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| Current File : //usr/share/gap/lib/ring.gi |
#############################################################################
##
#W ring.gi GAP library Thomas Breuer
##
##
#Y Copyright 1997, Lehrstuhl D für Mathematik, RWTH Aachen, Germany
#Y (C) 1998 School Math and Comp. Sci., University of St Andrews, Scotland
#Y Copyright (C) 2002 The GAP Group
##
## This file contains generic methods for rings.
##
#############################################################################
##
#M PrintObj( <R> ) . . . . . . . . . . . . . . . . . . . . . . print a ring
##
InstallMethod( PrintObj,
"for a ring",
true,
[ IsRing ], 0,
function( R )
Print( "Ring( ... )" );
end );
InstallMethod( PrintObj,
"for a ring with generators",
true,
[ IsRing and HasGeneratorsOfRing ], 0,
function( R )
Print( "Ring( ", GeneratorsOfRing( R ), " )" );
end );
#############################################################################
##
#M PrintObj( <R> ) . . . . . . . . . . . . . . . . . . print a ring-with-one
##
InstallMethod( PrintObj,
"for a ring-with-one",
true,
[ IsRingWithOne ], 0,
function( R )
Print( "RingWithOne( ... )" );
end );
InstallMethod( PrintObj,
"for a ring-with-one with generators",
true,
[ IsRingWithOne and HasGeneratorsOfRingWithOne ], 0,
function( R )
Print( "RingWithOne( ", GeneratorsOfRingWithOne( R ), " )" );
end );
#############################################################################
##
#M ViewObj( <R> ) . . . . . . . . . . . . . . . . . . . . . . . view a ring
##
InstallMethod( ViewObj,
"for a ring",
true,
[ IsRing ], 0,
function( R )
Print( "<ring>" );
end );
InstallMethod( ViewObj,
"for a ring with known generators",
true,
[ IsRing and HasGeneratorsOfRing ], 0,
function( R )
Print( "<ring with ", Length( GeneratorsOfRing( R ) ),
" generators>" );
end );
#############################################################################
##
#M ViewObj( <R> ) . . . . . . . . . . . . . . . . . . view a ring-with-one
##
InstallMethod( ViewObj,
"for a ring-with-one",
true,
[ IsRingWithOne ], 0,
function( R )
Print( "<ring-with-one>" );
end );
InstallMethod( ViewObj,
"for a ring-with-one with known generators",
true,
[ IsRingWithOne and HasGeneratorsOfRingWithOne ], 0,
function( R )
Print( "<ring-with-one, with ", Length( GeneratorsOfRingWithOne( R ) ),
" generators>" );
end );
#############################################################################
##
#M IsAnticommutative( <R> ) . . . . . . . . . for a ring in characteristic 2
##
InstallImmediateMethod( IsAnticommutative,
IsRing and IsCommutative and HasCharacteristic, 0,
function( R )
if Characteristic( R ) <> 2 then
TryNextMethod();
fi;
return true;
end );
#############################################################################
##
#M IsAnticommutative( <R> ) . . . . . test whether a ring is anticommutative
##
InstallMethod( IsAnticommutative,
"generic method for rings",
true,
[ IsRing ], 0,
function( R )
local elms, # list of elements
i, j; # loop variables
# Test if every element anti-commutes with all the others.
elms:= Enumerator( R );
for i in [ 2 .. Length( elms) ] do
for j in [ 1 .. i-1 ] do
if elms[i] * elms[j] <> AdditiveInverse( elms[j] * elms[i] ) then
return false;
fi;
od;
od;
# All elements anti-commute.
return true;
end );
#############################################################################
##
#M IsZeroSquaredRing(<R>) . . . . for anticomm. ring in odd characteristic
##
## In odd characteristic, any anticommutative ring is zero squared.
##
InstallImmediateMethod( IsZeroSquaredRing,
IsRing and IsAnticommutative and HasCharacteristic, 0,
function( R )
if Characteristic( R ) = 2 then
TryNextMethod();
fi;
return true;
end );
#############################################################################
##
#M IsZeroSquaredRing(<R>) . test whether the square of each element is zero
##
InstallMethod( IsZeroSquaredRing,
"for a ring",
true,
[ IsRing ], 0,
function( R )
local zero;
zero:= Zero( R );
return ForAll( R, r -> r^2 = zero );
end );
#############################################################################
##
#M IsZeroMultiplicationRing(<R>)
##
InstallImmediateMethod( IsZeroMultiplicationRing,
IsRingWithOne and HasIsTrivial, 0,
IsTrivial );
#############################################################################
##
#M IsCentral( <R>, <U> ) . . . . . . . . test if <U> is centralized by <R>
##
## For associative rings, we have to check $u a = a u$ only for ring
## generators $a$ and $u$ of $A$ and $U$, respectively.
##
InstallMethod( IsCentral,
"for two associative rings",
IsIdenticalObj,
[ IsRing and IsAssociative, IsRing and IsAssociative ],
IsCentralFromGenerators( GeneratorsOfRing, GeneratorsOfRing ) );
InstallMethod( IsCentral,
"for two associative rings-with-one",
IsIdenticalObj,
[ IsRingWithOne and IsAssociative,
IsRingWithOne and IsAssociative ],
IsCentralFromGenerators( GeneratorsOfRingWithOne,
GeneratorsOfRingWithOne ) );
#############################################################################
##
#M IsCentral( <R>, <x> ) . . . . . . . . test if <x> is centralized by <R>
##
InstallMethod( IsCentral,
"for an associative ring and an element",
IsCollsElms,
[ IsRing and IsAssociative, IsObject ],
IsCentralElementFromGenerators( GeneratorsOfRing ) );
InstallMethod( IsCentral,
"for an associative ring-with-one and an element",
IsCollsElms,
[ IsRingWithOne and IsAssociative, IsObject ],
IsCentralElementFromGenerators( GeneratorsOfRingWithOne ) );
#############################################################################
##
#M IsCommutative( <R> ) . . . . . . . . check whether a ring is commutative
##
## In characteristic 2, commutativity is the same as anticommutativity.
##
## If <R> is associative then we can restrict the check to ring generators.
##
InstallImmediateMethod( IsCommutative,
IsRing and IsAnticommutative and HasCharacteristic, 0,
function( R )
if Characteristic( R ) <> 2 then
TryNextMethod();
fi;
return true;
end );
InstallMethod( IsCommutative,
"for an associative ring",
true,
[ IsRing and IsAssociative ], 0,
IsCommutativeFromGenerators( GeneratorsOfRing ) );
InstallMethod( IsCommutative,
"for an associative ring-with-one",
true,
[ IsRingWithOne and IsAssociative ], 0,
IsCommutativeFromGenerators( GeneratorsOfRingWithOne ) );
#############################################################################
##
#M GeneratorsOfRing( <R> )
#M GeneratorsOfRingWithOne( <R> )
##
## `GeneratorsOfMagma' of a ring
## are also `GeneratorsOfRing'.
## `GeneratorsOfAdditiveMagmaWithInverses' of a ring
## are also `GeneratorsOfRing'.
##
## `GeneratorsOfMagmaWithOne' of a ring
## are also `GeneratorsOfRingWithOne'.
## `GeneratorsOfRing' of a ring-with-one
## are also `GeneratorsOfRingWithOne'.
##
InstallImmediateMethod( GeneratorsOfRing,
IsRing and HasGeneratorsOfMagma and IsAttributeStoringRep, 0,
GeneratorsOfMagma );
InstallImmediateMethod( GeneratorsOfRing,
IsRing and HasGeneratorsOfAdditiveMagmaWithInverses
and IsAttributeStoringRep, 0,
GeneratorsOfAdditiveMagmaWithInverses );
InstallImmediateMethod( GeneratorsOfRingWithOne,
IsRingWithOne and HasGeneratorsOfMagmaWithOne
and IsAttributeStoringRep, 0,
GeneratorsOfMagmaWithOne );
InstallImmediateMethod( GeneratorsOfRingWithOne,
IsRingWithOne and HasGeneratorsOfRing and IsAttributeStoringRep, 0,
GeneratorsOfRing );
InstallMethod( GeneratorsOfRing,
"for a ring",
true,
[ IsRing ], 0,
GeneratorsOfMagma );
InstallMethod( GeneratorsOfRing,
"for a ring-with-one with generators",
true,
[ IsRingWithOne and HasGeneratorsOfRingWithOne ], 0,
R -> Concatenation( GeneratorsOfRingWithOne( R ), [ One(R) ] ) );
#############################################################################
##
#M Representative( <R> ) . . . . . . . . . . . . . . . one element of a ring
##
InstallMethod( Representative,
"for a ring with generators",
true,
[ IsRing and HasGeneratorsOfRing ], 0,
RepresentativeFromGenerators( GeneratorsOfRing ) );
InstallMethod( Representative,
"for a ring-with-one with generators",
true,
[ IsRingWithOne and HasGeneratorsOfRingWithOne ], 0,
RepresentativeFromGenerators( GeneratorsOfRingWithOne ) );
#############################################################################
##
#M InterpolatedPolynomial( <R>, <x>, <y> ) . . . . . . . . . . interpolation
##
InstallOtherMethod( InterpolatedPolynomial, true,
[ IsRing, IsObject, IsObject ], 0,
function( R, x, y )
local a, t, k, i, p;
a := [];
t := ShallowCopy(y);
for i in [ 1 .. Length(x) ] do
for k in [ i-1, i-2 .. 1 ] do
t[k] := ( t[k+1] - t[k] ) / ( x[i] - x[k] );
od;
a[i] := t[1];
od;
p := a[Length(x)];
for i in [ Length(x)-1, Length(x)-2 .. 1 ] do
p := p * (Indeterminate(R)-x[i]) + a[i];
od;
return p;
end );
#############################################################################
##
#M RingByGenerators( <gens> ) . . . . . . . ring generated by a collection
##
InstallMethod( RingByGenerators,
"for a collection",
true,
[ IsCollection ], 0,
function( gens )
local R;
R:= Objectify( NewType( FamilyObj( gens ),
IsRing and IsAttributeStoringRep ),
rec() );
SetGeneratorsOfRing( R, gens );
return R;
end );
#############################################################################
##
#M DefaultRingByGenerators( <gens> ) . . . . ring containing a collection
##
InstallMethod( DefaultRingByGenerators,
"for a collection",
true,
[ IsCollection ], 0,
RingByGenerators );
#############################################################################
##
#F Ring( <z>, ... ) . . . . . . . . . . . . . ring generated by a collection
#F Ring( [ <z>, ... ] ) . . . . . . . . . . . ring generated by a collection
##
InstallGlobalFunction( Ring, function ( arg )
local R; # ring containing the elements of <arg>, result
# special case for one square matrix
if Length(arg) = 1
and IsMatrix( arg[1] ) and Length( arg[1] ) = Length( arg[1][1] )
then
R := RingByGenerators( arg );
# special case for list of elements
elif Length(arg) = 1 and IsList(arg[1]) then
R := RingByGenerators( arg[1] );
# other cases
else
R := RingByGenerators( arg );
fi;
# return the ring
return R;
end );
#############################################################################
##
#F DefaultRing( <z>, ... ) . . . . . . default ring containing a collection
#F DefaultRing( [ <z>, ... ] ) . . . . default ring containing a collection
##
InstallGlobalFunction( DefaultRing, function ( arg )
local R; # ring containing the elements of <arg>, result
# special case for one square matrix
if Length(arg) = 1
and IsMatrix( arg[1] ) and Length( arg[1] ) = Length( arg[1][1] )
then
R := DefaultRingByGenerators( arg );
# special case for list of elements
elif Length(arg) = 1 and IsList(arg[1]) then
R := DefaultRingByGenerators( arg[1] );
# other cases
else
R := DefaultRingByGenerators( arg );
fi;
# return the default ring
return R;
end );
#############################################################################
##
#F Subring( <R>, <gens> ) . . . . . . . . subring of <R> generated by <gens>
#F SubringNC( <R>, <gens> )
##
InstallGlobalFunction( Subring, function( R, gens )
local S;
if IsEmpty( gens ) then
Error( "<gens> must be a nonempty list" );
elif IsHomogeneousList( gens )
and IsIdenticalObj( FamilyObj(R), FamilyObj( gens ) )
and ForAll( gens, g -> g in R ) then
S:= RingByGenerators( gens );
SetParent( S, R );
return S;
fi;
Error( "<gens> must be a list of elements in <R>" );
end );
InstallGlobalFunction( SubringNC, function( R, gens )
local S;
S:= RingByGenerators( gens );
SetParent( S, R );
return S;
end );
#############################################################################
##
#F SubringWithOne( <R>, <gens> ) . . . . subring of <R> generated by <gens>
#F SubringWithOneNC( <R>, <gens> )
##
InstallGlobalFunction( SubringWithOne, function( R, gens )
local S;
if IsEmpty( gens ) then
S:= RingWithOneByGenerators( [ One( R ) ] );
SetParent( S, R );
return S;
elif IsHomogeneousList( gens )
and IsIdenticalObj( FamilyObj(R), FamilyObj( gens ) )
and ForAll( gens, g -> g in R ) then
S:= RingWithOneByGenerators( gens );
SetParent( S, R );
return S;
fi;
Error( "<gens> must be a list of elements in <R>" );
end );
InstallGlobalFunction( SubringWithOneNC, function( R, gens )
local S;
if IsEmpty( gens ) then
S:= Objectify( NewType( FamilyObj( R ),
IsRingWithOne and IsAttributeStoringRep ),
rec() );
SetGeneratorsOfRingWithOne( S, AsList( gens ) );
else
S:= RingWithOneByGenerators( gens );
fi;
SetParent( S, R );
return S;
end );
#############################################################################
##
#M RingWithOneByGenerators( <gens> ) . . ring-with-one gen. by a collection
##
InstallMethod( RingWithOneByGenerators,
"for a collection",
true,
[ IsCollection ], 0,
function( gens )
local R;
R:= Objectify( NewType( FamilyObj( gens ),
IsRingWithOne and IsAttributeStoringRep ),
rec() );
SetGeneratorsOfRingWithOne( R, gens );
return R;
end );
#############################################################################
##
#F RingWithOne( <z>, ... ) . . . . . ring-with-one generated by a collection
#F RingWithOne( [ <z>, ... ] ) . . . ring-with-one generated by a collection
##
InstallGlobalFunction( RingWithOne, function ( arg )
local R; # ring containing the elements of <arg>, result
# special case for one square matrix
if Length(arg) = 1
and IsMatrix( arg[1] ) and Length( arg[1] ) = Length( arg[1][1] )
then
R := RingWithOneByGenerators( arg );
# special case for list of elements
elif Length(arg) = 1 and IsList(arg[1]) then
R := RingWithOneByGenerators( arg[1] );
# other cases
else
R := RingWithOneByGenerators( arg );
fi;
# return the ring
return R;
end );
#############################################################################
##
#M IsAssociated( <R>, <r>, <s> ) . test if two ring elements are associated
##
InstallMethod( IsAssociated,
"for ring and two ring elements",
IsCollsElmsElms,
[ IsRing, IsRingElement, IsRingElement ], 0,
function( R, r, s )
local q;
# if a list of the units is already known, use it
if HasUnits( R ) then
return ForAny( Units( R ), u -> r = u * s );
elif s = Zero( R ) then
return r = Zero( R );
# or check if the quotient is a unit
else
q:= Quotient( R, r, s );
#T allowed?
return q <> fail and IsUnit( R, q );
fi;
end );
#############################################################################
##
#M IsAssociated( <r>, <s> ) . . . test if two ring elements are associated
##
InstallOtherMethod( IsAssociated,
"for two ring elements",
IsIdenticalObj,
[ IsRingElement, IsRingElement ], 0,
function( r, s )
return IsAssociated( DefaultRingByGenerators( [ r, s ] ), r, s );
end );
#############################################################################
##
#M IsSubset( <R>, <S> ) . . . . . . . . . . . . . . . . . . . for two rings
##
InstallMethod( IsSubset,
"for two rings",
IsIdenticalObj,
[ IsRing, IsRing and HasGeneratorsOfRing ],
function( R, S )
return IsSubset( R, GeneratorsOfRing( S ) );
end );
#############################################################################
##
#M IsSubset( <R>, <S> ) . . . . . . . . . . . . . . for two rings-with-one
##
InstallMethod( IsSubset,
"for two rings-with-one",
IsIdenticalObj,
[ IsRing, IsRingWithOne and HasGeneratorsOfRingWithOne ],
function( R, S )
local gens;
gens:= GeneratorsOfRingWithOne( S );
return IsSubset( R, gens ) and
( ( IsRingWithOne( R ) and not IsEmpty( gens ) )
or One( S ) in R );
end );
#############################################################################
##
#M Enumerator( <R> ) . . . . . . . . . . . . . set of the elements of a ring
##
## We must be careful to call `GeneratorsOfRing' only if ring generators are
## known; if we have only ideal generators then a different `Enumerator'
## method is used.
##
EnumeratorOfRing := function( R )
local elms, # elements of <R>, result
set, # set corresponding to <elms>
gens, # ring generators of <R>
elm, # one element of <elms>
gen, # one generator of <R>
new; # product or sum of <elm> and <gen>
# check that we can handle this ring
if HasIsFinite( R ) and not IsFinite( R ) then
TryNextMethod();
# otherwise use an orbit like algorithm
else
elms := [ Zero( R ) ];
set := ShallowCopy( elms );
gens := GeneratorsOfRing( R );
for elm in elms do
for gen in gens do
new := elm + gen;
if not new in set then
Add( elms, new );
AddSet( set, new );
fi;
new := elm * gen;
if not new in set then
Add( elms, new );
AddSet( set, new );
fi;
new := gen * elm;
if not new in set then
Add( elms, new );
AddSet( set, new );
fi;
od;
od;
fi;
return set;
end;
InstallMethod( Enumerator,
"generic method for a ring with known generators",
true,
[ IsRing and HasGeneratorsOfRing ], 0,
EnumeratorOfRing );
InstallMethod( Enumerator,
"generic method for a ring-with-one with known generators",
true,
[ IsRingWithOne and HasGeneratorsOfRingWithOne ], 0,
EnumeratorOfRing );
#############################################################################
##
#M Size( <R> ) . . . . . . . . . . . . characteristic zero ring is infinite
##
InstallMethod( Size,
"characteristic zero ring is infinite",
[ IsRing and HasGeneratorsOfRing and HasCharacteristic ],
function( R )
if Characteristic( R ) <> 0 then
TryNextMethod();
elif ForAll( GeneratorsOfRing( R ), IsZero ) then
return 1;
fi;
return infinity;
end );
#############################################################################
##
#M IsIntegralRing( <D> ) . . . . . . . . test if a ring is an integral ring
##
InstallMethod( IsIntegralRing,
"for a ring",
true,
[ IsRing ], 0,
function ( R )
local elms, zero, i, k;
if IsFinite( R ) then
if IsTrivial( R ) or not IsCommutative( R ) then
return false;
fi;
elms := Enumerator( R );
zero := Zero( R );
for i in [1..Length(elms)] do
if elms[i] = zero then continue; fi;
for k in [i+1..Length(elms)] do
if elms[k] = zero then continue; fi;
if elms[i] * elms[k] = zero then
return false;
fi;
od;
od;
return true;
else
TryNextMethod();
fi;
end );
#############################################################################
##
#M ClosureRing( <R>, <r> ) . . . . . . . . . . . . . closure with an element
##
InstallMethod( ClosureRing,
"for a ring and a ring element",
IsCollsElms,
[ IsRing, IsRingElement ], 0,
function( R, r )
# if possible test if the element lies in the ring already,
if HasGeneratorsOfRing( R )
and r in GeneratorsOfRing( R ) then
return R;
# otherwise make a new ring
else
return Ring( Concatenation( GeneratorsOfRing( R ), [ r ] ) );
fi;
end );
InstallMethod( ClosureRing,
"for a ring-with-one and a ring element",
IsCollsElms,
[ IsRingWithOne, IsRingElement ], 0,
function( R, r )
# if possible test if the element lies in the ring already,
if HasGeneratorsOfRingWithOne( R )
and r in GeneratorsOfRingWithOne( R ) then
return R;
# otherwise make a new ring-with-one
else
return RingWithOne( Concatenation( GeneratorsOfRingWithOne( R ),
[ r ] ) );
fi;
end );
InstallMethod( ClosureRing,
"for a ring containing the whole family, and a ring element",
IsCollsElms,
[ IsRing and IsWholeFamily, IsRingElement ],
SUM_FLAGS, # can't do better
function( R, r )
return R;
end );
#############################################################################
##
#M ClosureRing( <R>, <S> ) . . . . . . . . . . . . . . closure of two rings
##
InstallMethod( ClosureRing,
"for two rings",
IsIdenticalObj,
[ IsRing, IsRing ], 0,
function( R, S )
local r; # one generator
for r in GeneratorsOfRing( S ) do
R := ClosureRing( R, r );
od;
return R;
end );
InstallMethod( ClosureRing,
"for two rings-with-one",
IsIdenticalObj,
[ IsRingWithOne, IsRingWithOne ], 0,
function( R, S )
local r; # one generator
for r in GeneratorsOfRingWithOne( S ) do
R := ClosureRing( R, r );
od;
return R;
end );
InstallMethod( ClosureRing,
"for a ring cont. the whole family, and a collection",
IsIdenticalObj,
[ IsRing and IsWholeFamily, IsCollection ],
SUM_FLAGS, # can't do better
function( R, S )
return R;
end );
#############################################################################
##
#M ClosureRing( <R>, <C> ) . . . . . . . . . . . . . . . . . closure of ring
##
InstallMethod( ClosureRing,
"for ring and collection of elements",
IsIdenticalObj,
[ IsRing, IsCollection ], 0,
function( R, list )
local r; # one generator
for r in list do
R:= ClosureRing( R, r );
od;
return R;
end );
#############################################################################
##
#M Quotient( <r>, <s> ) . . . . . . . . . . . delegate to the default ring
##
InstallOtherMethod( Quotient,
"for two ring elements (delegate to three argument version",
IsIdenticalObj,
[ IsRingElement, IsRingElement ], 0,
function( r, s )
return Quotient( DefaultRing( [ r, s ] ), r, s );
end );
InstallMethod( Quotient,
"for a ring and two ring elements",
IsCollsElmsElms,
[ IsRing, IsRingElement, IsRingElement ], 0,
function( R, r, s )
local quo;
quo:= Inverse( s );
if quo <> fail then
quo:= r * quo;
if not quo in R then
quo:= fail;
fi;
fi;
return quo;
end );
#############################################################################
##
#M IsUnit( <r> ) . . . . . . . . . . . . . . . delegate to the default ring
#M IsUnit( <R>, <r> ) . . . . . . . . . . . . test if an element is a unit
##
InstallOtherMethod( IsUnit,
"for a ring element",
true,
[ IsRingElement ], 0,
r -> IsUnit( DefaultRing( [ r ] ), r ) );
InstallMethod( IsUnit,
"for a ring with known units and a ring element",
IsCollsElms,
[ IsRing and HasUnits, IsRingElement ], 0,
function ( R, r )
return r in Units( R );
end );
InstallMethod( IsUnit,
"for a ring and a ring element",
IsCollsElms,
[ IsRing, IsRingElement ], 0,
function ( R, r )
local one;
one:= One( R );
if one = fail then
return false;
else
# simply try to compute the inverse
return r <> Zero( R ) and Quotient( R, one, r ) <> fail;
#T allowed?
fi;
end );
#############################################################################
##
#M Units( <R> ) . . . . . . . . . . . . . . . . . . . . . . units of a ring
##
InstallMethod( Units,
"for a (finite) ring",
true,
[ IsRing ], 0,
function ( R )
local one,
units,
elm;
one:= One( R );
if one = fail then
return [];
elif not IsFinite( R ) then
TryNextMethod();
fi;
units:= GroupByGenerators( [], one );
for elm in Enumerator( R ) do
if IsUnit( R, elm ) and not elm in units then
units:= ClosureGroupDefault( units, elm );
fi;
od;
return units;
end );
#############################################################################
##
#M StandardAssociate( <r> )
##
InstallOtherMethod( StandardAssociate,
"for a ring element",
true,
[ IsRingElement ],
r -> StandardAssociate( DefaultRing( [ r ] ), r ) );
InstallMethod( StandardAssociate,
"for a ring and its zero element",
IsCollsElms,
[ IsRing, IsRingElement and IsZero ],
SUM_FLAGS, # can't do better
function ( R, r )
return r;
end );
InstallMethod( StandardAssociate,
"for a ring and a ring element (using StandardAssociateUnit)",
IsCollsElms,
[ IsRing, IsRingElement ],
function ( R, r )
local u;
u := StandardAssociateUnit( R, r );
if u <> fail then return u * r; fi;
TryNextMethod();
end );
#############################################################################
##
#M StandardAssociateUnit( <r> )
##
InstallOtherMethod( StandardAssociateUnit,
"for a ring element",
true,
[ IsRingElement ],
r -> StandardAssociateUnit( DefaultRing( [ r ] ), r ) );
InstallMethod( StandardAssociateUnit,
"for a ring and its zero element",
IsCollsElms,
[ IsRing, IsRingElement and IsZero ],
SUM_FLAGS, # can't do better
function ( R, r )
return One( R );
end );
#############################################################################
##
#M Associates( <r> ) . . . . . . . . . . . . . delegate to the default ring
#M Associates( <R>, <r> ) . . . . . . . . . . associates of a ring element
##
InstallOtherMethod( Associates,
"for a ring element",
true,
[ IsRingElement ], 0,
r -> Associates( DefaultRing( [ r ] ), r ) );
InstallMethod( Associates,
"for a ring and a ring element",
IsCollsElms,
[ IsRing, IsRingElement ], 0,
function( R, r )
return AsSSortedList( Enumerator( Units( R ) ) * r );
end );
#############################################################################
##
#M IsPrime( <r> ) . . . . . . . . . . . . . . delegate to the default ring
##
InstallOtherMethod( IsPrime,
"for a ring element",
true,
[ IsRingElement ], 0,
r -> IsPrime( DefaultRing( [ r ] ), r ) );
#############################################################################
##
#M IsIrreducibleRingElement( <r> ) . . . . . delegate to the default ring
##
InstallOtherMethod( IsIrreducibleRingElement,
"for a ring element",
true,
[ IsRingElement ], 0,
r -> IsIrreducibleRingElement( DefaultRing( [ r ] ), r ) );
#############################################################################
##
#M Factors( <r> ) . . . . . . . . . . . . . . delegate to the default ring
##
InstallOtherMethod( Factors,
"for a ring element",
true, [ IsRingElement ], 0,
r -> Factors( DefaultRing( [ r ] ), r ) );
#############################################################################
##
#M EuclideanDegree( <r> ) . . . . . . . . . . delegate to the default ring
##
InstallOtherMethod( EuclideanDegree,
"for a ring element",
true, [ IsRingElement ], 0,
r -> EuclideanDegree( DefaultRing( [ r ] ), r ) );
#############################################################################
##
#F EuclideanRemainder( <r>, <m> ) . . . . . . delegate to the default ring
#F EuclideanRemainder( <R>, <r>, <m> ) . . . . . . . . . euclidean remainder
##
InstallOtherMethod( EuclideanRemainder,
"for two ring elements",
IsIdenticalObj, [ IsRingElement, IsRingElement ], 0,
function( r, m )
return EuclideanRemainder( DefaultRing( [ r, m ] ), r, m );
end );
InstallMethod( EuclideanRemainder,
"for a Euclidean ring and two ring elements",
IsCollsElmsElms, [ IsEuclideanRing, IsRingElement, IsRingElement ], 0,
function( R, r, m )
return QuotientRemainder( R, r, m )[2];
end );
#############################################################################
##
#F EuclideanQuotient( <r>, <m> ) . . . . . . . delegate to the default ring
#F EuclideanQuotient( <R>, <r>, <m> ) . . . . . . . . . euclidean remainder
##
InstallOtherMethod( EuclideanQuotient,
"for two ring elements",
IsIdenticalObj, [ IsRingElement, IsRingElement ], 0,
function( r, m )
return EuclideanQuotient( DefaultRing( [ r, m ] ), r, m );
end );
InstallMethod( EuclideanQuotient,
"for a Euclidean ring and two ring elements",
IsCollsElmsElms, [ IsEuclideanRing, IsRingElement, IsRingElement ], 0,
function( R, r, m )
return QuotientRemainder( R, r, m )[1];
end );
#############################################################################
##
#M QuotientRemainder( <r>, <m> ) . . . . . . . delegate to the default ring
##
InstallOtherMethod( QuotientRemainder,
"for two ring elements",
IsIdenticalObj, [ IsRingElement, IsRingElement ], 0,
function( r, m )
return QuotientRemainder( DefaultRing( [ r, m ] ), r, m );
end );
#############################################################################
##
#M QuotientMod( <r>, <s>, <m> ) . . . . . . . delegate to the default ring
#M QuotientMod( <R>, <r>, <s>, <m> ) . . . . . quotient of two ring elements
#M modulo another
##
InstallOtherMethod( QuotientMod,
"for three ring elements",
IsFamFamFam,
[ IsRingElement, IsRingElement, IsRingElement ], 0,
function( r, s, m )
return QuotientMod( DefaultRing( [ r, s, m ] ), r, s, m );
end );
InstallMethod( QuotientMod,
"for a Euclidean ring and three ring elements",
IsCollsElmsElmsElms,
[ IsEuclideanRing, IsRingElement, IsRingElement, IsRingElement ], 0,
function( R, r, s, m )
local f, g, h, fs, gs, hs, q, t;
if not IsSubset( R, [ r, s, m ] ) then
Error( "<r>, <s>, <m> must lie in <R>" );
fi;
f := s; fs := 1;
g := m; gs := 0;
while g <> Zero( R ) do
t := QuotientRemainder( R, f, g );
h := g; hs := gs;
g := t[2]; gs := fs - t[1] * gs;
f := h; fs := hs;
od;
q := Quotient( R, r, f );
if q = fail then
return fail;
else
return EuclideanRemainder( R, fs * q, m );
fi;
end );
#############################################################################
##
#M PowerMod( <r>, <e>, <m> ) . . . . . . . . . delegate to the default ring
#M PowerMod( <R>, <r>, <e>, <m> ) . . . power of a ring element mod another
##
InstallOtherMethod( PowerMod,
"for ring element, integer, and ring element",
true, [ IsRingElement, IsInt, IsRingElement ], 0,
function( r, e, m )
return PowerMod( DefaultRing( [ r, m ] ), r, e, m );
end );
InstallMethod( PowerMod,
"for Euclidean ring, ring element, integer, and ring element",
true,
[ IsEuclideanRing, IsRingElement, IsInt, IsRingElement ], 0,
function( R, r, e, m )
local pow, f;
# handle special case
if e = 0 then
return One( R );
fi;
# reduce r initially
r := EuclideanRemainder( R, r, m );
# if e is negative then invert n modulo m with Euclids algorithm
if e < 0 then
r := QuotientMod( R, One( R ), r, m );
if r = fail then
Error( "<r> must be invertible modulo <m>" );
fi;
e := -e;
fi;
# now use the repeated squaring method (right-to-left)
pow := One( R );
f := 2 ^ (LogInt( e, 2 ) + 1);
while 1 < f do
pow := EuclideanRemainder( R, pow * pow, m );
f := QuoInt( f, 2 );
if f <= e then
pow := EuclideanRemainder( R, pow * r, m );
e := e - f;
fi;
od;
# return the power
return pow;
end );
#############################################################################
##
#F Gcd( <r1>, <r2>, ... )
#F Gcd( <list> )
#F Gcd( <R>, <r1>, <r2>, ... )
#F Gcd( <R>, <list> )
##
InstallGlobalFunction( Gcd, function ( arg )
local tested, R, ns, i, gcd;
# get and check the arguments (what a pain)
tested:= false;
if Length(arg)=2 and (not IsRing(arg[1])) and
IsIdenticalObj(FamilyObj(arg[1]),FamilyObj(arg[2]))
then # quick dispatch for two two ring elements. There is a
# fallback method that still supplies the ring if nothing special
return GcdOp(arg[1],arg[2]);
elif Length(arg) = 0 then
Error("usage: Gcd( [<R>,] <r1>, <r2>... )");
elif Length(arg) = 1 then
ns := arg[1];
elif Length(arg) = 2 and IsRing(arg[1]) then
R := arg[1];
ns := arg[2];
elif IsRing(arg[1]) then
R := arg[1];
ns := arg{ [2..Length(arg)] };
else
R := DefaultRing( arg );
ns := arg;
tested:= true;
fi;
if not IsList( ns ) or IsEmpty(ns) then
Error("usage: Gcd( [<R>,] <r1>, <r2>... )");
fi;
if not IsBound( R ) then
R := DefaultRing( ns );
elif not tested then
if not IsSubset( R, ns ) then
Error("<ns> must be a subset of <R>");
fi;
fi;
#T We do not want to require `R' to be euclidean,
#T for example multivariate polynomial rings are legal rings here.
#T (Perhaps a weaker test would be appropriate, for example for UFD?)
#T if not IsEuclideanRing( R ) then
#T Error("<R> must be a Euclidean ring");
#T fi;
# compute the gcd by iterating
gcd := StandardAssociate(R,ns[1]);
for i in [2..Length(ns)] do
gcd := GcdOp( R, gcd, ns[i] );
od;
# return the gcd
return gcd;
end );
#############################################################################
##
#M GcdOp( <r>, <s> ) . . . . . . . . . . . . . delegate to the default ring
#M GcdOp( <R>, <r>, <s> ) . . . . greatest common divisor of ring elements
##
InstallOtherMethod( GcdOp,
"for two ring elements",
IsIdenticalObj,
[ IsRingElement, IsRingElement ], 0,
function( r, s )
return Gcd( DefaultRing( [ r, s ] ), r, s );
end );
InstallMethod( GcdOp,
"for a Euclidean ring and two ring elements",
IsCollsElmsElms,
[ IsEuclideanRing, IsRingElement, IsRingElement ], 0,
function( R, r, s )
local gcd, u, v, w;
if not ( r in R and s in R ) then
Error( "<r> and <s> must lie in <R>" );
fi;
# perform a Euclidean algorithm
u := r;
v := s;
while v <> Zero( R ) do
w := v;
v := EuclideanRemainder( R, u, v );
u := w;
od;
gcd := u;
# norm the element
gcd := StandardAssociate( R, gcd );
# return the gcd
return gcd;
end );
#############################################################################
##
#F GcdRepresentation( <r1>, <r2>, ... )
#F GcdRepresentation( <list> )
#F GcdRepresentation( <R>, <r1>, <r2>, ... )
#F GcdRepresentation( <R>, <list> )
##
InstallGlobalFunction( GcdRepresentation, function ( arg )
local R, ns, i, gcd, rep, tmp;
# get and check the arguments (what a pain)
if Length(arg) = 0 then
Error("usage: GcdRepresentation( [<R>,] <r1>, <r2>... )");
elif Length(arg) = 1 then
ns := arg[1];
elif Length(arg) = 2 and IsRing(arg[1]) then
R := arg[1];
ns := arg[2];
elif IsRing(arg[1]) then
R := arg[1];
ns := arg{ [2..Length(arg)] };
else
R := DefaultRing( arg );
ns := arg;
fi;
if not IsList( ns ) or IsEmpty(ns) then
Error("usage: GcdRepresentation( [<R>,] <r1>, <r2>... )");
fi;
if not IsBound( R ) then
R := DefaultRing( ns );
else
if not IsSubset( R, ns ) then
Error("<ns> must be a subset of <R>");
fi;
fi;
if not IsEuclideanRing( R ) then
Error("<R> must be a Euclidean ring");
fi;
# compute the gcd by iterating
gcd := ns[1];
rep := [ One( R ) ];
for i in [2..Length(ns)] do
tmp := GcdRepresentationOp ( R, gcd, ns[i] );
gcd := tmp[1] * gcd + tmp[2] * ns[i];
rep := List( rep, x -> x * tmp[1] );
Add( rep, tmp[2] );
od;
# return the gcd representation
return rep;
end );
#############################################################################
##
#M GcdRepresentationOp( <r>, <s> ) . . . . . . delegate to the default ring
#M GcdRepresentationOp( <R>, <r>, <s> ) . . . . . representation of the gcd
##
InstallOtherMethod( GcdRepresentationOp,
"for two ring elements",
IsIdenticalObj, [ IsRingElement, IsRingElement ], 0,
function( r, s )
return GcdRepresentation( DefaultRing( [ r, s ] ), r, s );
end );
InstallMethod( GcdRepresentationOp,
"for a Euclidean ring and two ring elements",
IsCollsElmsElms,
[ IsEuclideanRing, IsRingElement, IsRingElement ], 0,
function( R, x, y )
local f, g, h, fx, gx, hx, q, t;
f := x; fx := One( R );
g := y; gx := Zero( R );
while g <> Zero( R ) do
t := QuotientRemainder( R, f, g );
h := g; hx := gx;
g := t[2]; gx := fx - t[1] * gx;
f := h; fx := hx;
od;
q := StandardAssociateUnit(R, f);
if y = Zero( R ) then
return [ q * fx, Zero( R ) ];
else
return [ q * fx, Quotient( R, q * (f - fx * x), y ) ];
fi;
end );
#############################################################################
##
#F Lcm( <r1>, <r2>, ... )
#F Lcm( <list> )
#F Lcm( <R>, <r1>, <r2>, ... )
#F Lcm( <R>, <list> )
##
InstallGlobalFunction( Lcm, function ( arg )
local tested, ns, R, lcm, i;
# get and check the arguments (what a pain)
tested:= false;
if Length(arg) = 0 then
Error("usage: Lcm( [<R>,] <r1>, <r2>... )");
elif Length(arg) = 1 then
ns := arg[1];
elif Length(arg) = 2 and IsRing(arg[1]) then
R := arg[1];
ns := arg[2];
elif IsRing(arg[1]) then
R := arg[1];
ns := arg{ [2..Length(arg)] };
else
R := DefaultRing( arg );
ns := arg;
tested:= true;
fi;
if not IsList( ns ) or IsEmpty(ns) then
Error("usage: Lcm( [<R>,] <r1>, <r2>... )");
fi;
if not IsBound( R ) then
R := DefaultRing( ns );
elif not tested then
if not IsSubset( R, ns ) then
Error("<ns> must be a subset of <R>");
fi;
fi;
#T We do not want to require `R' to be euclidean,
#T for example multivariate polynomial rings are legal rings here.
#T (Perhaps a weaker test would be appropriate, for example for UFD?)
#T if not IsEuclideanRing( R ) then
#T Error("<R> must be a Euclidean ring");
#T fi;
# compute the least common multiple
lcm := StandardAssociate(R,ns[1]);
for i in [2..Length(ns)] do
lcm := LcmOp( R, lcm, ns[i] );
od;
# return the lcm
return lcm;
end );
#############################################################################
##
#M LcmOp( <r>, <s> ) . . . . . . . . . . . . . delegate to the default ring
#M LcmOp( <R>, <r>, <s> ) . . . least common multiple of two ring elements
##
InstallOtherMethod( LcmOp,
"for two ring elements",
IsIdenticalObj,
[ IsRingElement, IsRingElement ], 0,
function( r, s )
return LcmOp( DefaultRing( [ r, s ] ), r, s );
end );
InstallMethod( LcmOp,
"for a Euclidean ring and two ring elements",
IsCollsElmsElms,
[ IsUniqueFactorizationRing, IsRingElement, IsRingElement ], 0,
function( R, r, s )
# compute the least common multiple
if r = Zero( R ) and s = Zero( R ) then
return r;
elif r in R and s in R then
return StandardAssociate( R, Quotient( R, r, GcdOp( R, r, s ) ) * s );
else
Error( "<r> and <s> must lie in <R>" );
fi;
end );
#############################################################################
##
#M \=( <R>, <S> ) . . . . . . . . . . . . . . . test if two rings are equal
##
InstallMethod( \=,
"for two rings with known generators",
IsIdenticalObj,
[ IsRing and HasGeneratorsOfRing, IsRing and HasGeneratorsOfRing ], 0,
function ( R, S )
if IsFinite( R ) then
if IsFinite( S ) then
#T really test this?
return Size( R ) = Size( S )
and IsSubset( S, GeneratorsOfRing( R ) );
else
return false;
fi;
elif IsFinite( S ) then
return false;
else
return IsSubset( S, GeneratorsOfRing( R ) )
and IsSubset( R, GeneratorsOfRing( S ) );
fi;
end );
# moved here from `integers` to avoid reading order change
#############################################################################
##
#M GcdOp( Integers, <n>, <m> ) . . . . . . . . . . . . . gcd of two integers
##
InstallRingAgnosticGcdMethod("integers", true,true,
[ IsIntegers, IsInt, IsInt ], 0,GcdInt);
#############################################################################
##
#E