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############################################################################
##
## elements/ffmat.gi
## Copyright (C) 2016-2022 James D. Mitchell
## Markus Pfeiffer
##
## Licensing information can be found in the README file of this package.
##
#############################################################################
##
#############################################################################
# This file contains the implementations of the operations etc declared in
# ffmat.gd for matrix objects over finite fields.
#############################################################################
#############################################################################
# The file is organised as follows:
#
# 1. Random matrices
# 2. Rows bases etc
# 3. Attributes etc for IsMatrixObjOverFiniteField
# 4. Helper functions
#############################################################################
# The following are only used in this file and so are declared here in the gi
# file.
BindGlobal("PlistRowBasisOverFiniteFieldFamily",
NewFamily("PlistRowBasisOverFiniteFieldFamily",
IsRowBasisOverFiniteField, CanEasilyCompareElements));
BindGlobal("PlistRowBasisOverFiniteFieldType",
NewType(PlistRowBasisOverFiniteFieldFamily,
IsRowBasisOverFiniteField
and IsPlistRowBasisOverFiniteFieldRep));
InstallMethod(IsMatrixObjOverFiniteField, "for a matrix obj",
[IsMultiplicativeElement],
function(m)
return IsMatrixObj(m)
and IsField(BaseDomain(m))
and IsFinite(BaseDomain(m))
and NrRows(m) = NrCols(m);
end);
InstallMethod(OneMutable, "for an FFE coll coll coll",
[IsFFECollCollColl], coll -> One(Representative(coll)));
###############################################################################
# 1. Random matrices
###############################################################################
# Note that: the following methods implement the RandomMatrix interface for
# IsMatrixOverSemiring in the Semigroups package and do not related to anything
# similar in the GAP library for IsMatrixObj (if they exist).
InstallMethod(RandomMatrixOp, "for a finite field and pos int",
[IsField and IsFinite, IsPosInt],
function(field, n)
local xy, i, j;
xy := List([1 .. n], x -> EmptyPlist(n));
for i in [1 .. n] do
for j in [1 .. n] do
xy[i][j] := Random(field);
od;
od;
return Matrix(field, xy);
end);
InstallMethod(RandomMatrixOp,
"for a finite field, dimension, and list of ranks",
[IsField and IsFinite, IsPosInt, IsList],
function(R, n, ranks)
local z, rk, mat, zv, conj, j;
if ForAny(ranks, x -> (x < 0) or (x > n)) then
ErrorNoReturn("the list of ranks has to consist of numbers > 0 and < n");
fi;
z := Zero(R);
# Choose a matrix of given rank
rk := Random(ranks);
if rk = 0 then
return ZeroMatrix(R, n, n);
fi;
mat := Unpack(Random(GL(rk, R)));
# Extend it to n x n
zv := [1 .. n - rk] * z;
for j in [1 .. rk] do
Append(mat[j], zv);
od;
zv := [1 .. n] * z;
for j in [1 .. n - rk] do
Add(mat, zv);
od;
# Swirl around
# Is Permuting rows/columns enough?
conj := Random(GL(n, R)); # PermutationMat(Random(Sym(n)), n, R);
return Matrix(R, mat ^ conj);
end);
InstallMethod(RandomMatrixOp,
"for a finite field, dimension, and pos int",
[IsField and IsFinite, IsPosInt, IsPosInt],
{R, n, rank} -> RandomMatrixOp(R, n, [rank]));
#############################################################################
# 2. Rows bases etc
#############################################################################
InstallMethod(NewRowBasisOverFiniteField,
"for IsPlistRowBasisOverFiniteFieldRep, a ring, and a list",
[IsPlistRowBasisOverFiniteFieldRep, IsRing, IsList],
function(_, basedomain, l)
local b;
b := Objectify(PlistRowBasisOverFiniteFieldType, rec(rows := l));
SetBaseDomain(b, basedomain);
return b;
end);
InstallMethod(Rank, "for a plist rowbasis",
[IsPlistRowBasisOverFiniteFieldRep],
v -> Length(v!.rows));
InstallMethod(\=, "for an rowbasis",
[IsPlistRowBasisOverFiniteFieldRep, IsPlistRowBasisOverFiniteFieldRep],
{x, y} -> BaseDomain(x) = BaseDomain(y) and x!.rows = y!.rows);
InstallMethod(\<, "for an rowbasis",
[IsPlistRowBasisOverFiniteFieldRep, IsPlistRowBasisOverFiniteFieldRep],
{x, y} -> Rank(x) < Rank(y) or (Rank(x) = Rank(y) and (x!.rows < y!.rows)));
InstallMethod(ViewString, "for a plist rowbasis",
[IsPlistRowBasisOverFiniteFieldRep],
function(v)
return STRINGIFY("<rowbasis of rank ",
Rank(v),
" over ",
BaseDomain(v),
">");
end);
InstallMethod(String, "for a plist rowbasis",
[IsPlistRowBasisOverFiniteFieldRep],
function(v)
return STRINGIFY("NewRowBasisOverFiniteField(",
"IsPlistRowBasisOverFiniteFieldRep, ",
BaseDomain(v),
", ",
v!.rows, ")");
end);
SEMIGROUPS.HashFunctionForPlistRowBasisOverFiniteField := function(x, data)
if Rank(x) = 0 then
return 1;
fi;
Assert(1, IsRecord(data));
return data.func(x!.rows, data.data);
end;
InstallMethod(ChooseHashFunction, "for plist row basis over finite field",
[IsPlistRowBasisOverFiniteFieldRep, IsInt],
function(x, hashlen)
local data;
if Rank(x) <> 0 then
data := ChooseHashFunction(x!.rows, hashlen);
else
data := hashlen;
fi;
return rec(func := SEMIGROUPS.HashFunctionForPlistRowBasisOverFiniteField,
data := data);
end);
#############################################################################
# 3. Attributes etc for IsMatrixObjOverFiniteField
#############################################################################
InstallMethod(RowSpaceBasis, "for a matrix obj over finite field",
[IsMatrixObj],
function(m)
if not IsMatrixObjOverFiniteField(m) then
TryNextMethod();
fi;
return ComputeRowSpaceAndTransformation(m)[1];
end);
InstallMethod(RowSpaceTransformation, "for a matrix obj over finite field",
[IsMatrixObj],
function(m)
if not IsMatrixObjOverFiniteField(m) then
TryNextMethod();
fi;
return ComputeRowSpaceAndTransformation(m)[2];
end);
InstallMethod(RowSpaceTransformationInv, "for a matrix obj over finite field",
[IsMatrixObj],
function(m)
if not IsMatrixObjOverFiniteField(m) then
TryNextMethod();
fi;
return ComputeRowSpaceAndTransformation(m)[3];
end);
# Should this go in a helper function, it also works similarly to the thing
# done below.
InstallMethod(RightInverse, "for a matrix obj over finite field",
[IsMatrixObj],
function(m)
local deg, u, rsp, zv, se, i;
if not IsMatrixObjOverFiniteField(m) then
TryNextMethod();
fi;
deg := NrRows(m);
u := One(BaseDomain(m));
rsp := Unpack(m);
zv := [1 .. deg] * Zero(BaseDomain(m));
for i in [1 .. deg] do
Append(rsp[i], zv);
rsp[i][deg + i] := u;
od;
se := SemiEchelonMat(rsp);
for i in [1 .. Length(se.vectors)] do
rsp[i] := ShallowCopy(se.vectors[i]);
od;
for i in [1 .. deg] do
if se.heads[i] = 0 then
rsp[i][i] := u;
rsp[i][deg + i] := Zero(BaseDomain(m));
fi;
od;
TriangulizeMat(rsp);
return Matrix(rsp{[1 .. deg]}{[deg + 1 .. 2 * deg]}, m);
end);
InstallMethod(LeftInverse, "for a matrix obj over finite field",
[IsMatrixObj],
function(m)
if not IsMatrixObjOverFiniteField(m) then
TryNextMethod();
fi;
return TransposedMat(RightInverse(TransposedMat(m)));
end);
#############################################################################
# 4. Helper functions
#############################################################################
InstallGlobalFunction(ComputeRowSpaceAndTransformation,
function(m)
local deg, bd, bas, tr, tri, sinv, rsp, zv, heads, tm, i;
Assert(1, IsMatrixObjOverFiniteField(m));
deg := NrRows(m);
bd := BaseDomain(m);
if IsZero(m) then
bas := [];
tr := IdentityMat(deg, bd);
tri := tr;
sinv := fail;
else
rsp := Unpack(m);
zv := [1 .. deg] * Zero(bd);
for i in [1 .. deg] do
Append(rsp[i], ShallowCopy(zv));
rsp[i][deg + i] := One(bd);
od;
TriangulizeMat(rsp);
heads := [];
bas := rsp{[1 .. deg]}{[1 .. deg]};
for i in [deg, deg - 1 .. 1] do
if IsZero(bas[i]) then
Remove(bas, i);
else
heads[PositionNonZero(bas[i])] := i;
fi;
od;
# Check whether this matrix has a semigroup inverse, i.e. a matrix t such
# that t * m * t = t and m * t * m = m. If it does this matrix is the
# transformation we computed otherwise we set fail
tm := TransposedMat(bas);
sinv := true;
for i in [1 .. deg] do
if not IsBound(heads[i]) then
if not IsZero(tm[i]) then
sinv := fail;
fi;
fi;
od;
# This is obviously totally ridiculous to do the same computation twice
if sinv = true then
sinv := RightInverse(m);
fi;
tr := rsp{[1 .. deg]}{[deg + 1 .. 2 * deg]};
tri := tr ^ (-1);
fi;
ConvertToVectorRep(bas);
MakeImmutable(bas);
bas := NewRowBasisOverFiniteField(IsPlistRowBasisOverFiniteFieldRep, bd, bas);
return [bas, tr, tri];
end);
[ Dauer der Verarbeitung: 0.28 Sekunden
(vorverarbeitet)
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