
// Make newform 6864.2.a.m in Magma, downloaded from the LMFDB on 24 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_6864_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_6864_2_a_m();".
// 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_6864_2_a_m();".
// 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
        Kf := Rationals();
    end if;
    return [Kf!elt[1] : elt in input];
end function;


// To make the character of type GrpDrchElt, type "MakeCharacter_6864_a();"
function MakeCharacter_6864_a()
    N := 6864;
    order := 1;
    char_gens := [2575, 1717, 4577, 4369, 2641];
    v := [1, 1, 1, 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_6864_a_Hecke(Kf)
    return MakeCharacter_6864_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], [2], [-4], [-1], [1], [-6], [0], [-8], [-2], [-4], [6], [-2], [4], [0], [6], [12], [14], [0], [0], [-6], [0], [4], [6], [-6], [-10], [8], [4], [-2], [-6], [-8], [12], [14], [-20], [-6], [20], [-2], [16], [-8], [14], [20], [22], [-16], [-22], [18], [16], [-4], [-20], [12], [30], [-6], [24], [2], [-28], [-22], [-8], [-10], [-12], [-10], [-2], [-4], [10], [16], [24], [26], [-22], [-8], [2], [12], [-10], [-10], [-24], [-8], [22], [16], [32], [6], [30], [14], [10], [36], [30], [0], [-30], [-8], [12], [14], [10], [-22], [-4], [-20], [24], [20], [12], [-16], [-24], [-6], [18], [12], [-2], [20], [-46], [-20], [-38], [44], [34], [12], [-18], [48], [10], [8], [-34], [-18], [0], [28], [-30], [-16], [32], [6], [-12], [38], [18], [14], [-12], [-48], [6], [46], [-48], [16], [-34], [32], [-32], [16], [22], [14], [26], [-38], [-32], [30], [-6], [-24], [-38], [-24], [-12], [14], [0], [-34], [-54], [20], [48], [6], [-6], [36], [48], [28], [0], [40], [6], [26], [18], [-28], [26], [60], [4], [-18], [-32], [8], [-42], [-30], [-18], [20], [-2], [0], [10], [16], [58], [-8], [26], [-24], [14], [-36], [12], [-26], [-58], [0], [22], [-34], [24], [10], [0], [2], [-4], [-28], [50], [-36], [18], [-6], [-2], [-50], [48], [-6], [8], [62], [-30], [-36], [46], [-4], [-52], [-26], [-4], [-46], [-42], [-40], [36], [24], [-38], [48], [-6], [-48], [50], [70], [28], [-50], [-20], [-60], [-10], [-38], [-40], [48], [-52], [46], [36], [-20], [2], [-20], [40], [10], [42], [60], [-40], [-28], [-4], [24], [-50], [-2], [48], [-36], [-60], [-8], [-24], [78], [6], [0], [-54], [30], [-52], [38], [-24], [54], [50], [-56], [-28], [54], [46], [46], [12], [42], [78], [-56], [6], [14], [32], [-46], [-32], [2], [28], [-28], [-42], [66], [36], [56], [20], [-48], [30], [8], [24], [2], [-38], [-84], [-46], [30], [28], [38], [-68], [14], [54], [-40], [-26], [-68], [80], [-70], [-22], [32], [28], [-4], [-6], [-20], [-18], [48], [54], [64], [50], [-14], [76], [-24], [26], [-20], [-48], [-70], [-54], [76], [-22], [54], [-76], [22], [-30], [80], [-56], [-48], [70], [62], [22], [32], [-84], [88], [-20], [-18], [86], [82], [-32], [22], [82], [-86], [8], [-54], [-36], [54], [-40], [16], [-42], [-8], [2], [82], [56], [-10], [26], [48], [-60], [98], [-24], [46], [-14], [24], [20], [76], [-6], [26], [68], [74], [4], [4], [0], [-50], [-88], [94], [36], [48], [-6], [-6], [-70], [-62], [-34], [-68], [62], [88], [0], [100], [70], [24], [-56], [-22], [42], [52], [44], [56], [58], [-84], [-38], [100], [-78], [-58], [-46], [-44], [-42], [-14], [-8], [70], [14], [-24], [-4], [-94], [-18], [-76], [44], [26], [-50], [-72], [40], [-98], [-16], [6], [78], [8], [36], [-54], [-46], [-36], [-58], [40], [-48]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_6864_a_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_6864_2_a_m();".
// 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_6864_2_a_m(:prec:=1)
    chi := MakeCharacter_6864_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(2999) - 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_6864_2_a_m();".
// 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_6864_2_a_m( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_6864_a();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,2,sign)));
    Vf := Kernel([<5,R![-2, 1]>,<7,R![4, 1]>,<17,R![6, 1]>,<19,R![0, 1]>],Snew);
    return Vf;
end function;