
// Make newform 2028.2.i.g in Magma, downloaded from the LMFDB on 23 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_2028_i();"
// To make the character of type GrpDrchElt with Codomain the HeckeField, type "MakeCharacter_2028_i_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_2028_2_i_g();".
// 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_2028_2_i_g();".
// 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_2028_i();"
function MakeCharacter_2028_i()
    N := 2028;
    order := 3;
    char_gens := [1015, 677, 1861];
    v := [3, 3, 2];
    // 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_2028_i_Hecke();"
function MakeCharacter_2028_i_Hecke(Kf)
    N := 2028;
    order := 3;
    char_gens := [1015, 677, 1861];
    char_values := [[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+1)]];
    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], [4, 0], [0, -2], [-4, 4], [0, 0], [0, -2], [0, -2], [0, 0], [6, -6], [10, 0], [10, -10], [8, -8], [0, -4], [4, 0], [-10, 0], [0, -8], [0, 14], [2, -2], [0, 16], [10, 0], [-16, 0], [0, 0], [-4, 4], [0, -2], [-10, 10], [-8, 0], [-12, 12], [2, 0], [0, -6], [-12, 12], [4, 0], [0, 8], [0, 0], [0, 0], [-18, 0], [2, 0], [0, 2], [-12, 12], [0, -2], [-12, 12], [2, 0], [0, 8], [-14, 14], [-12, 12], [0, -4], [12, -12], [14, -14], [0, -4], [2, 0], [-18, 0], [24, 0], [0, -2], [0, 20], [-30, 30], [16, -16], [0, 14], [-10, 10], [0, -22], [8, 0], [16, -16], [0, 8], [-22, 0], [24, 0], [-6, 0], [12, 0], [0, 2], [-30, 0], [0, -12], [18, -18], [-16, 16], [12, 0], [-4, 4], [0, 18], [6, -6], [0, 12], [-26, 0], [0, 18], [-12, 12], [0, 14], [20, -20], [2, 0], [-20, 20], [0, 18], [-16, 16], [-4, 0], [0, 12], [-18, 18], [0, 12], [-22, 0], [-28, 0], [-16, 16], [0, -26], [-12, 12], [6, 0], [0, -16], [12, -12], [-2, 0], [-16, 16], [-38, 0], [32, 0], [24, -24], [0, -28], [-6, 6], [-8, 0], [30, 0], [-8, 8], [36, 0], [24, 0], [22, -22], [0, 4], [-14, 14], [0, -48], [-14, 0], [0, -30], [0, -26], [0, 26], [24, -24], [46, -46], [0, 12], [10, -10], [-22, 22], [38, 0], [0, 20], [10, -10], [-2, 0], [0, 6], [0, 48], [44, 0], [-22, 0], [22, -22], [48, -48], [-8, 8], [-18, 18], [0, 40], [-22, 22], [0, -48], [0, -50], [0, 26], [-38, 38], [-6, 0], [24, -24], [0, -32], [20, 0], [14, -14], [0, -16], [14, 0], [-18, 0], [52, 0], [-4, 0], [0, 10], [-14, 14], [-32, 0], [-48, 48], [56, -56], [-56, 0], [0, -56], [0, 16], [-34, 0], [-8, 0], [8, -8], [0, 42], [-22, 0], [0, 28], [-12, 12], [-24, 0], [36, -36], [0, -22], [2, 0], [26, 0], [12, 0], [0, 42], [0, -40], [0, 54], [-32, 0], [0, 6], [-58, 58], [-8, 0], [24, -24], [26, -26], [26, 0], [20, 0], [2, 0], [60, 0], [12, -12], [0, -46], [-38, 0], [50, 0], [22, -22], [0, 60], [0, 2], [0, -48], [-16, 0], [-64, 64], [0, -52], [-30, 30], [-18, 0], [0, -30], [12, 0], [32, 0], [0, -12], [0, -12], [22, -22], [-34, 0], [-32, 32], [-14, 14], [10, 0], [0, 52], [-12, 12], [0, -8], [-46, 46], [-42, 0], [-16, 16], [0, -12], [0, 44], [-18, 0], [48, 0], [0, -18], [16, -16], [16, 0], [-6, 6], [50, 0], [-28, 0], [0, -10], [-36, 36], [-50, 0], [-58, 58], [0, 72], [0, -4], [24, 0], [54, -54], [0, 0], [-70, 70], [18, 0], [16, 0], [-24, 0], [0, 22], [24, -24], [0, 44], [24, -24], [-16, 16], [56, -56], [0, 48], [-62, 62], [0, 20], [-24, 0], [0, 30], [-48, 48], [0, 54], [-24, 24], [38, -38], [-16, 16], [56, 0], [-26, 26], [-54, 0], [0, 60], [0, -6], [-10, 10], [-66, 0], [0, 2], [28, 0], [-12, 12], [-70, 0], [-14, 14], [0, 60], [0, 36], [0, -60], [-24, 0], [0, -10], [0, 30], [-14, 0], [30, 0], [46, -46], [0, 0], [0, -10], [-54, 54], [0, 12], [38, 0], [0, -70], [0, 36], [0, 0], [-34, 34], [64, 0], [58, -58], [-14, 0], [48, 0], [30, 0], [-64, 0], [0, -70], [0, -30], [-22, 22], [0, -52], [-20, 20], [0, -72], [0, -50], [-18, 0], [4, 0], [-6, 6], [60, -60], [86, -86], [0, 22], [-60, 0], [16, -16]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_2028_i_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_2028_2_i_g();".
// 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_2028_2_i_g(:prec:=2)
    chi := MakeCharacter_2028_i();
    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_2028_2_i_g();".
// 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_2028_2_i_g( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_2028_i();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,2,sign)));
    Vf := Kernel([<5,R![-4, 1]>,<7,R![4, 2, 1]>],Snew);
    return Vf;
end function;