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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 * g_1 + alpha_2 * g_2 + cdots + alpha_k * 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 * 1 + alpha_2 * g_2 + cdots + alpha_k * 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 * g_1 + alpha_2 * g_2 + cdots + alpha_k * 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 * g_1 + alpha_2 * g_2 + cdots + alpha_k * 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 ) | ( operation )

`> Involution`( x ) | ( operation )

Returns: an element of a group ring

Let KG be a group ring and let f be a mapping G -> G, such that f^2 is the identity mapping on G. Then the involution of KG induced by f is defined by alpha_1 * g_1 + alpha_2 * g_2 + cdots + alpha_k * g_k -> alpha_1 * f(g_1) + alpha_2 * f(g_2) + cdots + alpha_k * f(g_k). This method returns the image of x under the involution of KG with respect to f.

In the second form the function returns the result of the so-called classical involution, which is the involution induced by the map x -> 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
    # let's check the action of involution on elements from the group G
    gap> l := List( MinimalGeneratingSet( G ), g -> g^Embedding( G, KG ) );
    [ (Z(2)^0)*f1, (Z(2)^0)*f2 ]
    gap> List( l, Involution );
    [ (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
    # let's check that elements of the group G are unitary
    gap> l:=List(MinimalGeneratingSet(G),g -> g^Embedding(G,KG));
    [ (Z(2)^0)*f1, (Z(2)^0)*f2 ]
    gap> List(l,IsUnitary);
    [ true, true ]

##### 4.2-10 IsUnit

`> IsUnit`( KG, x ) | ( method )

`> IsUnit`( x ) | ( method )

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

In the first form 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.

In the second form LAGUNA first 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 )

`> BicyclicUnitOfType1`( a, g ) | ( operation )

`> BicyclicUnitOfType2`( KG, a, g ) | ( operation )

`> BicyclicUnitOfType2`( 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(< a >), and the units u_a,g = 1 + (a-1) * g * alpha and v_a,g = 1 + alpha * g * (a-1) are called bicyclic units. 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(< a >), 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

#### 4.3 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, ..., b_n for the augmentation ideal A, for which there are indices i_1=1, ..., i_c-1 such that b_i_k, ..., 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)*f2, (Z(2)^0)*<identity>
            of ...+(Z(2)^0)*f1, (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> AugmentationIdealPowerSeries( KG );
    [ <algebra of dimension 15 over GF(2)>, <algebra of dimension 13 over GF(2)>,
      <algebra of dimension 11 over GF(2)>, <algebra of dimension 9 over GF(2)>,
      <algebra of dimension 7 over GF(2)>, <algebra of dimension 5 over GF(2)>,
      <algebra of dimension 3 over GF(2)>, <algebra of dimension 1 over GF(2)>,
      <algebra over GF(2)> ]
    gap> Length(last);
    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 )

`> LeftIdealBySubgroup`( 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)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2

##### 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)*f1+(Z(2)^0)*f2+(Z(2)^0)*f1*f2
    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 );
    [ <pc group of size 32768 with 15 generators>,
      <pc group of size 4096 with 12 generators>,
      Group([ f3*f5*f13*f15, f7, f15, f13*f15, f14*f15, f11*f13*f14*f15, f12,
          f9*f12, f10 ]),
      Group([ f3*f5*f13*f15, f7, f15, f13*f15, f14*f15, f11*f13*f14*f15 ]),
      Group([  ]) ]
    gap> f := Embedding( G, V );
    [ f1, f2, f3, f4 ] -> [ f2, f1, f3, f7 ]
    gap> G1 := Image( f, G );
    Group([ f2, f1, f3, f7 ])
    gap> H := Intersection( ucs[2], G1 );
    Group([ f3, f7, f3*f7 ])
    # H is the intersection of G and the 3rd centre of V(KG)
    gap> T:=PreImage( f, H );
    Group([ f3, f4, f3*f4 ])
    # and T is its preimage in G
    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>
    # now elements of U are already in KG
    gap> GeneratorsOfGroup( U )[5];
    (Z(2)^0)*f2+(Z(2)^0)*f3+(Z(2)^0)*f2*f3
    # in the next example the direct product structure is more clear
    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];
    Tuple( [ Z(3), (Z(3)^0)*<identity> of ... ] )
    gap> x in FH;
    false
    gap> x[1] * x[2] in FH;
    true # this is the way to get the corresponding element of FH

##### 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
    # in the next example the direct product structure is more clear
    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];
    Tuple( [ Z(3)^0, f1 ] )

##### 4.4-8 IsGroupOfUnitsOfMagmaRing

`> IsGroupOfUnitsOfMagmaRing`( U ) | ( property )

This property will be automatically set `true`, if U is a group generated by some units of a magma ring, including `Units(KG)` and `NormalizedUnitgroup(KG)`. Otherwise this property will not be bound.

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

##### 4.4-9 IsUnitGroupOfGroupRing

`> IsUnitGroupOfGroupRing`( U ) | ( property )

This property will be automatically set `true`, if U is the unit group of a p-modular group algebra, obtained either by `Units(KG)` or by `PcUnits(KG)`. Otherwise this property will not be bound.

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

##### 4.4-10 IsNormalizedUnitGroupOfGroupRing

`> IsNormalizedUnitGroupOfGroupRing`( U ) | ( property )

This property will be automatically set `true`, if U is the normalized unit group of a p-modular group algebra, obtained either by `NormalizedUnitGroup(KG)` or by `PcNormalizedUnitGroup(KG)`. Otherwise this property will not be bound.

    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 over GF(2), with 4 generators>
    gap> UnderlyingGroupRing( PcUnits( KG ) );
    <algebra-with-one over GF(2), with 4 generators>
    gap> UnderlyingGroupRing( NormalizedUnitGroup( KG ) );
    <algebra-with-one over GF(2), with 4 generators>
    gap> UnderlyingGroupRing( PcNormalizedUnitGroup( KG ) );
    <algebra-with-one over GF(2), with 4 generators>

##### 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 `BicyclicUnitOfType1` (4.2-12)), where g and h run over the elements of the underlying group, and h do not belongs to the normalizer of < g > 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, f6*f7 ])
    gap> IdGroup( BV );
    [ 4, 2 ]
    gap> Image( NaturalBijectionToPcNormalizedUnitGroup( KG ), BU ) = BV;
    true

##### 4.4-14 AugmentationIdealPowerFactorGroup

`> AugmentationIdealPowerFactorGroup`( KG, n ) | ( operation )

Returns: PcGroup

????????????????????????????????????????

##### 4.4-15 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> gb[1];
    [ (Z(2)^0)*<identity> of ..., (Z(2)^0)*f3,
      (Z(2)^0)*f1*f2+(Z(2)^0)*f2*f3+(Z(2)^0)*f1*f2*f3,
      (Z(2)^0)*f2+(Z(2)^0)*f1*f2+(Z(2)^0)*f1*f2*f3,
      (Z(2)^0)*<identity> of ...+(Z(2)^0)*f2+(Z(2)^0)*f3+(Z(2)^0)*f2*f3+(Z(2)^
        0)*f1*f2*f3, (Z(2)^0)*f2+(Z(2)^0)*f1*f3+(Z(2)^0)*f2*f3,
      (Z(2)^0)*<identity> of ...+(Z(2)^0)*f2+(Z(2)^0)*f3+(Z(2)^0)*f1*f2+(Z(2)^
        0)*f2*f3, (Z(2)^0)*f1+(Z(2)^0)*f2+(Z(2)^0)*f2*f3 ]
    gap> Length( last );
    8

#### 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 );
    MappingByFunction( <algebra-with-one over GF(2), with
    2 generators>, <Lie algebra over GF(
    2)>, <Operation "LieObject">, function( y ) ... end )
    gap> a := Random( FG );
    (Z(2)^0)*(1,2,3)+(Z(2)^0)*(1,3,2)+(Z(2)^0)*(1,3)
    gap> a * a;                     # product in the associative algebra
    (Z(2)^0)*()+(Z(2)^0)*(1,2,3)+(Z(2)^0)*(1,3,2)
    gap> b := a^t;
    LieObject( (Z(2)^0)*(1,2,3)+(Z(2)^0)*(1,3,2)+(Z(2)^0)*(1,3) )
    gap> b * b;                     # product in the Lie algebra (commutator) ...
    LieObject( <zero> of ... )      # ... must be zero!

##### 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 );
    MappingByFunction( <Lie algebra over GF(2)>, <algebra-with-one over GF(
    2), with 2 generators>, function( y ) ... end, <Operation "LieObject"> )
    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 );
    CompositionMapping( MappingByFunction( <algebra-with-one over GF(2), with
    2 generators>, <Lie algebra over GF(
    2)>, <Operation "LieObject">, function( y ) ... end ), <mapping: SymmetricGrou\
    p( [ 1 .. 3 ] ) -> AlgebraWithOne( GF(2), ... ) > )
    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;
    true  # This is always the case for Lie algebras of group algebras!

##### 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;
    true  # This is always the case for Lie algebras of group algebras!

##### 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 never be used for Lie algebras!
    true                    # It gives a result, but (probably) not the desired one.
    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.
    true
    gap> List( LieDerivedSeries( L ), Dimension ); # This is very slow.
    [ 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.
    true
    gap> List( LieLowerCentralSeries( L ), Dimension ); # This is very slow.
    [ 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 [R02] 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 );
    <pc group of size 2 with 1 generators>
    <algebra-with-one over GF(2), with 1 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 2 over GF(2)> )
    gap> StructureConstantsTable( B );
    [ [ [ [  ], [  ] ], [ [  ], [  ] ] ], [ [ [  ], [  ] ], [ [  ], [  ] ] ], -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 [S91], 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 [D92] 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([ f1, f1*f3, f1*f4, f1*f3*f4 ]), Group([ f2, f2*f3, f2*f4, f2*f3*f4 ]),
      Group([ f1*f2, f1*f2*f3, f1*f2*f4, f1*f2*f3*f4 ]) ]

##### 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 [D92]), 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>

##### 4.6-7 LAGUNABuildManual

`> LAGUNABuildManual`( ) | ( function )

This function is used to build the manual in the following formats: DVI, PDF, PS, HTML and text for online help. We recommend that the user should have a recent and fairly complete TeX distribution. Since LAGUNA is distributed together with its manual, it is not necessary for the user to use this function. Normally it is intended to be used by the developers only. This is the only function of LAGUNA which requires UNIX/Linux environment.

##### 4.6-8 LAGUNABuildManualHTML

`> LAGUNABuildManualHTML`( ) | ( function )

This fuction is used to build the manual only in HTML format. This does not depend on the availability of the TeX installation and works under Windows and MacOS as well. Since LAGUNA is distributed together with its manual, it is not necessary for the user to use this function. Normally it is intended to be used by the developers only.

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