44, 22], [2, 0], [-20, -20], [0, 42], [-6, 6], [27, -54], [0, 7], [5, -10], [-24, 0], [-30, 15], [0, -44], [-33, -33], [-19, 0], [-34, -34], [39, -39], [10, -20], [0, -36], [33, 33], [0, 44], [19, -19], [66, 0], [-62, 62], [44, -22], [-64, 32], [-23, 46], [-4, 0], [0, 3], [4, 4], [-32, 64], [31, -31], [-20, 40], [-11, 22], [24, -12], [-6, 6], [-19, 0], [-39, 39], [0, 0], [0, 64], [-16, 32], [23, -23], [-20, 20], [-8, -8], [51, 0], [-44, 0], [16, -32], [-72, 36], [-13, -13], [0, 36], [-42, 42], [42, 42], [38, -38], [0, 34], [5, 5], [-62, 31], [-12, 0], [-24, 12], [-24, -24], [-68, 34], [-42, 42], [16, 16], [24, 24], [-20, 40], [-37, 37], [-18, 0], [12, -6], [0, 38], [-10, -10], [18, 0], [50, -25], [-64, 0], [-36, 36], [27, -54], [29, -29], [46, -23], [0, 50], [-24, 12], [39, -78], [44, -22], [0, -33], [-59, 0], [-20, 40], [-7, -7], [0, 4], [0, 58], [26, 26], [64, -32], [23, -46], [-94, 47], [0, -6], [-6, 6], [32, 32], [-12, 0], [-17, -17], [26, -52], [-66, 0], [77, 0], [-25, 50], [-36, 18], [0, -33], [54, -54], [0, 42], [-25, -25], [40, -20], [0, 26], [-15, 0], [80, 0], [-39, 39], [72, -72], [-26, -26], [0, -1], [1, -2], [-2, 2]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_3900_cd_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_3900_2_cd_a();".
// This may take a long time!  To see verbose output, uncomment the SetVerbose lines below.
// The precision argument determines an initial guess on how many Fourier coefficients to use.
// This guess is increased enough to uniquely determine the newform.
function MakeNewformModFrm_3900_2_cd_a(:prec:=2)
    chi := MakeCharacter_3900_cd();
    f_vec := qexpCoeffs();
    Kf := Universe(f_vec);
    // SetVerbose("ModularForms", true);
    // SetVerbose("ModularSymbols", true);
    S := CuspidalSubspace(ModularForms(chi, 2));
    S := BaseChange(S, Kf);
    maxprec := NextPrime(1999) - 1;
    while true do
        trunc_vec := Vector(Kf, [0] cat [f_vec[i]: i in [1..prec]]);
        B := Basis(S, prec + 1);
        S_basismat := Matrix([AbsEltseq(g): g in B]);
        if Rank(S_basismat) eq Min(NumberOfRows(S_basismat), NumberOfColumns(S_basismat)) then
            S_basismat := ChangeRing(S_basismat,Kf);
            f_lincom := Solution(S_basismat,trunc_vec);
            f := &+[f_lincom[i]*Basis(S)[i] : i in [1..#Basis(S)]];
            return f;
        end if;
        error if prec eq maxprec, "Unable to distinguish newform within newspace";
        prec := Min(Ceiling(1.25 * prec), maxprec);
    end while;
end function;


// To make the Hecke irreducible modular symbols subspace (type ModSym)
// containing the newform, type "MakeNewformModSym_3900_2_cd_a();".
// This may take a long time!  To see verbose output, uncomment the SetVerbose line below.
// The default sign is -1.  You can change this with the optional parameter "sign".
function MakeNewformModSym_3900_2_cd_a( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_3900_cd();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,2,sign)));
    Vf := Kernel([<7,R![12, 6, 1]>,<11,R![12, -6, 1]>,<17,R![9, -3, 1]>],Snew);
    return Vf;
end function;