
// Make newform 43.2.a.b in Magma, downloaded from the LMFDB on 19 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_43_a();"
// To make the coeffs of the qexp of the newform in the Hecke field type "qexpCoeffs();"
// To make the newform (type ModFrm), type "MakeNewformModFrm_43_2_a_b();".
// 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_43_2_a_b();".
// 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 := [-2, 0, 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_43_a();"
function MakeCharacter_43_a()
    N := 43;
    order := 1;
    char_gens := [3];
    v := [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;

function MakeCharacter_43_a_Hecke(Kf)
    return MakeCharacter_43_a();
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, 1], [0, -1], [2, -1], [-2, 1], [-1, 2], [1, 2], [5, 2], [-2, -2], [1, -4], [0, 3], [-3, 0], [0, -6], [-1, -2], [1, 0], [6, 0], [11, -2], [-2, 2], [4, 3], [1, 6], [-6, -2], [-12, 3], [2, -2], [9, 4], [-6, -3], [-1, -2], [3, -2], [9, 6], [-6, -4], [-3, 12], [-4, 2], [1, -2], [4, -4], [-6, 6], [-3, -6], [0, -6], [14, -3], [-10, 0], [-16, -3], [-3, 8], [18, -4], [-6, -1], [-4, -8], [8, 10], [-1, -12], [0, 10], [2, -4], [14, 10], [14, 7], [-12, -4], [-9, 12], [2, -2], [6, -6], [4, 0], [9, -10], [-6, -3], [-14, -10], [-3, 0], [-9, 12], [-2, -7], [-3, 4], [7, -8], [16, 4], [7, -14], [3, 12], [-4, 15], [23, -2], [20, -1], [-5, 0], [-6, -1], [-16, 3], [3, -2], [-27, -4], [-10, -14], [0, -6], [19, -8], [-12, 6], [6, 16], [4, -18], [-21, -6], [10, 19], [-14, 7], [-10, 4], [-5, -20], [26, 3], [3, -6], [22, 10], [12, -15], [-12, -9], [20, -16], [-18, 9], [18, -4], [-1, 4], [28, 4], [12, -1], [-30, -3], [6, 26], [9, 6], [24, 5], [-18, 15], [-11, -14], [9, 0], [-9, 0], [17, -16], [19, 4], [4, 5], [26, -5], [14, -14], [-2, 5], [31, -4], [-8, 12], [-16, 12], [4, -2], [3, -12], [12, -12], [10, -26], [-14, 11], [-18, 6], [-4, -2], [18, 2], [5, -8], [-17, 0], [-4, 9], [14, 2], [-21, 18], [-14, 9], [-24, 4], [-7, 22], [0, 16], [-8, 12], [0, -12], [-14, 18], [-16, -22], [-16, -3], [12, 6], [-12, -22], [8, -26], [18, 7], [-10, -14], [24, -8], [0, 4], [36, 9], [-21, 22], [-49, 4], [4, 4], [4, -14], [-22, 19], [7, -28], [2, -4], [0, -12], [-2, 32], [17, -14], [-9, -32], [-25, -12], [42, -12], [-3, 12], [0, 3], [-21, -6], [42, 2], [-10, 29], [-9, 16], [-3, 2], [0, 17], [-7, 24], [11, 10], [-12, 8], [-30, 23], [-4, -11], [10, 5]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_43_a_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_43_2_a_b();".
// 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_43_2_a_b(:prec:=2)
    chi := MakeCharacter_43_a();
    f_vec := qexpCoeffs();
    Kf := Universe(f_vec);
    // SetVerbose("ModularForms", true);
    // SetVerbose("ModularSymbols", true);
    S := CuspidalSubspace(ModularForms(chi, 2));
    S := BaseChange(S, Kf);
    maxprec := NextPrime(997) - 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_43_2_a_b();".
// 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_43_2_a_b( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_43_a();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,2,sign)));
    Vf := Kernel([<2,R![-2, 0, 1]>],Snew);
    return Vf;
end function;