
// Make newform 847.2.a.h in Magma, downloaded from the LMFDB on 25 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_847_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_847_2_a_h();".
// 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_847_2_a_h();".
// 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_847_a();"
function MakeCharacter_847_a()
    N := 847;
    order := 1;
    char_gens := [122, 365];
    v := [1, 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_847_a_Hecke(Kf)
    return MakeCharacter_847_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 := [[1, 1], [-1, 1], [1, 0], [-1, 0], [0, 0], [0, -2], [0, 5], [3, 2], [2, -5], [4, -1], [-3, 2], [4, -4], [5, -10], [7, -9], [-6, 1], [-3, -1], [8, -5], [7, -1], [-4, 7], [-13, 5], [13, -2], [-7, -1], [1, 6], [5, -3], [7, 0], [8, -11], [7, -3], [1, 6], [4, 4], [2, 3], [2, 8], [-14, 8], [-11, 7], [-7, 8], [7, -3], [-5, -8], [10, -16], [-5, 6], [-4, 4], [16, 1], [-5, -2], [0, 12], [17, 2], [11, -7], [8, 6], [-7, 2], [-15, 1], [-6, -13], [8, -13], [8, 2], [22, -12], [14, -5], [-7, 15], [23, 0], [-3, 9], [6, -18], [15, -3], [-2, -11], [-15, 13], [19, -10], [-7, 8], [11, 0], [17, -10], [9, 0], [14, -7], [-10, -9], [0, -10], [-21, 8], [-11, -14], [-26, 11], [21, -5], [15, -1], [18, -24], [21, 9], [16, -26], [-21, 22], [-28, 10], [-7, -10], [-20, -4], [14, -6], [20, -16], [2, 2], [12, -3], [-17, -13], [-4, -1], [0, -5], [8, -7], [2, 3], [8, -17], [21, -2], [6, 17], [-23, -2], [-19, 22], [-24, -3], [-7, 5], [20, -24], [1, 19], [-11, 2], [-11, 9], [3, 6], [11, 15], [11, 2], [19, -21], [-20, 28], [-17, 28], [1, -14], [1, 0], [0, 18], [-16, 0], [-10, 6], [-27, -11], [-9, -16], [2, -12], [-36, 6], [3, -24], [23, -14], [9, 12], [0, -10], [17, -2], [-29, 4], [-29, 9], [14, 12], [27, -15], [11, 17], [0, -30], [-8, 17], [1, 13], [7, 16], [3, 25], [-48, 8], [12, 1], [18, 5], [24, -28], [-16, -2], [2, 28], [-43, 9], [39, -2], [-15, 9], [31, -2], [-22, 17], [-16, -24], [-27, 48], [-22, 16], [6, -22], [-35, 21], [17, -22], [38, 5], [12, -5], [16, -9], [-43, 8], [-13, 0], [16, 3], [10, -35], [-43, 23], [-17, -2], [-39, 10], [5, -16], [24, 0], [27, -40], [-11, 26], [10, 6], [5, -2], [-12, 25], [-6, 36], [-17, -10], [-13, -1], [-10, -15], [33, -37]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_847_a_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_847_2_a_h();".
// 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_847_2_a_h(:prec:=2)
    chi := MakeCharacter_847_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_847_2_a_h();".
// 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_847_2_a_h( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_847_a();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,2,sign)));
    Vf := Kernel([<2,R![1, -3, 1]>,<3,R![-1, 1, 1]>],Snew);
    return Vf;
end function;