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# 1 Introduction to the AtlasRep Package

### Sections

1. An Atlas of Group Representations
2. The GAP Interface to the Atlas of Group Representations
3. Web Services for the AtlasRep Package
4. Local or Remote Installation of the AtlasRep Package
5. Installing the AtlasRep Package
6. Maintaining the Local Data of the AtlasRep Package
7. User Parameters for the AtlasRep Package
8. Loading the AtlasRep Package
9. Extending the Atlas Database
10. What's New in Version 1.2?
11. What's New in Version 1.1?
12. Acknowledgements

The aim of the GAP 4 package AtlasRep is to provide a link between GAP and the ``ATLAS of Group Representations'', a database that comprises representations of many almost simple groups and information about their maximal subgroups. This database has been available independent of GAP at http://web.mat.bham.ac.uk/atlas/v2.0

The AtlasRep package consists of this database (see An Atlas of Group Representations) and a GAP interface (see The GAP Interface to the Atlas of Group Representations); the latter is extended by further information available via the internet (see Web Services for the AtlasRep Package).

Information about installing and customizing the package can be found in the sections Local or Remote Installation of the AtlasRep Package, Installing the AtlasRep Package, Maintaining the Local Data of the AtlasRep Package, User Parameters for the AtlasRep Package, Loading the AtlasRep Package, and Extending the Atlas Database.

Finally, the sections What's New in Version 1.2? and What's New in Version 1.1? list the changes w.r.t. previous releases of the package, and Section Acknowledgements acknowledges contributions of non-authors to the package.

## 1.1 An Atlas of Group Representations

The ATLAS of Group Representations consists of matrices over various rings, permutations, and shell scripts encoding so-called straight line programs (see BSW01, SWW00, and Straight Line Programs in the GAP Reference Manual). These programs can be used to compute certain elements in a group G from its standard generators (see Wil96 and Standard Generators of Groups in the GAP Reference Manual) of G, for example generators of maximal subgroups of G or representatives of conjugacy classes of G.

The ATLAS of Group Representations has been prepared by Robert Wilson, Peter Walsh, Jonathan Tripp, Ibrahim Suleiman, Stephen Rogers, Richard Parker, Simon Norton, Steve Linton, and John Bray (in reverse alphabetical order).

The information was computed and composed using computer algebra systems such as MeatAxe (see Rin98), Magma (see CP96), and GAP (in reverse alphabetical order). Part of the constructions have been documented in the literature on almost simple groups, or the results have been used in such publications, see for example the references in CCN85 and BN95.

If you use the ATLAS of Group Representations to solve a problem then please send a short email to R.A.Wilson@bham.ac.uk about it. The ATLAS of Group Representations database should be referenced with the entry AGR in the bibliography of this manual.

If your work made use of functions of the GAP interface (see The GAP Interface to the Atlas of Group Representations) then you should also reference this interface, via the entry AtlasRep in the bibliography of this manual.

For referencing the GAP system in general, use the entry GAP4 in the bibliography of this manual.

## 1.2 The GAP Interface to the Atlas of Group Representations

The GAP interface to the ATLAS of Group Representations consists of essentially two parts. First, there is the user interface which allows the user to get an overview of the contents of the database, and to access the data in GAP format; this is described in Chapter The User Interface of the AtlasRep Package. Second, there is administrational information which is important only for users interested in the actual implementation (e. g., for modifying the package) or in using it together with the C-MeatAxe standalone (see Rin98); this is described in Chapter Technicalities of the AtlasRep Package.

Information concerning the C-MeatAxe, including the manual Rin98, can be found at http://www.math.rwth-aachen.de/LDFM/homes/MTX

A GAP 4 Package that provides access to the functionality of the C-MeatAxe is in preparation.

The GAP interface should be regarded as preliminary. Hopefully it will become more user-friendly when the ATLAS of Group Representations will be integrated into a larger GAP database of groups and their representations, character tables, and tables of marks.

The interface and this manual have been provided by Thomas Breuer. Comments, bug reports, and hints for improving the interface can be sent to sam@math.rwth-aachen.de.

## 1.3 Web Services for the AtlasRep Package

The home page of the AtlasRep package is http://www.math.rwth-aachen.de/~Thomas.Breuer/atlasrep

Besides package archives and introductory package information, it provides

-
  the current file with the table of contents (`gap/atlasprm.g`), which is needed for those who use `wget` for accessing data files (see User Parameters for the AtlasRep Package),
-
  the list of changes of server files in HTML format (cf. Maintaining the Local Data of the AtlasRep Package), and
-
  an overview of the data available via the GAP interface to the ATLAS of Group Representations, in HTML format; this is similar to the information shown by `DisplayAtlasInfo` (see DisplayAtlasInfo); further information can be found on the home page of the ATLAS (see the introduction to this chapter).

## 1.4 Local or Remote Installation of the AtlasRep Package

There are two possibilities to install the AtlasRep package.

Local installation
  You can install the whole database physically as a part of the local copy of GAP; see User Parameters for the AtlasRep Package for the amount of disk space needed in this case.

Remote installation
  If your computer is connected to a network that provides access to the ATLAS data (for example the internet) then you can alternatively install just the functions of the package. It is then left to these functions to fetch the requested data automatically from remote servers when they are required for the first time; these data are then stored in the local copy, later access to them needs no network transfer.

This is controlled by the component `remote` of the global variable `AtlasOfGroupRepresentationsInfo` (see AtlasOfGroupRepresentationsInfo).

The latter possibility is presently not used by other GAP packages, so it may be regarded as an important feature of the AtlasRep package. Anyhow it requires a few sentences of explanation.

The possibility of a remote installation reflects in particular the fact that the ATLAS of Group Representations is designed as an open database, it is expected to grow. As soon as the developers of the ATLAS of Group Representations add new information to the servers, these data become available in remote installations of the package after updating the corresponding table of contents (see Updating the Tables of Contents of the AtlasRep Package).

Currently there is just one remote server. As soon as new servers become available, or a server name is changed (see Global Variables Used by the AtlasRep Package), which makes it necessary to adjust the installation of the AtlasRep package, this will be announced in the GAP Forum (see Further Information about GAP in the GAP Tutorial). The same holds when upgrades of the package become available. Additionally, the GAP servers will provide this information.

## 1.5 Installing the AtlasRep Package

To install the package unpack the archive file in a directory in the `pkg` directory of your local copy of GAP 4. This might be the `pkg` directory of the GAP 4 home directory, see Section Installing a GAP Package of the GAP 4 Reference Manual for details. It is however also possible to keep an additional `pkg` directory in your private directories, see Section GAP Root Directory of the GAP 4 Reference Manual. The latter possibility must be chosen if you do not have write access to the GAP root directory.

Data files (in the directories `pkg/atlasrep/datagens` and `pkg/atlasrep/dataword`) that are available from an earlier version of the package are in principle kept; see AtlasOfGroupRepresentationsTestTableOfContentsRemoteUpdates for necessary updates.

The package consists entirely of GAP code plus a few scripts, no external binaries need to be compiled for the package itself.

If one wants to use the remote installation of the package (see Local or Remote Installation of the AtlasRep Package), one needs either `wget` or the perl module `libnet`.

Most Linux distributions provide the perl module `libnet` via add-ons. (The module is called `libnet-perl` in Debian Linux and `perl-libnet` in RedHat Linux.) In the case that the `libnet` module is missing on your machine, use `man perlmodinstall` for information on how to install it; if you have to adjust the environment variable `PERLLIB`, call `man perlrun` for details.

If one does not want to use the remote installation of the package then the value of the component `remote` of the global variable `AtlasOfGroupRepresentationsInfo` (see AtlasOfGroupRepresentationsInfo) must be set to `false`. For this variant the system program `ls` is needed, so currently this may be not available for example under the Windows operating system.

After unpacking the archive, it should be checked whether the subdirectories `datagens` and `dataword` of the `atlasrep` directory have write permissions for those users who will download files from the servers. The recommended permissions under UNIX are set as follows.

    you@unix> chmod 1777 atlasrep/data*
    you@unix> ls -ld atlasrep/data*
    drwxrwxrwt   3 you      you          1024 Oct 31 12:34 datagens
    drwxrwxrwt   3 you      you          1024 Oct 31 12:34 dataword

For checking the installation of the package, you should start GAP, load the package (see Loading the AtlasRep Package), and then call

    gap> ReadPackage( "atlasrep", "tst/testinst.g" );

If the installation is o.k. then the GAP prompt appears without anything else being printed; otherwise the output lines tell you what should be changed.

More testfiles are available in the `tst` directory of the package (see Sanity Checks for the Atlas of Group Representations for details).

Both dvi and pdf versions of the package manual are available (as `manual.dvi` and `manual.pdf` respectively) in the `doc` directory of the package, an HTML version can be found in the `htm` directory.

## 1.6 Maintaining the Local Data of the AtlasRep Package

It may be useful if a system administrator runs the following function from time to time, and then removes the files listed in the printed messages. (Other users may not have the permissions to remove files.)

- `AtlasOfGroupRepresentationsTestTableOfContentsRemoteUpdates( ) F`

  It is checked whether files in the local data directories must be updated because of changes of the corresponding files in the remote data directories. (Such changes are restricted to bugfixes, their number is expected to be very small. A list of all changes of data files on the server is available in the internet, see Web Services for the AtlasRep Package.)

  For each local file that must be updated, a line of the following form is printed.

      #I  update local file `...'

  The necessity of updating a file is detected from the fact that the time of its last modification on the server is later than that of the local copy. (This means that `touch`ing files in the local directories will cheat this function.)

  The return value is the list of names of all files that need to be updated.

  ## 1.7 User Parameters for the AtlasRep Package

  This section lists some global parameters which can be set by users.

  Accessing data files with perl or wget

  When GAP uses the feature to access files on remote servers, it calls one of the system programs `perl` or `wget`. This works under UNIX, but may cause problems when used with other operating systems. For example, in principle the system programs needed are available for MS-Windows, but it may require some work to install them; hints for that can be found on the homepage of the AtlasRep package (see Web Services for the AtlasRep Package).

  The `perl` alternative is used by default because only with this variant the listing of the current server contents is possible (see Updating the Tables of Contents of the AtlasRep Package). One can inhibit this (and hence force using `wget`) by setting the component `wget` of `AtlasOfGroupRepresentationsInfo` (see AtlasOfGroupRepresentationsInfo) to `true`; this is necessary in the case that the perl module `libnet` is not available.

  Local or remote access

  One further change may be useful if in principle there is a network connection to a remote server but GAP shall not attempt to access remote data. In this case, the value of the `remote` component of the global variable `AtlasOfGroupRepresentationsInfo` (see AtlasOfGroupRepresentationsInfo) should be set from the default `true` to `false`.

  Independent of the default value of `AtlasOfGroupRepresentationsInfo` (see AtlasOfGroupRepresentationsInfo), each user can individually change the `remote`, `servers`, and `wget` components of this record, for example using a `.gaprc` file (see The .gaprc file in the GAP Reference Manual). Note that if in the GAP installation, the `remote` value is set to `false` then it might be impossible to actually store files fetched from a remote server in the data directories of the installed package; in such cases, it is advisable for the user in question to install the package or just its data directories in a private directory, see GAP Root Directory in the GAP Reference Manual for details. Removing entries from the `servers` list means to disable access to the corresponding servers. Adding entries makes of course sense only if corresponding servers really exist, for example in a local network.

  Compressed or uncompressed data files

  When used with UNIX, GAP can read `gzip`ped files (see Saving and Loading a Workspace in the GAP Reference Manual). If the component `compress` of `AtlasOfGroupRepresentationsInfo` has the value `true` then each MeatAxe format file that is fetched from a remote server is afterwards compressed with `gzip`. This saves a lot of space if many MeatAxe format files are accessed. (Note that data files in other formats are very small.) For example, at the time of the release of version 1.2 there were about 7 000 data files in MeatAxe format, which needed 1 300 MB in uncompressed and about 230 MB in compressed form. The default value for the component `compress` is `false`.

  Reading large matrices over finite fields

  Matrices over finite fields in GAP can be represented in a compressed format that needs less space than the corresponding text file. Such a file can be read either line by line (which is the default) or as a whole; the latter is faster but needs more space than the former. For example, a 4 370 by 4 370 matrix over the field with 2 elements (as occurs for an irreducible representation of the Baby Monster) requires less than 3 MB space in GAP but the corresponding text file is more than 19 MB large, which means that when one reads the file with the fast variant, GAP will temporarily grow by more than this value. One can change the mode via the global variable `CMeatAxe.FastRead` (see Global Variables Used by the AtlasRep Package).

  Updates of the table of contents

  For replacing the package file with the table of contents by the current version, see Updating the Tables of Contents of the AtlasRep Package.

  ## 1.8 Loading the AtlasRep Package

  The AtlasRep package may be loaded automatically when GAP is started, or it has to be loaded within GAP as follows.

      gap> LoadPackage( "atlasrep" );
      true

  See Loading a GAP Package in the GAP Reference Manual for details about these alternatives.

  ## 1.9 Extending the Atlas Database

  Users who have computed new representations that might be interesting for inclusion into the ATLAS of Group representations can send the data in question to R.A.Wilson@bham.ac.uk.

  It is also possible to store ``private'' representations and straight line programs in local directories, and to use them in the same way as the ``official'' data. See Chapter Private Extensions of the AtlasRep Package for details.

  ## 1.10 What's New in Version 1.2?

  Not much.

  The release of Version 1.2 became necessary first of all in order to provide a package version that is compatible with GAP 4.4, since some cross-references into the GAP Reference Manual were broken due to changes of section names. Additionally, several web addresses concerning the package itself were changed and thus had to be adjusted.

  This opportunity was used

  --
    to upgrade the administrational part for loading the package to the mechanism that is recommended for GAP 4.4,

  --
    to extend the test suite, which now covers more consistency checks using the GAP Character Table Library CTblLib,

  --
    to make the function `ScanMeatAxeFile` more robust, due to the fact that the GAP function `PermList` now returns `fail` instead of raising an error,

  --
    to change the way how representations with prescribed properties are accessed (the new function `OneAtlasGeneratingSetInfo` is now preferred to the former `OneAtlasGeneratingSet`, and `AllAtlasGeneratingSetInfos` has been added in order to provide programmatic access in parallel to the human readable descriptions printed by `DisplayAtlasInfo`),

  --
    and last but not least to include the current table of contents of the underlying database.

  For AtlasRep users, the new feature of GAP 4.4 is particularly interesting that due to better kernel support, reading large matrices over finite fields is now faster than it was in GAP 4.3.

  ## 1.11 What's New in Version 1.1?

  The biggest change w.r.t. Version 1.1 is the addition of private extensions (see Chapter Private Extensions of the AtlasRep Package). It includes a new ``free format'' for straight line programs (see The Effect of Private Extensions on the User Interface). Unfortunately, this feature requires the system program `ls`, so it may be not available for example under the Windows operating system.

  In order to admit the addition of other types of data, the implementation of several functions has been changed. Data types are described in Data Types and Data Types Used in the Atlas of Group Representations. An example of a new data type are quaternionic representations (see Filenames Used in the Atlas of Group Representations). The user interface itself (see Chapter The User Interface of the AtlasRep Package) remained the same.

  As an alternative to `perl`, one can use `wget` now for transferring data files (see User Parameters for the AtlasRep Package).

  Data files can be read much more efficiently in GAP 4.3 than in GAP 4.2. In Version 1.1 of the AtlasRep package, this feature is used for reading matrices and permutations in MeatAxe text format with `ScanMeatAxeFile` (see ScanMeatAxeFile). As a consequence, (at least) GAP 4.3 is required for AtlasRep Version 1.1.

  The new `compress` component of the global variable `AtlasOfGroupRepresentationsInfo` (see AtlasOfGroupRepresentationsInfo) allows one to store data files automatically in `gzip`ped form.

  For matrix representations in characteristic zero, invariant forms and generators for the centralizer algebra are now accessible in GAP if they are contained in the source files --this information had been ignored in Version 1.0 (see AtlasOfGroupRepresentationsTestTableOfContentsRemoteUpdates for necessary updates).

  Additional information is now available via the internet (see Web Services for the AtlasRep Package).

  The update facilities have been extended (see Updating the Tables of Contents of the AtlasRep Package).

  The manual is now distributed also in pdf and HTML format; on the other hand, the PostScript format manual is no longer contained in the archives.

  Apart from these changes, a few minor bugs in the handling of MeatAxe files have been fixed, typos in the documentation have been corrected, and the syntax checks for ATLAS straight line programs (see Reading and Writing Atlas Straight Line Programs) have been improved.

  ## 1.12 Acknowledgements

  The perl script for fetching remote data has been kindly provided by Frank Lübeck and Max Neunhöffer. Thanks also to Greg Gamble and Alexander Hulpke for technical hints concerning ``standard'' perl. Ulrich Kaiser helped with preparing the package for Windows.

  The idea of supporting private extensions of the package (see Chapter Private Extensions of the AtlasRep Package) is due to Klaus Lux. He used a preliminary version of AtlasRep Version 1.1, and helped to fix several bugs.

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