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### 4 LAGUNA functions

#### 4.1 General functions for group algebras

##### 4.1-1 IsGroupAlgebra
 ‣ IsGroupAlgebra( KG ) ( property )

A group ring over a field is called a group algebra. For a group ring KG, IsGroupAlgebra returns true, if the underlying ring of KG is a field; false is returned otherwise. This property will be set automatically for every group ring created by the function GroupRing.


gap> IsGroupAlgebra( GroupRing( GF( 2 ), DihedralGroup( 16 ) ) );
true
gap> IsGroupAlgebra( GroupRing( Integers, DihedralGroup( 16 ) ) );
false



##### 4.1-2 IsFModularGroupAlgebra
 ‣ IsFModularGroupAlgebra( KG ) ( property )

A group algebra $$KG$$ over a field $$K$$ is called modular, if the characteristic of the field $$K$$ divides the order of some element in $$G$$. For a group algebra KG of a finite group $$G$$, IsModularGroupAlgebra returns true, if KG is modular according to this definition; false is returned otherwise. This property will be set automatically for every group algebra, created by the function GroupRing.


gap> IsFModularGroupAlgebra( GroupRing( GF( 2 ), SymmetricGroup( 6 ) ) );
true
gap> IsFModularGroupAlgebra( GroupRing( GF( 2 ), CyclicGroup( 3 ) ) );
false



##### 4.1-3 IsPModularGroupAlgebra
 ‣ IsPModularGroupAlgebra( KG ) ( property )

A group algebra $$KG$$ is said to be $$p$$-modular, if $$K$$ is a field of characteristic $$p$$ and $$G$$ is a finite $$p$$-group for the same prime $$p$$. For a group algebra KG of a finite group $$G$$, IsPModularGroupAlgebra returns true, if KG is $$p$$-modular according to this definition; false is returned otherwise. This property will be set automatically for every group algebra, created by the function GroupRing.


gap> IsPModularGroupAlgebra( GroupRing( GF( 2 ), DihedralGroup( 16 ) ) );
true
gap> IsPModularGroupAlgebra( GroupRing( GF( 2 ), SymmetricGroup( 6 ) ) );
false



##### 4.1-4 UnderlyingGroup
 ‣ UnderlyingGroup( KG ) ( attribute )

Returns: the underlying group of a group ring

This attribute stores the underlying group of a group ring KG. In fact, it refers to the attribute UnderlyingMagma which returns the same result, and was introduced for group rings for convenience, and for teaching purposes.


gap> KG := GroupRing( GF ( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> G := UnderlyingGroup( KG );
<pc group of size 16 with 4 generators>



##### 4.1-5 UnderlyingRing
 ‣ UnderlyingRing( KG ) ( attribute )

Returns: the underlying ring of a group ring

This attribute stores the underlying ring of a group ring KG. In fact, it refers to the attribute LeftActingDomain which returns the same result, and was introduced for group rings for convenience, and for teaching purposes.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> UnderlyingRing( KG );
GF(2)



##### 4.1-6 UnderlyingField
 ‣ UnderlyingField( KG ) ( attribute )

Returns: the underlying field of a group algebra

This attribute stores the underlying field of a group algebra KG. In fact, it refers to the attribute LeftActingDomain which returns the same result, and was introduced for group algebras for convenience, and for teaching purposes.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> UnderlyingField( KG );
GF(2)



#### 4.2 Operations with group algebra elements

##### 4.2-1 Support
 ‣ Support( x ) ( attribute )

Returns: support of x as a list of elements of the underlying group

Returns the support of a group ring element x. The support of a non-zero element $$x = \alpha_1 \cdot g_1 + \alpha_2 \cdot g_2 + \cdots + \alpha_k \cdot g_k$$ of a group ring is the list of elements $$g_i \in G$$ for which the coefficient $$\alpha_i$$ is non-zero. The support of the zero element of a group ring is defined to be the empty list. This method is also applicable to elements of magma rings.


# First we create an element x to use in in the series of examples.
# We map the minimal generating system of the group G to its group algebra
# and denote their images as a and b
gap> G:=DihedralGroup(16);; KG:=GroupRing(GF(2),G);;
gap> l := List( MinimalGeneratingSet( G ), g -> g^Embedding( G, KG ) );
[ (Z(2)^0)*f1, (Z(2)^0)*f2 ]
gap> a := l[1]; b := l[2]; e := One( KG ); # we denote the identity by e
(Z(2)^0)*f1
(Z(2)^0)*f2
(Z(2)^0)*<identity> of ...
gap> x := ( e + a ) * ( e + b );
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> Support( x );
[ <identity> of ..., f1, f2, f1*f2 ]



##### 4.2-2 CoefficientsBySupport
 ‣ CoefficientsBySupport( x ) ( attribute )

Returns: coefficients of support elements as list of elements of the underlying ring

Returns a list that contains the coefficients corresponding to the elements of Support( x ) in the same order as the elements appear in Support( x ). This method is also applicable to elements of magma rings.


gap> x;
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> CoefficientsBySupport( x );
[ Z(2)^0, Z(2)^0, Z(2)^0, Z(2)^0 ]



##### 4.2-3 TraceOfMagmaRingElement
 ‣ TraceOfMagmaRingElement( x ) ( attribute )

Returns: an element of the underlying ring

Returns the trace of a group ring element x. By definition, the trace of an element $$x = \alpha_1 \cdot 1 + \alpha_2 \cdot g_2 + \cdots + \alpha_k \cdot g_k$$ is equal to $$\alpha_1$$, that is, the coefficient of the identity element in $$G$$. The trace of the zero element is zero. This method is also applicable to elements of magma rings.


gap> x;
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> TraceOfMagmaRingElement( x );
Z(2)^0



##### 4.2-4 Length
 ‣ Length( x ) ( attribute )

The length of an element of a group ring x is defined as the number of elements in its support. This method is also applicable to elements of magma rings.


gap> x;
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> Length( x );
4



##### 4.2-5 Augmentation
 ‣ Augmentation( x ) ( attribute )

Returns: the sum of coefficients of a group ring element

The augmentation of a group ring element $$x = \alpha_1 \cdot g_1 + \alpha_2 \cdot g_2 + \cdots + \alpha_k \cdot g_k$$ is the sum of its coefficients $$\alpha_1 + \alpha_2 + \cdots + \alpha_k$$. The method is also applicable to elements of magma rings.


gap> x;
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> Augmentation( x );
0*Z(2)



##### 4.2-6 PartialAugmentations
 ‣ PartialAugmentations( KG, x ) ( operation )

Returns: a list of partial augmentations and a list of conjugacy class representatives

The partial augmentation of an element $$x = \alpha_1 \cdot g_1 + \alpha_2 \cdot g_2 + \cdots + \alpha_k \cdot g_k$$ of the group ring $$KG$$, corresponding to the conjugacy class of an element $$g$$ from the underlying group $$G$$ is the sum of coefficients $$\alpha_i$$ taken over all $$g_i$$ such that $$g_i$$ is conjugated to $$g$$. The function returns a list of two lists, the first one is a list of partial augmentations, and the second is a list of representatives of appropriate conjugacy classes of elements of the group $$G$$.


gap> y := x + a*b^2;
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2+(Z(2)^
0)*f1*f3
gap> PartialAugmentations( KG, y );
[ [ Z(2)^0, 0*Z(2), Z(2)^0, Z(2)^0 ], [ <identity> of ..., f1, f2, f1*f2 ] ]



##### 4.2-7 Involution
 ‣ Involution( x[[, f], s] ) ( operation )

Returns: an element of a group ring

Let $$KG$$ be a group ring, $$f$$ be a homomorphism from the group $$G$$ to the unit group of the ring $$K$$. Furthermore, let $$s$$ be a mapping $$G \rightarrow G$$, such that $$s^2$$ is the identity mapping on $$G$$ and for every element $$g \in G$$ $$f(g*s(g))$$ equals $$f(s(g)*g)$$ and equals the identity element of the ring $$K$$. Then the involution of $$KG$$ induced by $$f$$ and $$s$$ is defined by $$\alpha_1 \cdot g_1 + \alpha_2 \cdot g_2 + \cdots + \alpha_k \cdot g_k \mapsto \alpha_1 \cdot f(g_1) \cdot s(g_1) + \alpha_2 \cdot f(g_2) \cdot s(g_2) + \cdots + \alpha_k \cdot f(g_k) \cdot s(g_k)$$.

The method returns the image of x under the involution of $$KG$$ induced by $$f$$ and $$s$$. If the mapping $$f$$ is omitted, $$f$$ is assumed to map everything to the identity element of the ring $$K$$. If both mappings are omitted, it returns the result of so-called classical involution, induced by the mapping $$x \mapsto x^{-1}$$.


gap> x;
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> Involution( x );
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f1*f2+(Z(2)^0)*f2*f3*f4
gap> l := List( MinimalGeneratingSet( G ), g -> g^Embedding( G, KG ) );
[ (Z(2)^0)*f1, (Z(2)^0)*f2 ]
gap> List( l, Involution ); # check how involution acts on elements of G
[ (Z(2)^0)*f1, (Z(2)^0)*f2*f3*f4 ]
gap> List( l, g -> g^-1 );
[ (Z(2)^0)*f1, (Z(2)^0)*f2*f3*f4 ]



##### 4.2-8 IsSymmetric
 ‣ IsSymmetric( x ) ( attribute )

An element of a group ring is called symmetric if it is fixed under the classical involution. This property is checked here.


gap> IsSymmetric( x );
false
gap> IsSymmetric( x * Involution( x ) );
true



##### 4.2-9 IsUnitary
 ‣ IsUnitary( x ) ( attribute )

A unit of a group ring is called unitary if the classical involution inverts it. This property is checked here.


gap> IsUnitary(x);
false
gap> l:=List(MinimalGeneratingSet(G),g -> g^Embedding(G,KG));
[ (Z(2)^0)*f1, (Z(2)^0)*f2 ]
gap> List(l,IsUnitary); # check that elements of G are unitary
[ true, true ]



##### 4.2-10 IsUnit
 ‣ IsUnit( [KG, ]x ) ( method )

This method improves a standard GAP functionality for modular group algebras.

In the two-argument version the method returns true if x is an invertible element of the modular group algebra KG and false otherwise. This can be done very quickly by checking whether the augmentation of the element x is non-zero.

If the first argument is omitted, then LAGUNA constructs the group $$H$$ generated by the support of x, and, if this group is a finite $$p$$-group, then checks whether the coefficients of x belong to a field $$F$$ of characteristic $$p$$. If this is the case, then IsUnit( FH, x ) is called; otherwise, standard GAP method is used.


gap> x;
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> IsUnit( KG, x ); # clearly, is not a unit due to augmentation zero
false
gap> y := One( KG ) + x; # this should give a unit
(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> IsUnit( KG, y );
true



##### 4.2-11 InverseOp
 ‣ InverseOp( x ) ( method )

Returns: the inverse element of an element of a group ring

This method improves a standard GAP functionality for modular group algebras. It calculates the inverse of a group algebra element. The user can also invoke this function by typing  x^-1 .


gap> y;
(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
gap> y^-1;
(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f3+(Z(2)^0)*f4+(Z(2)^0)*f1*f2+(Z(2)^
0)*f1*f3+(Z(2)^0)*f1*f4+(Z(2)^0)*f2*f4+(Z(2)^0)*f1*f2*f4+(Z(2)^0)*f2*f3*f4+(
Z(2)^0)*f1*f2*f3*f4
gap> y * y^-1;
(Z(2)^0)*<identity> of ...



##### 4.2-12 BicyclicUnitOfType1
 ‣ BicyclicUnitOfType1( [KG, ]a, g ) ( operation )
 ‣ BicyclicUnitOfType2( [KG, ]a, g ) ( operation )

Returns: an element of a group ring

let $$a$$ be an element of order $$n$$ of a group $$G$$. We put $$\alpha = 1 + a + a^2 + ... +a^{n-1}$$. Then $$(a-1)*g*\alpha$$ and $$\alpha*g*(a-1)$$ are nilpotent of index two for any element $$g$$ of the group $$G$$ not containing in the normalizer $$N_G(\langle a \rangle)$$, and the units $$u_{a,g} = 1 + (a-1) * g * \alpha$$ and $$v_{a,g} = 1 + \alpha * g * (a-1)$$ are called bicyclic units of the 1st and 2nd type respectively. Note that $$u_{a,g}$$ and $$v_{a,g}$$ may coincide for some $$a$$ and $$g$$, but in general this does not hold. In the three-argument version these methods construct bicyclic units of both types when a and g are elements of the underlying group $$G$$ of a group ring KG. The two-argument version accepts images of elements a and g from the underlying group in the group ring $$KG$$ obtained using the mapping Embedding( G, KG ). Note that it is not actually checked that $$g$$ is not contained in $$N_G(\langle a \rangle)$$, because this is verified in BicyclicUnitGroup (4.4-13).


gap> G := SmallGroup(32,6);
<pc group of size 32 with 5 generators>
gap> KG := GroupRing( GF(2), G );
<algebra-with-one over GF(2), with 5 generators>
gap> g := MinimalGeneratingSet( G );
[ f1, f2 ]
gap> g[1] in Normalizer( G, Subgroup( G, [g[2]] ) );
false
gap> g[2] in Normalizer( G, Subgroup( G, [g[1]] ) );
false
gap> g := List( g, x -> x^Embedding( G, KG ) );
[ (Z(2)^0)*f1, (Z(2)^0)*f2 ]
gap> BicyclicUnitOfType1(g[1],g[2]) = BicyclicUnitOfType2(g[1],g[2]);
false



##### 4.2-13 BassCyclicUnit
 ‣ BassCyclicUnit( [ZG, ]g, k ) ( operation )

Returns: an element of a group ring

Let g be an element of order $$n$$ of the group $$G$$, and 1 < k < $$n$$ be such that k and $$n$$ are coprime, then k^Phi($$n$$) is congruent to 1 modulo $$n$$. The unit b(g,k)= ( \sum_{j=0}^{k-1} g^j )^Phi(n) + ( (1-k^Phi(n))/n ) * Hat(g), where Hat(g) = g + g^2 + ... + g^n, is called a Bass cyclic unit of the integral group ring ZG.

The three-argument version constructs the Bass cyclic unit $$b(g,k)$$ for the element g from the underlying group $$G$$ of the group ring ZG. The two-argument version accepts the image of g in the group ring $$ZG$$ obtained using the mapping Embedding( G, KG ).

Remark that when $$G$$ is a finite nilpotent group, the group generated by the Bass cyclic units contain a subgroup of finite index in the centre of the unit group of ZG [JPS96].


gap> S := SymmetricGroup( 5 );;
gap> ZS := GroupRing( Integers, S );;
gap> f := Embedding( S, ZS );;
gap> BassCyclicUnit( ZS, (1,3,2,5,4) , 3 );
(1)*()+(-2)*(1,2,4,3,5)+(-2)*(1,3,2,5,4)+(3)*(1,4,5,2,3)+(1)*(1,5,3,4,2)
gap> BassCyclicUnit( (1,3,2,5,4)^f, 3 );
(1)*()+(-2)*(1,2,4,3,5)+(-2)*(1,3,2,5,4)+(3)*(1,4,5,2,3)+(1)*(1,5,3,4,2)



#### 4.3 Important attributes of group algebras

##### 4.3-1 AugmentationHomomorphism
 ‣ AugmentationHomomorphism( KG ) ( attribute )

Returns: a homomorphism from a group ring to the underlying ring

The mapping which maps an element of a group ring $$KG$$ to its augmentation is a homomorphism from $$KG$$ onto the ring $$K$$; see Augmentation (4.2-5). This attribute stores this homomorphism for the group ring KG.

Please note that for calculation of the augmentation of an element of a group ring the user is strongly recommended to use Augmentation (4.2-5) which works much faster than AugmentationHomomorphism.


gap> F := GF( 2 ); G := SymmetricGroup( 3 ); FG := GroupRing( F, G );
GF(2)
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> e := Embedding( G,FG );
<mapping: SymmetricGroup( [ 1 .. 3 ] ) -> AlgebraWithOne( GF(2), ... ) >
gap> x := (1,2)^e; y := (1,3)^e;
(Z(2)^0)*(1,2)
(Z(2)^0)*(1,3)
gap> a := AugmentationHomomorphism( FG );
[ (Z(2)^0)*(1,2,3), (Z(2)^0)*(1,2) ] -> [ Z(2)^0, Z(2)^0 ]
gap> x^a; y^a; ( x + y )^a; # this is slower
Z(2)^0
Z(2)^0
0*Z(2)
gap> Augmentation(x); Augmentation(y); Augmentation( x + y ); # this is faster
Z(2)^0
Z(2)^0
0*Z(2)



##### 4.3-2 AugmentationIdeal
 ‣ AugmentationIdeal( KG ) ( attribute )

Returns: an ideal of a group ring

If $$KG$$ is a group ring, then its augmentation ideal $$A$$ is generated by all elements of the form $$g-1$$, where $$g \in G$$ \ { $$1$$ }. The augmentation ideal consists of all elements of $$FG$$ with augmentation $$0$$; see Augmentation (4.2-5). This method changes a standard GAP functionality for modular group algebras and returns the augmentation ideal of a modular group algebra KG.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> AugmentationIdeal( KG );
<two-sided ideal in <algebra-with-one over GF(2), with 4 generators>,
(dimension 15)>



##### 4.3-3 RadicalOfAlgebra
 ‣ RadicalOfAlgebra( KG ) ( attribute )

Returns: an ideal of a group algebra

This method improves a standard GAP functionality for modular group algebras of finite $$p$$-groups. Since in this case the radical of the group algebra coincides with its augmentation ideal, this method simply checks if the algebra KG is a $$p$$-modular group algebra, and, if yes, it returns the augmentation ideal; otherwise, the standard GAP method will be used.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> RadicalOfAlgebra( KG );
<two-sided ideal in <algebra-with-one over GF(2), with 4 generators>,
(dimension 15)>
gap> RadicalOfAlgebra( KG ) = AugmentationIdeal( KG );
true



##### 4.3-4 WeightedBasis
 ‣ WeightedBasis( KG ) ( attribute )

Returns: a record of two components: weighted basis elements and their weights

The argument KG must be a $$p$$-modular group algebra.

For a group algebra $$KG$$, let $$A$$ denote the augmentation ideal, and assume that $$c$$ is the smallest number such that $$A^c=0$$. Then a weighted basis of $$KG$$ is some basis $$b_1, \ldots, b_n$$ for the augmentation ideal $$A$$, for which there are indices $$i_1=1, \ldots, i_{c-1}$$ such that $$b_{i_k}, \ldots, b_n$$ is a basis for $$A^k$$. The weight of an element $$b_i$$ of a weighted basis is the unique integer $$w$$ such that $$b_i$$ belongs to $$w$$-th power of $$A$$ but does not belong to its $$(w+1)$$-th power.

Note that this function actually constructs a basis for the augmentation ideal of KG and not for KG itself. Since the augmentation ideal has co-dimension 1 in KG, a basis for KG can be easily obtained by adjoining the identity element of the group.

The method returns a record whose basis entry is the basis and the weights entry is a list of the corresponding weights the of basis elements. See Section 3.3 for more details.


gap> KG := GroupRing( GF( 2 ), ElementaryAbelianGroup( 4 ) );
<algebra-with-one over GF(2), with 2 generators>
gap> WeightedBasis( KG );
rec(
weightedBasis := [ (Z(2)^0)*<identity> of ...+(Z(2)^0)*f1,
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f2,
(Z(2)^0)*<identity> of ...+(Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2 ],
weights := [ 1, 1, 2 ] )



##### 4.3-5 AugmentationIdealPowerSeries
 ‣ AugmentationIdealPowerSeries( KG ) ( attribute )

Returns: a list of ideals of a group algebra

The argument KG is a $$p$$-modular group algebra. The method returns a list whose elements are the terms of the augmentation ideal filtration of KG, that is AugmentationIdealPowerSeries(A)[i] is the $$i$$-th power of the augmentation ideal of KG.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> s := AugmentationIdealPowerSeries( KG );;
gap> s[2];
<algebra of dimension 13 over GF(2)>
gap> List(s,Dimension);
[ 15, 13, 11, 9, 7, 5, 3, 1, 0 ]
gap> Length(s);
9



##### 4.3-6 AugmentationIdealNilpotencyIndex
 ‣ AugmentationIdealNilpotencyIndex( KG ) ( attribute )

For the $$p$$-modular group algebra KG the method returns the smallest number $$n$$ such that $$A^n=0$$, where $$A$$ is the augmentation ideal of KG. This can be done using Jenning's theory without the explicit calculations of the powers of the augmentation ideal.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> AugmentationIdealNilpotencyIndex( KG );
9



##### 4.3-7 AugmentationIdealOfDerivedSubgroupNilpotencyIndex
 ‣ AugmentationIdealOfDerivedSubgroupNilpotencyIndex( KG ) ( attribute )

For the $$p$$-modular group algebra KG this attribute stores the nilpotency index of the augmentation ideal of $$KG'$$ where $$G'$$ denotes the derived subgroup of $$G$$.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> AugmentationIdealOfDerivedSubgroupNilpotencyIndex( KG );
4
gap> D := DerivedSubgroup( UnderlyingGroup( KG ) );
Group([ f3, f4 ])
gap> KD := GroupRing( GF( 2 ), D );
<algebra-with-one over GF(2), with 2 generators>
gap> AugmentationIdealNilpotencyIndex( KD );
4



##### 4.3-8 LeftIdealBySubgroup
 ‣ LeftIdealBySubgroup( KG, H ) ( operation )
 ‣ RightIdealBySubgroup( KG, H ) ( operation )
 ‣ TwoSidedIdalBySubgroup( KG, H ) ( operation )

Returns: an ideal of a group ring

Let KG be a group ring of a group $$G$$ over the ring $$K$$, and H be a subgroup of $$G$$. Then the set $$J_l(H)$$ of all elements of KG of the form

$\sum_{h \in H} x_h(h-1)$

is the left ideal in KG generated by all elements $$h-1$$ with $$h$$ in $$H$$. The right ideal $$J_r(H)$$ is defined analogously. These operations are used to consrtuct such ideals, taking into account the fact, that the ideal $$J_l(H)$$ is two-sided if and only if H is normal in $$G$$. An attempt of constructing two-sided ideal for a non-normal subgroup H will lead to an error message.


gap> KG := GroupRing( GF(2), DihedralGroup(16) );
<algebra-with-one over GF(2), with 4 generators>
gap> G := DihedralGroup(16);
<pc group of size 16 with 4 generators>
gap> KG := GroupRing( GF(2), G );
<algebra-with-one over GF(2), with 4 generators>
gap> D := DerivedSubgroup( G );
Group([ f3, f4 ])
gap> LeftIdealBySubgroup( KG, D );
<two-sided ideal in <algebra-with-one over GF(2), with 4 generators>,
(dimension 12)>
gap> H := Subgroup( G, [ GeneratorsOfGroup(G)[1] ]);
Group([ f1 ])
gap> IsNormal( G, H );
false
gap> LeftIdealBySubgroup( KG, H );
<left ideal in <algebra-with-one over GF(2), with 4 generators>, (dimension 8
)>



#### 4.4 Computations with the unit group

##### 4.4-1 NormalizedUnitGroup
 ‣ NormalizedUnitGroup( KG ) ( attribute )

Returns: a group generated by group algebra elements

Determines the normalized unit group of a $$p$$-modular group algebra KG over the field of $$p$$ elements. Returns the normalized unit group as the group generated by certain elements of KG; see Section 3.3 for more details.

For efficient computations the user is recommended to use PcNormalizedUnitGroup (4.4-2).


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> V := NormalizedUnitGroup( KG );
<group of size 32768 with 15 generators>
gap> u := GeneratorsOfGroup( V )[4];
(Z(2)^0)*f3



##### 4.4-2 PcNormalizedUnitGroup
 ‣ PcNormalizedUnitGroup( KG ) ( attribute )

Returns: a group given by power-commutator presentation

The argument KG is a $$p$$-modular group algebra over the field of $$p$$ elements. PcNormalizedUnitGroup returns the normalized unit group of KG given by a power-commutator presentation. The generators in this polycyclic presentation correspond to the weighted basis elements of KG. For more details, see Section 3.3.


gap> W := PcNormalizedUnitGroup( KG );
<pc group of size 32768 with 15 generators>
gap> w := GeneratorsOfGroup( W )[4];
f4



##### 4.4-3 NaturalBijectionToPcNormalizedUnitGroup
 ‣ NaturalBijectionToPcNormalizedUnitGroup( KG ) ( attribute )

Returns: a homomorphism of groups

The normalised unit group of a $$p$$-modular group algebra $$KG$$ over the field of $$p$$ elements can be computed using two methods, namely NormalizedUnitGroup (4.4-1) and PcNormalizedUnitGroup (4.4-2). These two methods return two different objects, and they can be used for different types of computations. The elements of NormalizedUnitGroup(KG) are represented in their natural group algebra representation, and hence they can easily be identified in the group algebra. However, the more quickly constructed NormalizedUnitGroup(KG) is often not suitable for further fast calculations. Hence one will have to use PcNormalizedUnitGroup(KG) if one wants to find some group theoretic properties of the normalized unit group. This method returns the bijection from NormalizedUnitGroup(KG) onto PcNormalizedUnitGroup(KG). This bijection can be used to map the result of a computation in PcNormalizedUnitGroup(KG) into NormalizedUnitGroup(KG).


gap> f := NaturalBijectionToPcNormalizedUnitGroup( KG );
MappingByFunction( <group of size 32768 with 15 generators>, <pc group of size\
32768 with 15 generators>, function( x ) ... end )
gap> u := GeneratorsOfGroup( V )[4];;
gap> u^f;
f4
gap> GeneratorsOfGroup( V )[4]^f = GeneratorsOfGroup( W )[4];
true



##### 4.4-4 NaturalBijectionToNormalizedUnitGroup
 ‣ NaturalBijectionToNormalizedUnitGroup( KG ) ( attribute )

Returns: a homomorphism of groups

For a $$p$$-modular group algebra KG over the field of $$p$$ elements this function returns the inverse of the mapping NaturalBijectionToPcNormalizedUnitGroup (4.4-3)


gap> t := NaturalBijectionToNormalizedUnitGroup(KG);;
gap> w := GeneratorsOfGroup(W)[4];;
gap> w^t;
(Z(2)^0)*f3
gap> GeneratorsOfGroup( W )[4]^t = GeneratorsOfGroup( V )[4];
true



##### 4.4-5 Embedding
 ‣ Embedding( H, V ) ( operation )

Returns: a homomorphism from an underlying group to a normalized unit group in pc-presentation

Let H be a subgroup of a group $$G$$ and V be the normalized unit group of the group algebra $$KG$$ given by the power-commutator presentation (see PcNormalizedUnitGroup (4.4-2). Then Embedding( H, V ) returns the homomorphism from H to V, which is the composition of Embedding( H, KG ) and NaturalBijectionToPcNormalizedUnitGroup( KG ).


gap> G := DihedralGroup( 16 );
<pc group of size 16 with 4 generators>
gap> KG := GroupRing( GF( 2 ), G );
<algebra-with-one over GF(2), with 4 generators>
gap> V:=PcNormalizedUnitGroup( KG );
<pc group of size 32768 with 15 generators>
gap> ucs := UpperCentralSeries( V );;
gap> f := Embedding( G, V );
[ f1, f2, f3, f4 ] -> [ f1, f2, f4, f8 ]
gap> G1 := Image( f, G );
Group([ f1, f2, f4, f8 ])
gap> H := Intersection( ucs[2], G1 ); # compute intersection in V(KG)
Group([ f4, f8, f4*f8 ])
gap> T:=PreImage( f, H );             # find its preimage in G
Group([ f3, f4, f3*f4 ])
gap> IdGroup( T );
[ 4, 1 ]



##### 4.4-6 Units
 ‣ Units( KG ) ( attribute )

Returns: the unit group of a group ring

This improves a standard GAP functionality for modular group algebras of finite $$p$$-groups over the field of $$p$$ elements. It returns the unit group of KG as a direct product of Units(K) and NormalizedUnitGroup(KG), where the latter is generated by certain elements of KG; see Chapter 3 for more details.


gap> U := Units( KG );
#I  LAGUNA package: Computing the unit group ...
<group of size 32768 with 15 generators>
gap> GeneratorsOfGroup( U )[5]; # now elements of U are already in KG
(Z(2)^0)*f1+(Z(2)^0)*f3+(Z(2)^0)*f1*f3
gap> FH := GroupRing( GF(3), SmallGroup(27,3) );
<algebra-with-one over GF(3), with 3 generators>
gap> T := Units( FH );
#I  LAGUNA package: Computing the unit group ...
<group of size 5083731656658 with 27 generators>
gap> x := GeneratorsOfGroup( T )[1];
DirectProductElement( [ Z(3), (Z(3)^0)*<identity> of ... ] )
gap> x in FH;
false
gap> x[1] * x[2] in FH; # how to get the corresponding element of FH
true



##### 4.4-7 PcUnits
 ‣ PcUnits( KG ) ( attribute )

Returns: a group given by power-commutator presentation

Returns the unit group of KG as a direct product of Units(K) and PcNormalizedUnitGroup(KG), where the latter is a group given by a polycyclic presentation. See Section 3.4 for more details.


gap> W := PcUnits( KG );
<pc group of size 32768 with 15 generators>
gap> GeneratorsOfGroup( W )[5];
f5
gap> FH := GroupRing( GF(3), SmallGroup(27,3) );
<algebra-with-one over GF(3), with 3 generators>
gap> T := PcUnits(FH);
<group of size 5083731656658 with 27 generators>
gap> x := GeneratorsOfGroup( T )[2];
DirectProductElement( [ Z(3)^0, f1 ] )



##### 4.4-8 IsGroupOfUnitsOfMagmaRing
 ‣ IsGroupOfUnitsOfMagmaRing( U ) ( filter )

This property is set if U is a group generated by some units of a magma ring, including Units(KG) and NormalizedUnitgroup(KG).


gap> IsGroupOfUnitsOfMagmaRing( NormalizedUnitGroup( KG ) );
true
gap> IsGroupOfUnitsOfMagmaRing( Units( KG ) );
true



##### 4.4-9 IsUnitGroupOfGroupRing
 ‣ IsUnitGroupOfGroupRing( U ) ( filter )

This filter is set if U is the unit group of a $$p$$-modular group algebra, obtained either by Units(KG) or by PcUnits(KG). bound.


gap> IsUnitGroupOfGroupRing( Units( KG ) );
true
gap> IsUnitGroupOfGroupRing( PcUnits( KG ) );
true



##### 4.4-10 IsNormalizedUnitGroupOfGroupRing
 ‣ IsNormalizedUnitGroupOfGroupRing( U ) ( filter )

This filter is set if U is the normalized unit group of a $$p$$-modular group algebra, obtained either by NormalizedUnitGroup(KG) or by PcNormalizedUnitGroup(KG).


gap> IsNormalizedUnitGroupOfGroupRing( NormalizedUnitGroup( KG ) );
true
gap> IsNormalizedUnitGroupOfGroupRing( PcNormalizedUnitGroup( KG ) );
true



##### 4.4-11 UnderlyingGroupRing
 ‣ UnderlyingGroupRing( U ) ( attribute )

Returns: a group ring

If U is the (normalized) unit group of a $$p$$-modular group algebra $$KG$$ obtained using one of the functions Units(KG), PcUnits(KG), NormalizedUnitGroup(KG) or PcNormalizedUnitGroup(KG), then the attribute UnderlyingGroupRing stores $$KG$$.


gap> UnderlyingGroupRing( Units( KG ) );
<algebra-with-one of dimension 16 over GF(2)>
gap> UnderlyingGroupRing( PcUnits( KG ) );
<algebra-with-one of dimension 16 over GF(2)>
gap> UnderlyingGroupRing( NormalizedUnitGroup( KG ) );
<algebra-with-one of dimension 16 over GF(2)>
gap> UnderlyingGroupRing( PcNormalizedUnitGroup( KG ) );
<algebra-with-one of dimension 16 over GF(2)>



##### 4.4-12 UnitarySubgroup
 ‣ UnitarySubgroup( U ) ( attribute )

Returns: the subgroup of the unit group

Let U be the normalized unit group of a group ring in either natural (see NormalizedUnitGroup (4.4-1)) or power-commutator (see PcNormalizedUnitGroup (4.4-2)) presentation. The attribute stores the unitary subgroup of U, generated by all unitary units of U (see IsUnitary (4.2-9)). The method is straightforward, so it is not recommended to run it for large groups.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 8 ) );
<algebra-with-one over GF(2), with 3 generators>
gap> U := NormalizedUnitGroup( KG );
<group of size 128 with 7 generators>
gap> HU := UnitarySubgroup( U );
<group with 5 generators>
gap> IdGroup( HU );
[ 64, 261 ]
gap> V := PcNormalizedUnitGroup( KG );
<pc group of size 128 with 7 generators>
gap> HV := UnitarySubgroup( V );
Group([ f1, f2, f5, f6, f7 ])
gap> IdGroup( HV );
[ 64, 261 ]
gap> Image(NaturalBijectionToPcNormalizedUnitGroup( KG ), HU ) = HV;
true



##### 4.4-13 BicyclicUnitGroup
 ‣ BicyclicUnitGroup( U ) ( attribute )

Returns: the subgroup of the unit group, generated by bicyclic units

Let U be the normalized unit group of a group ring in either natural (see NormalizedUnitGroup (4.4-1)) or power-commutator (see PcNormalizedUnitGroup (4.4-2)) presentation. The attribute stores the subgroup of U, generated by all bicyclic units $$u_{g,h}$$ and $$v_{g,h}$$ (see BicyclicUnitOfType1 (4.2-12) and BicyclicUnitOfType2 (4.2-12)), where $$g$$ and $$h$$ run over the elements of the underlying group, and $$h$$ do not belongs to the normalizer of $$\langle g \rangle$$ in $$G$$.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 8 ) );
<algebra-with-one over GF(2), with 3 generators>
gap> U := NormalizedUnitGroup( KG );
<group of size 128 with 7 generators>
gap> BU := BicyclicUnitGroup( U );
<group with 2 generators>
gap> IdGroup( BU );
[ 4, 2 ]
gap> V := PcNormalizedUnitGroup( KG );
<pc group of size 128 with 7 generators>
gap> BV := BicyclicUnitGroup( V );
Group([ f5*f6, f5*f7 ])
gap> IdGroup( BV );
[ 4, 2 ]
gap> Image( NaturalBijectionToPcNormalizedUnitGroup( KG ), BU ) = BV;
true



##### 4.4-14 GroupBases
 ‣ GroupBases( KG ) ( attribute )

Returns: a list of lists of group rings elements

The subgroup $$B$$ of the normalized unit group of the group algebra $$KG$$ is called a group basis, if the elements of $$B$$ are linearly independent over the field $$K$$ and $$KB=KG$$. If KG is a $$p$$-modular group algebra, then GroupBases returns a list of representatives of the conjugacy classes of the group bases of the group algebra KG in its normalised unit group.


gap> D8 := DihedralGroup( 8 );
<pc group of size 8 with 3 generators>
gap> K := GF(2);
GF(2)
gap> KD8 := GroupRing( GF( 2 ), D8 );
<algebra-with-one over GF(2), with 3 generators>
gap> gb := GroupBases( KD8 );;
gap> Length( gb );
32
gap> Length( gb[1] );
8
gap> gb[1][1];
(Z(2)^0)*<identity> of ...
gap> ForAll(gb, b -> IdGroup(Group(b))=[8,3]);
true



#### 4.5 The Lie algebra of a group algebra

##### 4.5-1 LieAlgebraByDomain
 ‣ LieAlgebraByDomain( A ) ( method )

This method takes a group algebra as its argument, and constructs its associated Lie algebra in which the product is the bracket operation: $$[a,b]=ab-ba$$. It is recommended that the user never calls this method. The Lie algebra for an associative algebra should normally be created using LieAlgebra( A ). When LieAlgebra is first invoked, it constructs the Lie algebra for A using LieAlgebraByDomain. After that it stores this Lie algebra and simply returns it if LieAlgebra is called again.


gap> G := SymmetricGroup(3);; FG := GroupRing( GF( 2 ), G );
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>



##### 4.5-2 IsLieAlgebraByAssociativeAlgebra
 ‣ IsLieAlgebraByAssociativeAlgebra( L ) ( category )

This category signifies that the Lie algebra L was constructed as the Lie algebra associated with an associative algebra (this piece of information cannot be obtained later).


gap> KG := GroupRing( GF(3), DihedralGroup(16) );
<algebra-with-one over GF(3), with 4 generators>
gap> L := LieAlgebra ( KG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(3)>
gap> IsLieAlgebraByAssociativeAlgebra( L );
true



##### 4.5-3 UnderlyingAssociativeAlgebra
 ‣ UnderlyingAssociativeAlgebra( L ) ( attribute )

Returns: the underlying associative algebra of a Lie algebra

If a Lie algebra L is constructed from an associative algebra, then it remembers this underlying associative algebra as one of its attributes.


gap> KG := GroupRing( GF(2), DihedralGroup(16) );
<algebra-with-one over GF(2), with 4 generators>
gap> L := LieAlgebra ( KG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> UnderlyingAssociativeAlgebra( L );
<algebra-with-one over GF(2), with 4 generators>
gap> last = KG;
true



##### 4.5-4 NaturalBijectionToLieAlgebra
 ‣ NaturalBijectionToLieAlgebra( A ) ( attribute )

Returns: a mapping

The natural linear bijection between the (isomorphic, but not equal) underlying vector spaces of an associative algebra A and its associated Lie algebra is stored as an attribute of A. Note that this is a vector space isomorphism between two algebras, but not an algebra isomorphism.


gap> F := GF( 2 ); G := SymmetricGroup( 3 ); FG := GroupRing( F, G );
GF(2)
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> t := NaturalBijectionToLieAlgebra( FG );;
#I  LAGUNA package: Constructing Lie algebra ...
gap> a := Random( FG );
(Z(2)^0)*()+(Z(2)^0)*(2,3)+(Z(2)^0)*(1,2)+(Z(2)^0)*(1,2,3)
gap> a * a;                     # product in the associative algebra
(Z(2)^0)*()+(Z(2)^0)*(2,3)+(Z(2)^0)*(1,2)+(Z(2)^0)*(1,2,3)
gap> b := a^t;
LieObject( (Z(2)^0)*()+(Z(2)^0)*(2,3)+(Z(2)^0)*(1,2)+(Z(2)^0)*(1,2,3) )
gap> b * b; # product in the Lie algebra (commutator) - must be zero!
LieObject( <zero> of ... )



##### 4.5-5 NaturalBijectionToAssociativeAlgebra
 ‣ NaturalBijectionToAssociativeAlgebra( L ) ( attribute )

This is the inverse of the previous linear bijection, stored as an attribute of the Lie algebra L.


gap> G := SymmetricGroup(3); FG := GroupRing( GF( 2 ), G );
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> s := NaturalBijectionToAssociativeAlgebra( L );;
gap> InverseGeneralMapping( s ) = NaturalBijectionToLieAlgebra( FG );
true



##### 4.5-6 IsLieAlgebraOfGroupRing
 ‣ IsLieAlgebraOfGroupRing( L ) ( property )

If a Lie algebra L is constructed from an associative algebra which happens to be in fact a group ring, it has many nice properties that can be used for fast algorithms, so this information is stored as a property.


gap> F := GF( 2 ); G := SymmetricGroup( 3 ); FG := GroupRing( F, G );
GF(2)
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> IsLieAlgebraOfGroupRing( L );
true



##### 4.5-7 UnderlyingGroup
 ‣ UnderlyingGroup( L ) ( attribute )

Returns: the underlying group

The underlying group of a Lie algebra L that is constructed from a group ring is defined as the underlying group of this group ring; see UnderlyingGroup (4.1-4).


gap> F := GF( 2 ); G := SymmetricGroup( 3 ); FG := GroupRing( F, G );
GF(2)
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> UnderlyingGroup( L );
Sym( [ 1 .. 3 ] )
gap> LeftActingDomain( L );
GF(2)



##### 4.5-8 Embedding
 ‣ Embedding( U, L ) ( operation )

Returns: a mapping, which is a composition of two mappings

Let $$FG$$ be a group ring, let U be a submagma of $$G$$, and let L be the Lie algebra associated with $$FG$$. Then Embedding(U, L ) returns the obvious mapping from U to L (as the composition of the mappings Embedding( U, FG ) and NaturalBijectionToLieAlgebra( FG )).


gap> F := GF( 2 ); G := SymmetricGroup( 3 ); FG := GroupRing( F, G );
GF(2)
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> f := Embedding( G, L );;
gap> (1,2)^f + (1,3)^f;
LieObject( (Z(2)^0)*(1,2)+(Z(2)^0)*(1,3) )



##### 4.5-9 LieCentre
 ‣ LieCentre( L ) ( method )

Returns: a Lie algebra

The centre of the Lie algebra associated with a group ring corresponds to the centre of the underlying group ring, and it can be calculated very fast by considering the conjugacy classes of the group. This method returns the centre of L using this idea.


gap> G := SmallGroup( 256, 400 ); FG := GroupRing( GF( 2 ), G );
<pc group of size 256 with 8 generators>
<algebra-with-one over GF(2), with 8 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> C := LieCentre( L );
<Lie algebra of dimension 28 over GF(2)>
gap> D := LieDerivedSubalgebra( L );
#I  LAGUNA package: Computing the Lie derived subalgebra ...
<Lie algebra of dimension 228 over GF(2)>
gap> c := Dimension( C ); d := Dimension( D ); l := Dimension( L );
28
228
256
gap> c + d = l; # This is always the case for Lie algebras of group algebras!
true



##### 4.5-10 LieDerivedSubalgebra
 ‣ LieDerivedSubalgebra( L ) ( method )

Returns: a Lie algebra

If L is the Lie algebra associated with a group ring, then this method returns the Lie derived subalgebra of L. This can be done very fast using the conjugacy classes of the underlying group.


gap> G := SmallGroup( 256, 400 ); FG := GroupRing( GF( 2 ), G );
<pc group of size 256 with 8 generators>
<algebra-with-one over GF(2), with 8 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> C := LieCentre( L );
<Lie algebra of dimension 28 over GF(2)>
gap> D := LieDerivedSubalgebra( L );
#I  LAGUNA package: Computing the Lie derived subalgebra ...
<Lie algebra of dimension 228 over GF(2)>
gap> l := Dimension( L ); c := Dimension( C ); d := Dimension( D );
256
28
228
gap> c + d = l; # This is always the case for Lie algebras of group algebras!
true



##### 4.5-11 IsLieAbelian
 ‣ IsLieAbelian( L ) ( method )

The Lie algebra L of an associative algebra $$A$$ is Lie abelian, if and only if $$A$$ is abelian, so this method refers to IsAbelian( A ).


gap> G := SymmetricGroup( 3 ); FG := GroupRing( GF( 2 ), G);
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> IsAbelian( G );
false
gap> IsAbelian( L );    # This command should not be used for Lie algebras!
true
gap> IsLieAbelian( L ); # Instead, IsLieAbelian is the correct command.
false



##### 4.5-12 IsLieSolvable
 ‣ IsLieSolvable( L ) ( method )

In [PPS73] Passi, Passman, and Sehgal have classified all groups $$G$$ such that the Lie algebra associated with the group ring is solvable. This method uses their classification, making it considerably faster than the more elementary method which just calculates Lie commutators.


gap> G := SmallGroup( 256, 400 ); FG := GroupRing( GF( 2 ), G );
<pc group of size 256 with 8 generators>
<algebra-with-one over GF(2), with 8 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> IsLieSolvable( L );                       # This is very fast.
#I  LAGUNA package: Checking Lie solvability ...
true
gap> List( LieDerivedSeries( L ), Dimension ); # This is very slow.
#I  LAGUNA package: Computing the Lie derived subalgebra ...
[ 256, 228, 189, 71, 0 ]



##### 4.5-13 IsLieNilpotent
 ‣ IsLieNilpotent( L ) ( method )

In [PPS73] Passi, Passman, and Sehgal have classified all groups $$G$$ such that the Lie algebra associated with the group ring is Lie nilpotent. This method uses their classification, making it considerably faster than the more elementary method which just calculates Lie commutators.


gap> G := SmallGroup( 256, 400 ); FG := GroupRing( GF( 2 ), G );
<pc group of size 256 with 8 generators>
<algebra-with-one over GF(2), with 8 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> IsLieNilpotent( L );                           # This is very fast.
#I  LAGUNA package: Checking Lie nilpotency ...
true
gap> List( LieLowerCentralSeries( L ), Dimension ); # This is very slow.
#I  LAGUNA package: Computing the Lie derived subalgebra ...
[ 256, 228, 222, 210, 191, 167, 138, 107, 76, 54, 29, 15, 6, 0 ]



##### 4.5-14 IsLieMetabelian
 ‣ IsLieMetabelian( L ) ( property )

In [LR86] Levin and Rosenberger have classified all groups $$G$$ such that the Lie algebra associated with the group ring is Lie metabelian. This method uses their classification, making it considerably faster than the more elementary method which just calculates Lie commutators.


gap> G := SmallGroup( 256, 400 ); FG := GroupRing( GF( 2 ), G );
<pc group of size 256 with 8 generators>
<algebra-with-one over GF(2), with 8 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> IsLieMetabelian( L );
false



##### 4.5-15 IsLieCentreByMetabelian
 ‣ IsLieCentreByMetabelian( L ) ( property )

In [Ros02] the third author of this package classified all groups $$G$$ such that the Lie algebra associated with the group ring is Lie centre-by-metabelian. This method uses the classification, making it considerably faster than the more elementary method which just calculates Lie commutators.


gap> G := SymmetricGroup( 3 ); FG := GroupRing( GF( 2 ), G );
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> IsLieMetabelian( L );
false
gap> IsLieCentreByMetabelian( L );
true



##### 4.5-16 CanonicalBasis
 ‣ CanonicalBasis( L ) ( method )

Returns: basis of a Lie algebra

The canonical basis of a group algebra $$FG$$ is formed by the elements of $$G$$. Here L is the Lie algebra associated with $$FG$$, and the method returns the images of the elements of $$G$$ in L.


gap> G := SymmetricGroup( 3 ); FG := GroupRing( GF( 2 ), G );
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> B := CanonicalBasis( L );
CanonicalBasis( <Lie algebra of dimension 6 over GF(2)> )
gap> Elements( B );
[ LieObject( (Z(2)^0)*() ), LieObject( (Z(2)^0)*(2,3) ),
LieObject( (Z(2)^0)*(1,2) ), LieObject( (Z(2)^0)*(1,2,3) ),
LieObject( (Z(2)^0)*(1,3,2) ), LieObject( (Z(2)^0)*(1,3) ) ]



##### 4.5-17 IsBasisOfLieAlgebraOfGroupRing
 ‣ IsBasisOfLieAlgebraOfGroupRing( B ) ( property )

A basis B has this property if the preimages of the basis vectors in the group algebra form a group. It can be verified if a basis has this property. This is important for the speed of the calculation of the structure constants table; see StructureConstantsTable (4.5-18).


gap> G := SymmetricGroup( 3 ); FG := GroupRing( GF( 2 ), G );
Sym( [ 1 .. 3 ] )
<algebra-with-one over GF(2), with 2 generators>
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> B := CanonicalBasis( L );
CanonicalBasis( <Lie algebra of dimension 6 over GF(2)> )
gap> IsBasisOfLieAlgebraOfGroupRing( B );
true



##### 4.5-18 StructureConstantsTable
 ‣ StructureConstantsTable( B ) ( method )

A very fast implementation for calculating the structure constants table for the Lie algebra L associated with a group ring with respect to its canonical basis B using its special structure; see CanonicalBasis (4.5-16).


gap> G := CyclicGroup( 2 );; FG := GroupRing( GF( 2 ), G );;
gap> L := LieAlgebra( FG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> B := CanonicalBasis( L );
CanonicalBasis( <Lie algebra of dimension 2 over GF(2)> )
gap> StructureConstantsTable( B );
#I  LAGUNA package: Computing the structure constants table ...
[ [ [ [  ], [  ] ], [ [  ], [  ] ] ], [ [ [  ], [  ] ], [ [  ], [  ] ] ], -1,
0*Z(2) ]



##### 4.5-19 LieUpperNilpotencyIndex
 ‣ LieUpperNilpotencyIndex( KG ) ( attribute )

In a modular group algebra $$KG$$ the upper Lie power series is defined as follows: $$KG^{(1)}=KG$$, $$KG^{(n+1)}$$ is the associative ideal, generated by $$[KG^{(n)},KG]$$. The upper Lie nilpotency index $$t^L(G)$$ of the group algebra $$KG$$ is defined to be the smallest number $$n$$ such that $$KG^{(n)}=0$$. It can be calculated very fast using Lie dimension subgroups [Sha91], that is, using only information about the underlying group; see LieDimensionSubgroups (4.6-4). This is why it is stored as an attribute of the group algebra KG rather than that of its associated Lie algebra.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> LieUpperNilpotencyIndex( KG );
5



##### 4.5-20 LieLowerNilpotencyIndex
 ‣ LieLowerNilpotencyIndex( KG ) ( attribute )

In a modular group algebra $$KG$$ the lower Lie power series is defined as follows: $$KG^{[n]}$$ is the associative ideal, generated by all (left-normed) Lie-products $$[x_1, x_2, \dots, x_n]$$, $$x_i \in KG$$. The lower Lie nilpotency index $$t_L(G)$$ of the group algebra $$KG$$ is defined to be the minimal smallest $$n$$ such that $$KG^{[n]}=0$$. In [Du92] the Jennings' conjecture was proved, which means that the nilpotency class of the normalized unit group of the modular group algebra $$KG$$ is equal to $$t_L(G)-1$$.

This allows to express lower Lie nilpotency index via the nilpotency class of the normalized unit group, and with its polycyclic presentation, provided by LAGUNA, this will be faster than elementary calculations with Lie commutators. As the previous attribute, this index is also stored as an attribute of the group algebra KG.


gap> KG := GroupRing( GF( 2 ), DihedralGroup( 16 ) );
<algebra-with-one over GF(2), with 4 generators>
gap> LieLowerNilpotencyIndex( KG );
5



##### 4.5-21 LieDerivedLength
 ‣ LieDerivedLength( L ) ( attribute )

Let $$L$$ be a Lie algebra. The Lie derived series of $$L$$ is defined as follows: $$\delta^{[0]}(L) = L$$ and $$\delta^{[n]}(L) = [\delta^{[n-1]}(L), \delta^{[n-1]}(L)]$$. $$L$$ is called Lie solvable if there exists an integer $$m$$ such that $$\delta^{[m]}(L) = 0$$. In this case the integer $$m$$ is called the Lie derived length of $$L$$, and it is returned by this function.


gap> KG := GroupRing( GF ( 2 ), DihedralGroup( 16 ) );;
gap> L := LieAlgebra( KG );
#I  LAGUNA package: Constructing Lie algebra ...
<Lie algebra over GF(2)>
gap> LieDerivedLength( L );
#I  LAGUNA package: Computing the Lie derived subalgebra ...
3



#### 4.6 Other commands

##### 4.6-1 SubgroupsOfIndexTwo
 ‣ SubgroupsOfIndexTwo( G ) ( attribute )

Returns a list of subgroups of $$G$$ with index two. Such subgroups are important for the investigation of the Lie structure of the group algebra $$KG$$ in the case of characteristic 2.


gap> SubgroupsOfIndexTwo( DihedralGroup( 16 ) );
[ Group([ f3, f4, f1 ]), Group([ f3, f4, f2 ]), Group([ f3, f4, f1*f2 ]) ]



##### 4.6-2 DihedralDepth
 ‣ DihedralDepth( U ) ( method )

For a finite 2-group U, the function returns its dihedral depth, which is defined to be the maximal number $$d$$ such that U contains a subgroup isomorphic to the dihedral group of order $$2^{d+1}$$.


gap> KD8 := GroupRing( GF(2), DihedralGroup( 8 ) );
<algebra-with-one over GF(2), with 3 generators>
gap> UD8 := PcNormalizedUnitGroup( KD8 );
<pc group of size 128 with 7 generators>
gap> DihedralDepth( UD8 );
2



##### 4.6-3 DimensionBasis
 ‣ DimensionBasis( G ) ( method )

Returns: record with two components: dimensionBasis' (list of group elements) and weights' (list of weights)

For a finite $$p$$-group G, returns its Jennings basis as it was described in Section 3.3.


gap> G := DihedralGroup( 16 );
<pc group of size 16 with 4 generators>
gap> DimensionBasis( G );
rec( dimensionBasis := [ f1, f2, f3, f4 ], weights := [ 1, 1, 2, 4 ] )



##### 4.6-4 LieDimensionSubgroups
 ‣ LieDimensionSubgroups( G ) ( attribute )

Returns: list of subgroups

For a finite $$p$$-group G, returns the series of its Lie dimension subgroups. The $$m$$-th Lie dimension subgroup $$D_{(m)}$$ is the intersection of the group $$G$$ and $$1+KG^{(m)}$$, where $$KG^{(m)}$$ is the $$m$$-th term of the upper Lie power series of $$KG$$; see LieUpperNilpotencyIndex (4.5-19)


gap> G := DihedralGroup( 16 );
<pc group of size 16 with 4 generators>
gap> LieDimensionSubgroups( G );
[ <pc group of size 16 with 4 generators>, Group([ f3, f4 ]), Group([ f4 ]),
Group([ <identity> of ... ]) ]



##### 4.6-5 LieUpperCodimensionSeries
 ‣ LieUpperCodimensionSeries( KG ) ( attribute )
 ‣ LieUpperCodimensionSeries( G ) ( attribute )

Returns: list of subgroups

A notion of upper Lie codimension subgroups was introduced in [CS06]. For a finite $$p$$-group G, $$C_i$$ is the set of all elements $$g$$ in G, such that the Lie commutator $$[ g, g_1, ..., g_i ]$$ of the length $$i+1$$ is equal to zero for all $$g_1, ..., g_i$$ from G, and $$C_0 = {1}$$. By Du's theorem (see [Du92]), $$C_i$$ coincides with the intersection of $$G$$ and the i-th term of the upper central series $${1}=Z_0 < Z_1 < Z_2 < ... < Z_n = V(KG)$$ of the normalized unit group $$V(KG)$$. This fact is used in LAGUNA to speed up computation of this series. Since $$V(KG)$$ is involved in computation, for the first time the argiment should be the group ring KG, but later you can also apply it to the group G itself.


gap> G := DihedralGroup(16);
<pc group of size 16 with 4 generators>
gap> KG := GroupRing( GF(2), G );
<algebra-with-one over GF(2), with 4 generators>
gap> LieUpperCodimensionSeries( KG );
[ Group([ f1, f2, f3, f4 ]), Group([ f3, f4, f3*f4 ]), Group([ f4 ]),
Group([ f4 ]), Group([  ]) ]
gap> LieUpperCodimensionSeries( G );
[ Group([ f1, f2, f3, f4 ]), Group([ f3, f4, f3*f4 ]), Group([ f4 ]),
Group([ f4 ]), Group([  ]) ]



##### 4.6-6 LAGInfo
 ‣ LAGInfo ( info class )

LAGInfo is a special Info class for LAGUNA algorithms. It has 5 levels: 0, 1 (default), 2, 3 and 4. To change info level to k, use command SetInfoLevel(LAGInfo, k).


gap> SetInfoLevel( LAGInfo, 2 );
gap> KD8 := GroupRing( GF( 2 ), DihedralGroup( 8 ) );
<algebra-with-one over GF(2), with 3 generators>
gap> UD8 := PcNormalizedUnitGroup( KD8 );
#I  LAGInfo: Computing the pc normalized unit group ...
#I  LAGInfo: Calculating weighted basis ...
#I  LAGInfo: Calculating dimension basis ...
#I  LAGInfo: dimension basis finished !
#I  LAGInfo: Weighted basis finished !
#I  LAGInfo: Computing the augmentation ideal filtration...
#I  LAGInfo: Filtration finished !
#I  LAGInfo: finished, converting to PcGroup
<pc group of size 128 with 7 generators>


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