
// Make newform 1600.2.a.m in Magma, downloaded from the LMFDB on 24 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_1600_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_1600_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_1600_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_1600_a();"
function MakeCharacter_1600_a()
    N := 1600;
    order := 1;
    char_gens := [1151, 901, 577];
    v := [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_1600_a_Hecke(Kf)
    return MakeCharacter_1600_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], [0], [0], [0], [0], [4], [-8], [0], [0], [-10], [0], [12], [-10], [0], [0], [-4], [0], [-10], [0], [0], [-16], [0], [0], [-10], [8], [2], [0], [0], [-6], [16], [0], [0], [8], [0], [-14], [0], [-12], [0], [0], [-4], [0], [18], [0], [-24], [-28], [0], [0], [0], [0], [30], [16], [0], [30], [0], [32], [0], [26], [0], [28], [-10], [0], [4], [0], [0], [-24], [28], [0], [-32], [0], [-10], [-16], [0], [0], [36], [0], [0], [34], [12], [2], [-6], [0], [30], [0], [24], [0], [0], [-14], [-8], [-38], [0], [0], [0], [0], [0], [0], [0], [-10], [-22], [0], [42], [0], [-28], [0], [26], [0], [-48], [0], [-16], [0], [-10], [0], [-36], [32], [0], [0], [-50], [0], [0], [-44], [0], [-50], [24], [52], [0], [0], [-10], [30], [0], [0], [-4], [0], [0], [0], [-52], [-38], [-50], [44], [0], [-52], [-10], [0], [-50], [0], [0], [-54], [0], [-36], [8], [0], [0], [-12], [-50], [0], [0], [0], [0], [0], [-46], [-48], [58], [0], [56], [0], [0], [8], [0], [0], [-12], [30], [44], [0], [-22], [0], [64], [0], [-10], [0], [-62], [0], [26], [0], [0], [-4], [-32], [0], [-50], [52], [0], [-54], [0], [16], [0], [0], [-10], [0], [-64], [-50], [-44], [-32], [0], [70], [0], [68], [30], [0], [68], [0], [0], [70], [0], [72], [-50], [0], [0], [0], [-10], [0], [-62], [0], [4], [30], [0], [-50], [0], [0], [-46], [-16], [0], [0], [0], [-76], [0], [0], [70], [0], [0], [66], [76], [0], [0], [0], [0], [0], [70], [64], [0], [0], [0], [0], [0], [-68], [2], [0], [-6], [76], [0], [-78], [0], [-52], [72], [0], [0], [30], [-36], [-8], [0], [70], [-22], [0], [-76], [58], [0], [-64], [0], [32], [0], [0], [-10], [70], [0], [0], [0], [0], [-62], [0], [0], [56], [-28], [0], [34], [70], [0], [-16], [0], [-84], [-86], [0], [-76], [0], [0], [24], [68], [0], [0], [0], [88], [0], [-90], [0], [84], [0], [-50], [82], [0], [0], [-90], [0], [0], [-64], [-46], [0], [-72], [-10], [0], [-56], [30], [0], [0], [0], [-4], [-90], [-92], [0], [0], [0], [0], [74], [16], [-90], [0], [84], [-88], [-94], [0], [44], [0], [30], [0], [0], [52], [0], [-88], [70], [0], [-50], [64], [0], [0], [8], [0], [-12], [58], [0], [0], [0], [96], [92], [0], [70], [0], [0], [0], [-14], [0], [92], [0], [0], [-96], [-94], [8], [-22], [-56], [0], [-32], [0], [0], [0], [-68], [0], [0], [66], [-44], [0], [0], [0], [104], [0], [-10], [0], [-50], [-86], [-104], [0], [88], [34], [0], [28], [98], [0], [0], [-96], [-68], [0], [0], [-32], [-38], [0], [0], [88], [0], [106], [-108], [0], [0], [-24], [-92], [0], [74], [0], [0]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_1600_a_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_1600_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_1600_2_a_m(:prec:=1)
    chi := MakeCharacter_1600_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_1600_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_1600_2_a_m( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_1600_a();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,2,sign)));
    Vf := Kernel([<3,R![0, 1]>,<7,R![0, 1]>,<11,R![0, 1]>,<13,R![-4, 1]>],Snew);
    return Vf;
end function;