GAP (UnitLib) - Chapter 2: UnitLib functions

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### 2. UnitLib functions

Since the main purpose of UnitLib is the storage of large amount of data, it has only two main user functions: to read the description of V(KG) for the given catalogue number of G in the Small Groups Libary of the GAP system, and to save the description of V(KG) if the user would like to store it for further usage for the group that is not contained in the library.

Examples below contain some functions from the LAGUNA package [BK+], see their description in the LAGUNA manual.

To use the UnitLib package first you need to load it as follows:

    gap> LoadPackage("unitlib");
    ----------------------------------------------------------------------------
    Loading  UnitLib 1.0 (Library of normalized unit groups of modular group algebras)
    by Alexander Konovalov (http://homepages.vub.ac.be/~okonoval/) and
       Elena Yakimenko (k-algebra@zsu.zp.ua).
    ----------------------------------------------------------------------------
    true
    gap>

In case of non-UNIX system, a warning will be displayed about the non-availability of the library of normalized unit groups for groups of order 128.

#### 2.1 MainFunctions

##### 2.1-1 PcNormalizedUnitGroupSmallGroup

`> PcNormalizedUnitGroupSmallGroup`( s, n ) | ( function )

Returns: PcGroup

Let s be a power of prime p and n is an integer from `[ 1 .. NrSmallGroups(s) ]`. `PcNormalizedUnitGroupSmallGroup(s,n)` returns the normalized unit group V(KG) of the modular group algebra KG, where G is `SmallGroup(s,n)` and K is a field of p elements.

    gap> PcNormalizedUnitGroupSmallGroup(128,161);
    <pc group of size 170141183460469231731687303715884105728 with 127 generators>

The result returned by `PcNormalizedUnitGroupSmallGroup` will be equivalent to the following sequence of commands:

    gap> G := SmallGroup( s, n );
    gap> p := PrimePGroup( G );
    gap> K := GF( p );
    gap> KG := GroupRing( K, G );
    gap> PcNormalizedUnitGroup( KG );

Nevertheless, `PcNormalizedUnitGroupSmallGroup` is not just a shortcut for such computation. It reads the description of the normalized unit group from the UnitLib library and then reconstructs all its other necessary attributes and properties. Thus, if you would like to obtain the group algebra KG or the field K and the group G, you should extract them from V(KG), which should be constructed first.

    gap> V:=PcNormalizedUnitGroup(GroupRing(GF(2),SmallGroup(8,3)));
    <pc group of size 128 with 7 generators>
    gap> V1:=PcNormalizedUnitGroupSmallGroup(8,3);
    <pc group of size 128 with 7 generators>
    gap> V1=V;     # two isomorphic groups but not identical objects
    false
    gap> IdGroup(V)=IdGroup(V1);
    true
    gap> IsomorphismGroups(V,V1);
    [ f1, f2, f3, f4, f5, f6, f7 ] -> [ f1, f2, f3, f4, f5, f6, f7 ]
    gap> KG:=UnderlyingGroupRing(V1);  # now the correct way
    <algebra-with-one over GF(2), with 3 generators>
    gap> V1=PcNormalizedUnitGroup(KG); # V1 is an attribite of KG
    true
    gap> K:=UnderlyingField(KG);
    GF(2)
    gap> G:=UnderlyingGroup(KG);
    <pc group of size 8 with 3 generators>

If the order s is not a power of prime, an error message will appear. If s is bigger than 128, you will get a warning telling that the library does not contain V(KG) for G of such order, and you can use only data stored by yourself in your `unitlib/userdata` directory with the help of the function `SavePcNormalizedUnitGroup` (2.1-2).

##### 2.1-2 SavePcNormalizedUnitGroup

`> SavePcNormalizedUnitGroup`( G ) | ( property )

Returns: true

Let G be a finite p-group of order s from the Small Groups Library of the GAP system, constructed with the help of `SmallGroup(s,n)`. Then `SavePcNormalizedUnitGroup(G)` creates the file with the name of the form `us_n.g` in the directory `unitlib/userdata`, and returns `true` if this file was successfully generated. This file contains the description of the normalized unit group V(KG) of the group algebra of the group G over the field of p elements.

If the order of G is greater than 128, after this you can construct the group V(KG) using `PcNormalizedUnitGroupSmallGroup` (2.1-1) similarly to the previous section. The preliminary warning will be displayed, telling that for such orders you can use only those groups that were already computed and saved by you to the `unitlib/userdata` directory. If there will be no such file there, you will get an error message, otherwise the computation will begin.

If the order of G is less or equal to 128, then the file will be created in the `unitlib/userdata` directory, but UnitLib will continue to use the file with the same name from the appropriate directory in `unitlib/data`. You can compare these two files to make it sure that they are the same.

WARNINGS:

1. It is important to apply this function to the underlying group G and not to the normalized unit group V(KG).

2. The user should use as an argument only groups from the Small Groups Library of the GAP system, constructed with the help of `SmallGroup(s,n)`, otherwise the consistency of data may be lost.

    gap> SavePcNormalizedUnitGroup( SmallGroup( 256, 56092 ) );
    true
    gap> PcNormalizedUnitGroupSmallGroup( 256, 56092 );
    WARNING : the library of V(KG) for groups of order
    256 is not available yet !!!
    You can use only groups from the unitlib/userdata directory
    in case if you already computed their descriptions
    (See the manual for SavePcNormalizedUnitGroup)

    Description of V(KG) for G=SmallGroup(256,
    56092) accepted, started its generation
    <pc group of size
    57896044618658097711785492504343953926634992332820282019728792003956564819968
      with 255 generators>

#### 2.2 Service tools

##### 2.2-1 UNITLIBBuildManual

`> UNITLIBBuildManual`( ) | ( 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 UnitLib 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 UnitLib which requires UNIX/Linux environment.

##### 2.2-2 UNITLIBBuildManualHTML

`> UNITLIBBuildManualHTML`( ) | ( 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 UnitLib 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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