
// Make newform 27.3.b.b in Magma, downloaded from the LMFDB on 25 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_27_b();"
// To make the character of type GrpDrchElt with Codomain the HeckeField, type "MakeCharacter_27_b_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_27_3_b_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_27_3_b_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 := [1, 0, 1];
        Kf := NumberField(Polynomial([elt : elt in poly]));
        AssignNames(~Kf, ["nu"]);
    end if;
    Rf_num := [[1, 0], [0, 3]];
    Rf_basisdens := [1, 1];
    Rf_basisnums := ChangeUniverse([[z : z in elt] : elt in Rf_num], Kf);
    Rfbasis := [Rf_basisnums[i]/Rf_basisdens[i] : i in [1..Degree(Kf)]];
    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_27_b();"
function MakeCharacter_27_b()
    N := 27;
    order := 2;
    char_gens := [2];
    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;

// To make the character of type GrpDrchElt with Codomain the HeckeField, type "MakeCharacter_27_b_Hecke();"
function MakeCharacter_27_b_Hecke(Kf)
    N := 27;
    order := 2;
    char_gens := [2];
    char_values := [[-1, 0]];
    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 := 3;
    raw_aps := [[0, 1], [0, 0], [0, -1], [5, 0], [0, -5], [-10, 0], [0, 6], [-16, 0], [0, -4], [0, 10], [-1, 0], [20, 0], [0, 20], [50, 0], [0, -2], [0, -9], [0, -10], [-76, 0], [-10, 0], [0, -30], [65, 0], [14, 0], [0, 1], [0, 30], [-85, 0], [0, -65], [170, 0], [0, 63], [164, 0], [0, 8], [-205, 0], [0, 5], [0, 46], [-28, 0], [0, -25], [77, 0], [-100, 0], [110, 0], [0, 26], [0, -59], [0, -75], [-16, 0], [0, 10], [215, 0], [0, -69], [-223, 0], [-316, 0], [-130, 0], [0, -14], [-226, 0], [0, 78], [0, -40], [14, 0], [0, 30], [0, 146], [0, -92], [0, 90], [299, 0], [140, 0], [0, -50], [-280, 0], [0, 86], [290, 0], [0, -160], [185, 0], [0, 61], [-238, 0], [-10, 0], [0, 23], [-256, 0], [0, -152], [0, -150], [-625, 0], [170, 0], [704, 0], [0, 206], [0, 175], [-70, 0], [0, -40], [269, 0], [0, -70], [644, 0], [0, 90], [-565, 0], [-211, 0], [0, 166], [0, -120], [365, 0], [0, -35], [215, 0], [0, 21], [0, 250], [110, 0], [0, 215], [-766, 0], [0, 276], [0, 185], [0, -150], [-250, 0], [-268, 0], [410, 0], [0, 213], [0, -67], [0, -80], [-946, 0], [830, 0], [0, 151], [0, -234], [0, -370], [869, 0], [530, 0], [-70, 0], [0, -184], [662, 0], [-331, 0], [0, -20], [440, 0], [0, -324], [0, 161], [0, -275], [-928, 0], [-985, 0], [0, 118], [0, 66], [-436, 0], [0, 45], [32, 0], [0, -300], [-175, 0], [1160, 0], [-1006, 0], [0, 38], [359, 0], [-430, 0], [0, 440], [1259, 0], [0, -174], [-460, 0], [0, -79], [0, 270], [272, 0], [0, 130], [1205, 0], [0, 6], [1442, 0], [0, -530], [590, 0], [0, -434], [-316, 0], [0, 396], [-550, 0], [0, -30], [-880, 0], [0, -94], [-1300, 0], [0, 70], [137, 0], [0, 220], [605, 0], [0, -535], [0, 181], [0, 48], [845, 0], [0, 135], [0, -82], [0, 346], [-1501, 0], [770, 0]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_27_b_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_27_3_b_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_27_3_b_b(:prec:=2)
    chi := MakeCharacter_27_b();
    f_vec := qexpCoeffs();
    Kf := Universe(f_vec);
    // SetVerbose("ModularForms", true);
    // SetVerbose("ModularSymbols", true);
    S := CuspidalSubspace(ModularForms(chi, 3));
    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_27_3_b_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_27_3_b_b( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_27_b();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,3,sign)));
    Vf := Kernel([<2,R![9, 0, 1]>],Snew);
    return Vf;
end function;