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#############################################################################
##
#A IsIrred.gi Frank Lübeck
##
## The files IsIrred.g{i,d} contain code to find irreducible
## polynomials over a finite field, polynomials handled as coefficient
## lists.
##
# only for prime degree d
# test that poly divides X^(q^d) - X and is prime to X^q - X
InstallGlobalFunction(IsIrreducibleCoeffListPrimeDegree, function(cs, q)
local d, v, vp;
d := Length(cs)-1;
v := cs{[1,2]};
v[1] := Zero(v[1]);
v[2] := One(v[2]);
vp := PowerModCoeffs(v, q, cs);
if Length(GcdCoeffs(vp-v, cs)) > 1 then
return false;
fi;
vp := PowerModCoeffs(v, q^d, cs);
if vp <> v then
return false;
fi;
return true;
end);
# utiltiy for X^(p^k) mod cs - compute (X^i)^p mod cs for i < Length(cs)
# and then use matrix multiplication
InstallGlobalFunction(PMCspecial, function(v, q, cs)
local p, m, a, vv, pp, i;
p := SmallestRootInt(q);
if q = p or 2^Length(cs) > q then
return PowerModCoeffs(v, q, cs);
fi;
m := [[One(v[1])], PowerModCoeffs(v, p, cs)];
for i in [3..Length(cs)-1] do
a := ProductCoeffs(m[i-1], m[2]);
ReduceCoeffs(a, cs);
ShrinkRowVector(a);
Add(m, a);
od;
vv := v;
pp := 1;
while pp < q do
vv := List(vv, c-> c^p)*m;
ShrinkRowVector(vv);
pp := pp*p;
od;
return vv;
end);
## <#GAPDoc Label="IsIrreducibleCoeffList">
## <ManSection>
## <Func Name="IsIrreducibleCoeffList" Arg="coeffs, q" />
## <Returns><K>true</K> or <K>false</K></Returns>
## <Description>
## The argument <A>coeffs</A> must be a list of elements in a finite field
## with <A>q</A> elements (or some subfield of it).
## <P/>
## The function checks if the univariate polynomial <M>f</M> with coefficient
## list <A>coeffs</A> (ending with the leading coefficient) is irreducible
## over the field with <A>q</A> elements.
## <P/>
## The algorithm computes the greatest common divisor of <M>f</M> with
## <M>X^{{q^i}} - X</M> for <M>i = 1, 2, \ldots</M> up to half of the degree
## of <M>f</M>.
##
## <Example>gap> cs := Z(3)^0 * ConwayPol(3,8);
## [ Z(3), Z(3), Z(3), 0*Z(3), Z(3)^0, Z(3), 0*Z(3), 0*Z(3), Z(3)^0 ]
## gap> IsIrreducibleCoeffList(cs, 3);
## true
## gap> F := FF(17,4);; x := PrimitiveElement(F);;
## gap> cs := [x, x+x^0, 0*x, x^0];
## [ ZZ(17,4,[0,1,0,0]), ZZ(17,4,[1,1,0,0]), ZZ(17,4,[0]), ZZ(17,4,[1]) ]
## gap> while not IsIrreducibleCoeffList(cs, 17^4) do
## > cs[1] := cs[1] + One(F);
## > od;
## gap> cs;
## [ ZZ(17,4,[8,1,0,0]), ZZ(17,4,[1,1,0,0]), ZZ(17,4,[0]), ZZ(17,4,[1]) ]
## </Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
# for general polynomials, test for divisors of all lower degrees
InstallGlobalFunction(IsIrreducibleCoeffList, function(cs, q)
local d, v, z, o, vq, vqq, mat, vv, m, k, i;
if not IsMutable(cs) then
cs := ShallowCopy(cs);
fi;
d := Length(cs)-1;
if IsPrimeInt(d) and q < 5 then
return IsIrreducibleCoeffListPrimeDegree(cs, q);
fi;
v := cs{[1,2]};
z := Zero(v[1]);
o := One(v[1]);
v[1] := z;
v[2] := o;
# remark: a precomputation of v^i mod cs for i = d..2d-1 and variants
# of PowerModCoeffs and ReduceCoeffs which use it could be useful
## vq := PowerModCoeffs(v, q, cs);
vq := PMCspecial(v, q, cs);
# test for linear factors
if Length(GcdCoeffs(vq-v, cs)) > 1 then
Info(InfoStandardFF, 2, 1,"\c");
return false;
fi;
# for very small q we take a few more q-th powers,
# many random polynomials have factors of very small degree
vqq := vq;
for k in [1..QuoInt(d, 2*Log2Int(q)+2)] do
vqq := PowerModCoeffs(vqq, q, cs);
if Length(GcdCoeffs(vqq-v, cs)) > 1 then
Info(InfoStandardFF, 2, k+1,"'\c");
return false;
fi;
od;
# precompute in rows of mat: 1^q, X^q, (X^2)^q, ... (x^(d-1))^q mod cs
mat := [0*cs{[1..d]}];
mat[1][1] := v[2];
Add(mat, vq);
for i in [2..d-1] do
vv := ProductCoeffs(mat[i], vq);
m := ReduceCoeffs(vv, cs);
if Length(vv) > m then
vv := vv{[1..m]};
fi;
Add(mat, vv);
od;
for vv in mat do
while Length(vv) > d do
Unbind(vv[Length(vv)]);
od;
while Length(vv) < d do
Add(vv, z);
od;
od;
# Now mat is the Berlekamp matrix, describing the Frobenius
# on the X^i-basis.
# We could compute the rank of mat - id (first row is zero),
# cs is irreducible iff the rank is d-1.
# But when is this good? The loop below stops for random polynomials
# after few vector-matrix multiplications plus gcd.
# now we get further q-powers by matrix multiplication
# in step i (<=d/2) we check for irreducible factors of degree i
## for i in [2..QuoInt(d,2)] do
for i in [2..Log2Int(d)] do
# checking for factors of degree <= Log2(d) first
# seems a good compromise in experiments
vq := vq*mat;
while Length(vq) > 0 and IsZero(vq[Length(vq)]) do
Unbind(vq[Length(vq)]);
od;
if Length(GcdCoeffs(vq-v, cs)) > 1 then
Info(InfoStandardFF, 3, i,"\c");
return false;
fi;
od;
## return true;
# if no small degree factor found we now compute rank
for i in [1..d] do
mat[i][i]:=mat[i][i]-One(mat[i][i]);
od;
return RankMat(mat) = d-1;
end);
# About 1/r of all monic polynomials of degree r with constant term a are
# irreducible.
# We try to find sparse polynomials. After each r tries we allow
# additional non-zero coefficients.
# See Algorithm 5.5 of the article StdFFCyc.
InstallGlobalFunction(StandardIrreducibleCoeffList, function(K, r, a)
local l, q, count, inc, d, st, k, qq;
# l is coefficient list, monic and constant coeff a
# first polynomial to try is X^r + X + a
l := NullMat(1, r+1, K)[1];
l[r+1] := One(K);
l[1] := a;
l[2] := One(K);
q := Size(K);
# inc is the expected number on non-zero coeffs
inc := 1;
while q^inc < 2*r do
inc := inc+1;
od;
# we allow non-zero coeffs up to position d
d := 0;
count := 0;
while not IsIrreducibleCoeffList(l, q) do
# after every r attempts allow inc more non-zero coeffs
Info(InfoStandardFF, 2, "reducible ",count,":",List(l, IntFFE),"\n");
if count mod r = 0 and d < r-1 then
d := d + inc;
if d >= r then
d := r-1;
fi;
qq := q^(d-1);
Info(InfoStandardFF, 2, "(d=",d,")\c");
fi;
st := StandardAffineShift(qq, count);
st := CoefficientsQadic(st, q);
while Length(st) < d-1 do
Add(st, 0);
od;
for k in [2..d] do
l[k] := ElementSteinitzNumber(K, st[k-1]);
od;
Info(InfoStandardFF, 3, "*\c");
count := count+1;
od;
Info(InfoStandardFF, 2, "found ",count,":",List(l, IntFFE),"\n");
return l;
end);
# we leave this undocumented, seems to be interesting only for
# quite large degrees?
isirrNTL := function(cs, K)
local d, q, prg, inp, res, p, prcoeffs, c, x;
d:=DirectoriesPackageLibrary("StandardFF","ntl");
q := Size(K);
if IsPrimeInt(q) then
prg := Filename(d, "isirrGFp");
if prg = fail then
return fail;
fi;
inp := "";
Append(inp, String(q));
Append(inp, "\n[\n");
Append(inp, JoinStringsWithSeparator(List(cs, a-> String(IntFFE(a))), "\n"));
Append(inp, "\n]\n");
res := IO_PipeThrough(prg,[],inp);
if res = "true\n" then
return true;
else
return false;
fi;
else
prg := Filename(d, "isirrGFq");
if prg = fail then
return fail;
fi;
p := SmallestRootInt(q);
if not Size(LeftActingDomain(K)) = p then
return fail;
fi;
inp := "";
Append(inp, String(p));
Add(inp, '\n');
prcoeffs := function(c)
local a;
Append(inp, "[\n");
for a in c do
Append(inp, String(IntFFE(a)));
Add(inp, '\n');
od;
Append(inp, "]\n");
end;
c := CoefficientsOfUnivariatePolynomial(DefiningPolynomial(K));
prcoeffs(c);
Append(inp,"[\n");
for x in cs do
c := ShallowCopy(x![1]);
if not IsList(c) then
c := [c];
fi;
ShrinkRowVector(c);
prcoeffs(c);
od;
Append(inp,"]\n");
res := IO_PipeThrough(prg,[],inp);
if res = "true\n" then
return true;
else
return false;
fi;
fi;
end;
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