
// Make newform 2695.2.a.b in Magma, downloaded from the LMFDB on 25 April 2024.
// To make the character of type GrpDrchElt, type "MakeCharacter_2695_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_2695_2_a_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_2695_2_a_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
        Kf := Rationals();
    end if;
    return [Kf!elt[1] : elt in input];
end function;


// To make the character of type GrpDrchElt, type "MakeCharacter_2695_a();"
function MakeCharacter_2695_a()
    N := 2695;
    order := 1;
    char_gens := [2157, 1816, 981];
    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_2695_a_Hecke(Kf)
    return MakeCharacter_2695_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], [2], [-1], [0], [-1], [-4], [4], [8], [0], [-6], [6], [-6], [0], [-4], [6], [10], [14], [-12], [12], [-12], [8], [8], [16], [14], [2], [-8], [-2], [4], [10], [-6], [16], [12], [-10], [16], [6], [8], [2], [-4], [16], [-20], [-4], [-2], [0], [-10], [2], [-14], [4], [-2], [12], [14], [18], [24], [8], [-6], [-10], [0], [-18], [4], [26], [6], [-28], [0], [-12], [26], [14], [-2], [-28], [-6], [12], [-28], [-2], [16], [-26], [-14], [32], [10], [6], [-22], [-38], [-4], [6], [-30], [32], [-6], [36], [24], [-10], [-34], [-4], [-24], [6], [20], [-32], [-4], [8], [36], [14], [42], [-28], [-26], [4], [18], [0], [-10], [-36], [-22], [-42], [16], [12], [16], [-24], [-18], [-42], [-6], [24], [30], [22], [42], [-34], [-20], [10], [-38], [-36], [-8], [-42], [10], [-2], [-18], [-30], [-32], [12], [-16], [-20], [-10], [20], [28], [-18], [16], [18], [-6], [12], [38], [52], [-4], [10], [-18], [-4], [8], [50], [36], [-18], [18], [48], [-12], [-12], [-4], [-8], [-22], [-56], [-28], [48], [-18], [32], [18], [-14], [-58], [-52], [-12], [-38], [-42], [-20], [58], [24], [34], [10], [-6], [52], [-6], [12], [0], [12], [-40], [-58], [-12], [-36], [38], [26], [62], [30], [24], [18], [12], [-56], [50], [36], [30], [-42], [-62], [-24], [-12], [6], [-48], [22], [-12], [-30], [18], [-30], [20], [2], [-50], [34], [-26], [-28], [18], [12], [14], [16], [-20], [24], [-66], [34], [60], [30], [-20], [-28], [26], [42], [-56], [52], [-52], [26], [68], [48], [58], [-74], [28], [50], [-14], [48], [-2], [-28], [8], [2], [10], [44], [64], [-54], [78], [44], [0], [-78], [36], [-8], [6], [64], [-36], [42], [32], [2], [-20], [40], [-36], [-28], [44], [30], [-6], [18], [-30], [-20], [14], [26], [-44], [26], [80], [24], [22], [-32], [-50], [74], [-8], [16], [-12], [68], [8], [-32], [76], [-2], [-30], [-14], [16], [58], [-58], [-86], [24], [42], [-36], [-10], [54], [-24], [56], [16], [-34], [-40], [-36], [-88], [-26], [8], [-14], [48], [-22], [-56], [86], [30], [8], [16], [76], [-6], [-40], [2], [22], [44], [-38], [12], [76], [6], [-82], [-32], [-74], [-24], [-74], [30], [-70], [-16], [16], [-68], [-2], [34], [36], [-58], [-60], [54], [46], [16], [0], [14], [-60], [-50], [-28], [24], [-34], [-32], [38], [-18], [-44], [30], [4], [-70], [-44], [70], [60], [18], [24], [-28], [-28], [78], [-74], [92], [-64], [58], [16], [14], [-64], [-66], [-24], [-62], [50], [8], [-52], [-56], [-28], [-14], [6], [0], [-58], [-56], [6], [96], [-22], [36], [-42], [-2], [-82], [40], [62], [-96], [10], [-76], [-66], [-40], [-62], [36], [-22], [32], [-98], [-8], [32], [62], [2], [74], [22], [56], [-26], [-40], [-44], [42], [90], [-16], [58], [14], [16], [38], [-32], [-88], [-12], [-10], [-74], [28], [22], [6], [-44]];
    aps := ConvertToHeckeField(raw_aps);
    chi := MakeCharacter_2695_a_Hecke(Universe(aps));
    return ExtendMultiplicatively(weight, aps, chi);
end function;


// To make the newform (type ModFrm), type "MakeNewformModFrm_2695_2_a_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_2695_2_a_b(:prec:=1)
    chi := MakeCharacter_2695_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_2695_2_a_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_2695_2_a_b( : sign := -1)
    R<x> := PolynomialRing(Rationals());
    chi := MakeCharacter_2695_a();
    // SetVerbose("ModularSymbols", true);
    Snew := NewSubspace(CuspidalSubspace(ModularSymbols(chi,2,sign)));
    Vf := Kernel([<2,R![1, 1]>,<3,R![-2, 1]>],Snew);
    return Vf;
end function;