
// Make newform 4.13.b.a in Magma, downloaded from the LMFDB on 25 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_4_b();"
// To make the character of type GrpDrchElt with Codomain the HeckeField, type "MakeCharacter_4_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_4_13_b_a();".
// 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_4_13_b_a();".
// 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_4_b();"
function MakeCharacter_4_b()
    N := 4;
    order := 2;
    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;

// To make the character of type GrpDrchElt with Codomain the HeckeField, type "MakeCharacter_4_b_Hecke();"
function MakeCharacter_4_b_Hecke(Kf)
    N := 4;
    order := 2;
    char_gens := [3];
    char_values := [[-1]];
    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 := 13;
    raw_aps := [[-64], [0], [23506], [0], [0], [6911282], [-47295038], [0], [0], [-173439758], [0], [-2050092718], [-2285065118], [0], [0], [-43462597358], [0], [-47844884878], [0], [0], [-119852347678], [0], [0], [907573615522], [502341690242], [777161886802], [0], [0], [-3144016483918], [-2904548221118], [0], [0], [-6652334060318], [0], [10974187187602], [0], [28692667177202], [0], [0], [-13835599520078], [0], [-56782125120238], [0], [103216035686402], [76653364882642], [0], [0], [0], [0], [-5716934973358], [302045519665762], [0], [-391820763971518], [0], [422035496729602], [0], [-10885816261838], [0], [759223655583122], [-867355027558238], [0], [208462022834002], [0], [0], [-1668303121885918], [-278475496957838], [0], [2903143874604482], [0], [-3592202427075598], [2074262246925442], [0], [0], [-1393501949264878], [0], [0], [6896995142432722], [-6760092340132558], [6865545886347202], [-2005440142324318], [0], [-10196724328605358], [0], [6063748806922562], [0], [0], [-10205701060172798], [-11567069843565598], [17120300253059122], [0], [0], [0], [0], [0], [0], [0], [-31167170157669518], [38765388996220642], [0], [29161241176420082], [0], [13828909561703602], [0], [54711123959017762], [0], [64780070315800322], [0], [-55729395265460798], [0], [-73471898110946398], [0], [-99945917600883118], [-57178092008820638], [0], [0], [-45357593548184318], [0], [0], [153923699262248242], [0], [-158605698523924078], [161953050827617922], [172430025067011922], [0], [0], [-154525848888845198], [158344912134236882], [0], [0], [195906750299685362], [0], [0], [0], [-246310509592508398], [-370216776511360478], [252886657578197762], [-17353436323128878], [0], [46326582775232242], [-292925707110793118], [0], [604911643121261842], [0], [0], [-285821946565468558], [0], [-88075136111330158], [-58578347349839198], [0], [0], [-699851513830182478], [797942664212682562], [0], [0], [0], [0], [0], [-862087554514226558], [-244566160248028318], [-923587223045815118], [0], [723103585905777442], [0], [0], [53769238192829122], [0], [0], [-1300247575314582958]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_4_b_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_4_13_b_a();".
// 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_4_13_b_a(:prec:=1)
    chi := MakeCharacter_4_b();
    f_vec := qexpCoeffs();
    Kf := Universe(f_vec);
    // SetVerbose("ModularForms", true);
    // SetVerbose("ModularSymbols", true);
    S := CuspidalSubspace(ModularForms(chi, 13));
    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_4_13_b_a();".
// 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_4_13_b_a( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_4_b();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,13,sign)));
    Vf := Kernel([<3,R![0, 1]>],Snew);
    return Vf;
end function;