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# SPDX-License-Identifier: GPL-2.0-or-later
# MatricesForHomalg: Matrices for the homalg project
#
# Implementations
#
## Efficiency is not among the two purposes served by the implementation below:
## 1. Prove that Euclidean rings are computable in the sense of Barakat and Lange-Hegermann.
## A practical purpose of this proof is to make univariate polynomial rings over fields
## computable in GAP (i.e., without relying on external oracles).
## 2. Demonstrate a unified implementation of the Row (Reduced) Echelon Form (REF, RREF)
## and the (reduced) Hermite normal form algorithms.
##
InstallMethod( FullyDividePairTrafo,
[ IsRingElement, IsRingElement, IsHomalgRing ],
function( a, b, R )
local U, q, r;
U := HomalgIdentityMatrix( 2, R );
while not IsZero( b ) do
q := EuclideanQuotient( R, a, b );
U := HomalgMatrix( [ 0, 1, 1, -q ], 2, 2, R ) * U;
r := a - q * b;
a := b;
b := r;
od;
return U;
end );
##
InstallMethod( FullyDividePairTrafo,
[ IsRingElement, IsRingElement, IsHomalgRing and IsFieldForHomalg ],
function( a, b, R )
local U, q, r;
U := HomalgIdentityMatrix( 2, R );
while not IsZero( b ) do
q := EuclideanQuotient( R, a, b );
U := HomalgMatrix( [ 0, 1, 1, -q ], 2, 2, R ) * U;
r := a - q * b;
a := b;
b := r;
od;
return U;
end );
##
InstallMethod( FullyDividePairTrafoInflated,
[ IsRingElement, IsRingElement, IsInt, IsInt, IsInt, IsHomalgRing ],
function( a, b, i, j, n, R )
local U, u;
U := HomalgInitialIdentityMatrix( n, R );
u := FullyDividePairTrafo( a, b, R );
SetMatElm( U, i, i, MatElm( u, 1, 1 ) );
SetMatElm( U, i, j, MatElm( u, 1, 2 ) );
SetMatElm( U, j, i, MatElm( u, 2, 1 ) );
SetMatElm( U, j, j, MatElm( u, 2, 2 ) );
MakeImmutable( U );
return U;
end );
##
InstallMethod( FullyDivideColumnTrafo,
[ IsHomalgMatrix ],
function( col )
local R, m, U, nz, j, a, b, u;
R := HomalgRing( col );
m := NumberRows( col );
U := HomalgIdentityMatrix( m, R );
if IsZero( col ) then ## also takes care of 0x1 matrix
return U;
elif m = 1 then ## takes care of the 1x1 nonzero case
if HasIsFieldForHomalg( R ) and IsFieldForHomalg( R ) then
return HomalgMatrix( [ MatElm( col, 1, 1 )^-1 ], 1, 1, R );
fi;
return U;
fi;
## now we know that col is not zero with at least two rows
nz := NonZeroRows( col );
nz := ShallowCopy( nz );
## remove 1 from NonZeroRows( col )
if nz[1] = 1 then
## this would be enough for the ring case
Remove( nz, 1 );
## ensure normalizing the first column entry in the field case
if nz = [ ] and HasIsFieldForHomalg( R ) and IsFieldForHomalg( R ) then
nz := [ 2 ];
fi;
fi;
for j in nz do
a := MatElm( col, 1, 1 );
b := MatElm( col, j, 1 );
u := FullyDividePairTrafoInflated( a, b, 1, j, m, R );
col := u * col;
U := u * U;
od;
return U;
end );
##
InstallMethod( FullyDivideMatrixTrafo,
[ IsHomalgMatrix ],
function( mat )
local R, U, NZC, i, u;
R := HomalgRing( mat );
U := HomalgIdentityMatrix( NumberRows( mat ), R );
NZC := NonZeroColumns( mat );
i := 0;
while not NZC = [ ] do
u := FullyDivideColumnTrafo( CertainColumns( mat, [ NZC[1] ] ) );
mat := u * mat;
u := DiagMat( [ HomalgIdentityMatrix( i, R ), u ] );
U := u * U;
mat := CertainRows( mat, [ 2 .. NumberRows( mat ) ] );
i := i + 1;
NZC := NonZeroColumns( mat );
od;
return U;
end );
##
InstallMethod( DivideColumnTrafo,
[ IsHomalgMatrix, IsInt ],
function( col, r )
local R, m, U, nz, b, i, a, u, q;
R := HomalgRing( col );
m := NumberRows( col );
U := HomalgIdentityMatrix( m, R );
if m <= 1 then
return U;
fi;
## now we know that col is not zero with at least two rows
nz := NonZeroRows( col );
nz := Intersection( nz, [ 1 .. r - 1 ] );
b := MatElm( col, r, 1 );
for i in nz do
a := MatElm( col, i, 1 );
q := EuclideanQuotient( R, a, b );
u := HomalgInitialIdentityMatrix( m, R );
SetMatElm( u, i, r, -q );
col := u * col;
U := u * U;
od;
return U;
end );
##
InstallMethod( StrictlyFullyDivideMatrixTrafo,
[ IsHomalgMatrix ],
function( mat )
local R, U, steps, r, u;
R := HomalgRing( mat );
U := FullyDivideMatrixTrafo( mat );
mat := U * mat;
steps := PositionOfFirstNonZeroEntryPerRow( CertainRows( mat, NonZeroRows( mat ) ) );
mat := CertainColumns( mat, steps );
for r in [ 1 .. Length( steps ) ] do
u := DivideColumnTrafo( CertainColumns( mat, [ r ] ), r );
mat := u * mat;
U := u * U;
od;
return U;
end );
##
InstallMethod( CreateHomalgTable,
"for Euclidean rings",
[ IsEuclideanRing ],
function( ring )
local RP_General, RP, component;
if IsField( ring ) or ( HasIsFinite( ring ) and IsFinite( ring ) ) then ## leave for Gauss/GaussForHomalg
TryNextMethod( );
fi;
RP_General := ShallowCopy( CommonHomalgTableForRings );
RP := rec(
## Can optionally be provided by the RingPackage
## (homalg functions check if these functions are defined or not)
## (homalgTable gives no default value)
RowRankOfMatrix :=
function( M )
return Rank( Eval( M )!.matrix );
end,
ElementaryDivisors :=
function( arg )
local M;
M := arg[1];
return ElementaryDivisorsMat( ring, Eval( M )!.matrix );
end,
Gcd :=
function( a, b )
return Gcd( ring, [ a, b ] );
end,
CancelGcd :=
function( a, b )
local gcd;
gcd := Gcd( ring, [ a, b ] );
return [ a / gcd, b / gcd ];
end,
## Must be defined if other functions are not defined
ReducedRowEchelonForm :=
function( arg )
local M, nargs, U, H;
M := arg[1];
nargs := Length( arg );
U := StrictlyFullyDivideMatrixTrafo( M );
H := U * M;
SetIsUpperStairCaseMatrix( H, true );
if nargs > 1 and IsHomalgMatrix( arg[2] ) then ## not ReducedRowEchelonForm( M, "" )
## compute H and U
# assign U:
SetPreEval( arg[2], U );
U := arg[2];
ResetFilterObj( U, IsVoidMatrix );
SetNumberRows( U, NumberRows( M ) );
SetNumberColumns( U, NumberRows( M ) );
SetIsInvertibleMatrix( U, true );
fi;
return H;
end
);
for component in NamesOfComponents( RP_General ) do
RP.(component) := RP_General.(component);
od;
Objectify( TheTypeHomalgTable, RP );
return RP;
end );
