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#############################################################################
##
## This file is part of GAP, a system for computational discrete algebra.
## This file's authors include Thomas Breuer.
##
## Copyright of GAP belongs to its developers, whose names are too numerous
## to list here. Please refer to the COPYRIGHT file for details.
##
## SPDX-License-Identifier: GPL-2.0-or-later
##
## This file contains the definition of the family of Lie elements of a
## family of ring elements.

#############################################################################
##
## <#GAPDoc Label="[1]{liefam}">
## Let <C>x</C> be a ring element, then <C>LieObject(x)</C>
## (see <Ref Attr="LieObject"/>) wraps <C>x</C> up into an
## object that contains the same data (namely <C>x</C>). The multiplication
## <C>*</C> for Lie objects is formed by taking the commutator. More exactly,
## if <C>l1</C> and <C>l2</C> are the Lie objects corresponding to
## the ring elements <C>r1</C> and <C>r2</C>, then <C>l1 * l2</C>
## is equal to the Lie object corresponding to <C>r1 * r2 - r2 * r1</C>.
## Two rules for Lie objects are worth noting:
## 
## <List>
## <Item>
## An element is <E>not</E> equal to its Lie element.
## </Item>
## <Item>
## If we take the Lie object of an ordinary (associative) matrix
## then this is again a matrix;
## it is therefore a collection (of its rows) and a list.
## But it is <E>not</E> a collection of collections of its entries,
## and its family is <E>not</E> a collections family.
## </Item>
## </List>
## 
## Given a family <C>F</C> of ring elements, we can form its Lie family
## <C>L</C>. The elements of <C>F</C> and <C>L</C> are in bijection, only
## the multiplications via <C>*</C> differ for both families.
## More exactly, if <C>l1</C> and <C>l2</C> are the Lie elements
## corresponding to the elements <C>f1</C> and <C>f2</C> in <C>F</C>,
## we have <C>l1 * l2</C> equal to the Lie element corresponding to
## <C>f1 * f2 - f2 * f1</C>.
## Furthermore, the product of Lie elements <C>l1</C>, <C>l2</C> and
## <C>l3</C> is left-normed, that is <C>l1*l2*l3</C> is equal to
## <C>(l1*l2)*l3</C>.
## 
## The main reason to distinguish elements and Lie elements on the family
## level is that this helps to avoid forming domains that contain
## elements of both types.
## For example, if we could form vector spaces of matrices then at first
## glance it would be no problem to have both ordinary and Lie matrices
## in it, but as soon as we find out that the space is in fact an algebra
## (e.g., because its dimension is that of the full matrix algebra),
## we would run into strange problems.
## 
## Note that the family situation with Lie families may be not familiar.
## 
## <List>
## <Item>
## We have to be careful when installing methods for certain types
## of domains that may involve Lie elements.
## For example, the zero element of a matrix space is either an ordinary
## matrix or its Lie element, depending on the space.
## So either the method must be aware of both cases, or the method
## selection must distinguish the two cases.
## In the latter situation, only one method may be applicable to each
## case; this means that it is not sufficient to treat the Lie case
## with the additional requirement <C>IsLieObjectCollection</C> but that
## we must explicitly require non-Lie elements for the non-Lie case.
## </Item>
## <Item>
## Being a full matrix space is a property that may hold for a space
## of ordinary matrices or a space of Lie matrices.
## So methods for full matrix spaces must also be aware of Lie matrices.
## </Item>
## </List>
## <#/GAPDoc>
##

#############################################################################
##
#C IsLieObject( <obj> )
#C IsLieObjectCollection( <obj> )
##
## <#GAPDoc Label="IsLieObject">
## <ManSection>
## <Filt Name="IsLieObject" Arg='obj' Type='Category'/>
## <Filt Name="IsLieObjectCollection" Arg='obj' Type='Category'/>
## <Filt Name="IsRestrictedLieObject" Arg='obj' Type='Category'/>
## <Filt Name="IsRestrictedLieObjectCollection" Arg='obj' Type='Category'/>
##
## <Description>
## An object lies in <Ref Filt="IsLieObject"/> if and only if
## it lies in a family constructed by <Ref Attr="LieFamily"/>.
## <Example><![CDATA[
## gap> m:= [ [ 1, 0 ], [ 0, 1 ] ];;
## gap> lo:= LieObject( m );
## LieObject( [ [ 1, 0 ], [ 0, 1 ] ] )
## gap> IsLieObject( m );
## false
## gap> IsLieObject( lo );
## true
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareCategory( "IsLieObject",
 IsRingElement
 and IsZeroSquaredElement
 and IsJacobianElement );

DeclareCategoryCollections( "IsLieObject" );

DeclareSynonym( "IsRestrictedLieObject",
 IsLieObject and IsRestrictedJacobianElement);

DeclareCategoryCollections( "IsRestrictedLieObject" );

#############################################################################
##
#A LieFamily( <Fam> )
##
## <#GAPDoc Label="LieFamily">
## <ManSection>
## <Attr Name="LieFamily" Arg='Fam'/>
##
## <Description>
## is a family <C>F</C> in bijection with the family <A>Fam</A>,
## but with the Lie bracket as infix multiplication.
## That is, for <C>x</C>, <C>y</C> in <A>Fam</A>, the product of
## the images in <C>F</C> will be the image of <C>x * y - y * x</C>.
## 
## The standard type of objects in a Lie family <C>F</C> is
## <C><A>F</A>!.packedType</C>.
## 
## <Index Key="Embedding" Subkey="for Lie algebras"><C>Embedding</C></Index>
## The bijection from <A>Fam</A> to <C>F</C> is given by
## <C>Embedding( <A>Fam</A>, F )</C>
## (see <Ref Oper="Embedding" Label="for two domains"/>);
## this bijection respects addition and additive inverses.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute( "LieFamily", IsFamily );

#############################################################################
##
#A UnderlyingFamily( <Fam> )
##
## <#GAPDoc Label="UnderlyingFamily">
## <ManSection>
## <Attr Name="UnderlyingFamily" Arg='Fam'/>
##
## <Description>
## If <A>Fam</A> is a Lie family then <C>UnderlyingFamily( <A>Fam</A> )</C>
## is a family <C>F</C> such that <C><A>Fam</A> = LieFamily( F )</C>.
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute( "UnderlyingFamily", IsObject );

#############################################################################
##
#A LieObject( <obj> )
##
## <#GAPDoc Label="LieObject">
## <ManSection>
## <Attr Name="LieObject" Arg='obj'/>
##
## <Description>
## Let <A>obj</A> be a ring element. Then <C>LieObject( <A>obj</A> )</C> is the
## corresponding Lie object. If <A>obj</A> lies in the family <C>F</C>,
## then <C>LieObject( <A>obj</A> )</C> lies in the family <C>LieFamily( F )</C>
## (see <Ref Attr="LieFamily"/>).
## <Example><![CDATA[
## gap> m:= [ [ 1, 0 ], [ 0, 1 ] ];;
## gap> lo:= LieObject( m );
## LieObject( [ [ 1, 0 ], [ 0, 1 ] ] )
## gap> m*m;
## [ [ 1, 0 ], [ 0, 1 ] ]
## gap> lo*lo;
## LieObject( [ [ 0, 0 ], [ 0, 0 ] ] )
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute( "LieObject", IsRingElement );

#############################################################################
##
#A UnderlyingRingElement( <obj> )
##
## <#GAPDoc Label="UnderlyingRingElement">
## <ManSection>
## <Attr Name="UnderlyingRingElement" Arg='obj'/>
##
## <Description>
## Let <A>obj</A> be a Lie object constructed from a ring element
## <C>r</C> by calling <C>LieObject( r )</C>.
## Then <C>UnderlyingRingElement( <A>obj</A> )</C> returns
## the ring element <C>r</C> used to construct <A>obj</A>.
## If <C>r</C> lies in the family <C>F</C>, then <A>obj</A>
## lies in the family <C>LieFamily( F )</C>
## (see <Ref Attr="LieFamily"/>).
## <Example><![CDATA[
## gap> lo:= LieObject( [ [ 1, 0 ], [ 0, 1 ] ] );
## LieObject( [ [ 1, 0 ], [ 0, 1 ] ] )
## gap> m:=UnderlyingRingElement(lo);
## [ [ 1, 0 ], [ 0, 1 ] ]
## gap> lo*lo;
## LieObject( [ [ 0, 0 ], [ 0, 0 ] ] )
## gap> m*m;
## [ [ 1, 0 ], [ 0, 1 ] ]
## ]]></Example>
## </Description>
## </ManSection>
## <#/GAPDoc>
##
DeclareAttribute( "UnderlyingRingElement", IsLieObject );

#############################################################################
##
#F IsLieObjectsModule( <V> )
##
## <ManSection>
## <Func Name="IsLieObjectsModule" Arg='V'/>
##
## <Description>
## If a free <M>F</M>-module <A>V</A> is in the filter <C>IsLieObjectsModule</C> then
## this expresses that <A>V</A> consists of Lie objects (see <Ref ???="..."/>),
## and that <A>V</A> is handled via the mechanism of nice bases (see <Ref ???="..."/>)
## in the following way.
## Let <M>K</M> be the default field generated by the vector space generators of
## <A>V</A>.
## Then the <C>NiceFreeLeftModuleInfo</C> value of <A>V</A> is irrelevant,
## and the <C>NiceVector</C> value of <M>v \in <A>V</A></M> is defined as the underlying
## element for which <A>v</A> is obtained as <C>LieObject</C> value.
## </Description>
## </ManSection>
##
DeclareHandlingByNiceBasis( "IsLieObjectsModule",
 "for free left modules of Lie objects" );

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