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# 2 The GAP Character Table Library

### Sections

1. Contents of the GAP Character Table Library
2. Access to Library Character Tables
3. Generic Character Tables
4. Examples of Generic Character Tables
5. ATLAS Tables
6. Examples of the ATLAS Format for GAP Tables
7. CAS Tables
8. Organization of the Character Table Library
9. How to Extend the Character Table Library

This chapter informs you about

--
  the currently available character tables (see Contents of the GAP Character Table Library),
--
  how to access library tables (see Access to Library Character Tables),
--
  generic character tables (see Generic Character Tables and Examples of Generic Character Tables),
--
  the subsets of ATLAS tables (see ATLAS Tables and Examples of the ATLAS Format for GAP Tables) and CAS tables (see CAS Tables),
--
  the organization of the table library (see Organization of the Character Table Library), and
--
  how to extend the library (see How to Extend the Character Table Library).

The latter two sections are rather technical, they are thought only for those who want to maintain or extend the table library.

## 2.1 Contents of the GAP Character Table Library

This section gives a brief overview of the contents of the GAP character table library. For the details about, e.g., the structure of data files, see Organization of the Character Table Library.

The changes in the character table library since the first release of GAP 4 are listed in a file that can be fetched from http://www.math.rwth-aachen.de/~Thomas.Breuer/ctbllib/htm/ctbldiff.html

There are three different kinds of character tables in the GAP library, namely ordinary character tables, Brauer tables, and generic character tables. Note that the Brauer table and the corresponding ordinary table of a group determine the decomposition matrix of the group (and the decomposition matrices of its blocks). These decomposition matrices can be computed from the ordinary and modular irreducibles with GAP (see Operations Concerning Blocks in the GAP Reference Manual for details). A collection of DVI and PostScript files of the known decomposition matrices of almost simple groups in the GAP table library can also be found at http://www.math.rwth-aachen.de/~MOC/decomposition/

Ordinary Character Tables

Two different aspects are useful to list the ordinary character tables available in GAP, namely the aspect of the source of the tables and that of connections between the tables.

As for the source, there are first of all two big sources, namely the ATLAS of Finite Groups (see ATLAS Tables) and the CAS library of character tables (see NPP84). Many ATLAS tables are contained in the CAS library, and difficulties may arise because the succession of characters and classes in CAS tables and ATLAS tables are in general different, so see CAS Tables for the relations between these two variants of character tables of the same group. A large subset of the CAS tables is the set of tables of Sylow normalizers of sporadic simple groups as published in Ost86 --this may be viewed as another source of character tables. The library also contains the character tables of factor groups of space groups (computed by W. Hanrath, see Han88) that are part of HP89 via two microfiches; these tables are given in CAS format (see CAS Tables) on the microfiches, but they had not been part of the ``official'' CAS library.

To avoid confusion about the ordering of classes and characters in a given table, authorship and so on, the `InfoText` (see InfoText in the GAP Reference Manual) value of the table contains the information

`origin: ATLAS of finite groups`
  for ATLAS tables (see ATLAS Tables),

`origin: Ostermann`
  for tables contained in Ost86,

`origin: CAS library`
  for any table of the CAS table library that is contained neither in the ATLAS nor in Ost86, and

`origin: Hanrath library`
  for tables contained in the microfiches in HP89.

The `InfoText` value usually contains more detailed information, for example that the table in question is the character table of a maximal subgroup of an almost simple group. If the table was contained in the CAS library then additional information may be available via the `CASInfo` value (see CASInfo).

If one is interested in the aspect of connections between the tables, i.e., the internal structure of the library of ordinary tables, the contents can be listed up the following way.

We have

-
  all ATLAS tables (see ATLAS Tables), i.e., the tables of the simple groups which are contained in the ATLAS of Finite Groups, and the tables of cyclic and bicyclic extensions of these groups,
-
  most tables of maximal subgroups of sporadic simple groups (not all for B and M),
-
  some tables of maximal subgroups of other ATLAS tables, where the list of maximal subgroups is complete if the `Maxes` value for the table is known (see Maxes),
-
  the tables of most Sylow normalizers of sporadic simple groups, as printed in Ost86 (not J4N2, Co1N2, Fi22N2, and several for HN, Fi23, Fi24¢, B, M)
-
  some tables of element centralizers
-
  some tables of Sylow subgroups
-
  a few other tables, e.g. `W(F4)`

Note that class fusions stored on library tables are not guaranteed to be compatible for any two subgroups of a group and their intersection, and they are not guaranteed to be consistent w.r.t. the composition of maps.

Brauer Tables

The library contains all tables of the ATLAS of Brauer Tables (JLPW95), and many other Brauer tables of bicyclic extensions of simple groups which are known yet.

The Brauer tables in the library contain the information

    origin: modular ATLAS of finite groups

in their `InfoText` string (see InfoText in the GAP Reference Manual).

Generic Character Tables

See Generic Character Tables for an overview of generic tables available.

## 2.2 Access to Library Character Tables

This section describes how to access a specific character table (see CharacterTableFromLibrary), known character tables of maximal subgroups (see Maxes), and how to select character tables with prescribed properties (see AllCharacterTableNames, OneCharacterTableName).

- `CharacterTableFromLibrary(` tblname`) F`
- `CharacterTableFromLibrary(` series`,` param1`[,` param2`] ) F`

  If the only argument is a string tblname and if this is an admissible name (see below) of a library character table then `CharacterTableFromLibrary` returns this library table, otherwise `fail`.

  If `CharacterTableFromLibrary` is called with more than one argument then the first must be a string series specifying a series of groups which is implemented via a generic character table, for example `"Symmetric"` for symmetric groups; the remaining arguments specialise then the desired member of the series (see Generic Character Tables for a list of available generic tables). If no generic table with name series is available or if the parameters are not admissible then `CharacterTableFromLibrary` returns `fail`.

  A call of `CharacterTableFromLibrary` may cause to read some library files and to construct the table object from the data stored in these files, so fetching a library table may take more time than on expects.

  `CharacterTableFromLibrary` is called by `CharacterTable` if the first argument is a string, so one may also call `CharacterTable`.

  Admissible names for the ordinary character table t of the group G are

  -
    an ATLAS like name if t is an ATLAS table (see ATLAS Tables), for example `"M22"` for the table of the Mathieu group M22, `"L2(13).2"` for L2(13):2, and `"12_1.U4(3).2_1"` for 121.U4(3).21,

    (The difference to the name printed in the ATLAS is that subscripts and superscripts are omitted except if they are used to qualify integer values, and double dots are replaced by a single dot.)

  -
    the names that were admissible for tables of G in CAS if the CAS table library contained a table of G, for example `sl42` for the table of the alternating group A8,

    (But note that the GAP table may be different from that in CAS, see CAS Tables.)

  -
    some ``relative'' names, as follows.

    If G is the n--th maximal subgroup (in decreasing group order) of a group whose library table s is available in GAP and stores the `Maxes` value (see Maxes), and if name is an admissible name for s then ``name `M` n`` is admissible for t. For example, the name `"J3M2"` can be used to access the second maximal subgroup of the sporadic simple Janko group J3 which has the admissible name `J3`.

    If G is a nontrivial Sylow p normalizer in a sporadic simple group with admissible name name, --where nontrivial means that G is not isomorphic to a subgroup of p:(p-1)-- then ``name `N` p`` is an admissible name of t. For example, the name `"J4N11"` can be used to access the table of the Sylow 11 normalizer in the sporadic simple Janko group J4.

    In a few cases, the table of the Sylow p subgroup of G is accessible via the name ``name `Syl` p`` where name is an admissible name of the table of G. For example, `"A11Syl2"` is an admissible name for the table of the Sylow 2 subgroup of the alternating group A11.

    In a few cases, the table of an element centralizer in G is accessible via the name ``name `C` cl`` where name is an admissible name of the table of G. For example, `"M11C2"` is an admissible name for the table of an involution centralizer in the Mathieu group M11.

  The recommended way to access Brauer tables from the library is via the `mod` operator from the ordinary table and the desired characteristic (see BrauerTable and Operators for Character Tables in the GAP Reference Manual), so it is not necessary to define admissible names of Brauer tables.

  A generic character table (see Generic Character Tables) is accessible only by the name given by its `Identifier` value (see Identifier!for character tables in the GAP Reference Manual).

  Case is not significant for character table names. For example, both `"suzm3"` and `"SuzM3"` are admissible names for the third maximal subgroup of the sporadic simple Suzuki group.

      gap> s5:= CharacterTable( "A5.2" );
      CharacterTable( "A5.2" )
      gap> sym5:= CharacterTable( "Symmetric", 5 );
      CharacterTable( "Sym(5)" )
      gap> TransformingPermutationsCharacterTables( s5, sym5 );
      rec( columns := (2,3,4,7,5), rows := (1,7,3,4,6,5,2), group := Group(()) )

  The above two tables are tables of the symmetric group on five letters; the first is in ATLAS format (see ATLAS Tables), the second is constructed from the generic table for symmetric groups (see Generic Character Tables).

      gap> CharacterTable( "J5" );
      fail
      gap> CharacterTable( "A5" ) mod 2;
      BrauerTable( "A5", 2 )

- `Maxes(` tbl`) A`

  is a list of identifiers of the tables of all maximal subgroups of tbl. This is meaningful only for library tables, and there is no default method to compute the value.

  If the `Maxes` value of tbl is stored then it lists exactly one representative for each conjugacy class of maximal subgroups of the group of tbl, and the tables of these maximal subgroups are available in the GAP table library, and the fusions to tbl are stored on these tables.

      gap> tbl:= CharacterTable( "M11" );;
      gap> HasMaxes( tbl );
      true
      gap> maxes:= Maxes( tbl );
      [ "A6.2_3", "L2(11)", "3^2:Q8.2", "A5.2", "2.S4" ]
      gap> CharacterTable( maxes[1] );
      CharacterTable( "A6.2_3" )

- `FusionToTom(` tbl`) A`

  If this attribute is set for an ordinary character table tbl then the GAP Library of Tables of Marks contains the table of marks of the group of tbl, and the attribute value is a record with the following components.

  `name`
    the `Identifier` component of the table of marks of tbl,

  `map`
    the fusion map, and

  `text` (optional)
    a string describing the status of the fusion.

      gap> FusionToTom( CharacterTable( "A5" ) );
      rec( name := "A5", map := [ 1, 2, 3, 5, 5 ], text := "fusion map is unique" )

- `ProjectivesInfo(` tbl`) A`

  If this attribute is set for an ordinary character table tbl then the value is a list of records, each with the following components.

  `name`
    the `Identifier` value of the character table mult of the covering whose faithful irreducible characters are described by the record,

  `chars`
    a list of values lists of faithful projective irreducibles; only one representative of each family of Galois conjugates is contained in this list, and

  `map`
    a list of positions that maps each class of tbl to that preimage in mult for which the entries in `chars` give the values. In a sense, a projection map is an inverse of the factor fusion from the table of the covering to the given table (see ProjectionMap in the GAP Reference Manual).

      gap> ProjectivesInfo( CharacterTable( "A5" ) );
      [ rec( name := "2.A5",
            chars := [ [ 2, 0, -1, E(5)+E(5)^4, E(5)^2+E(5)^3 ], [ 2, 0, -1,
                    E(5)^2+E(5)^3, E(5)+E(5)^4 ], [ 4, 0, 1, -1, -1 ],
                [ 6, 0, 0, 1, 1 ] ], map := [ 1, 3, 4, 6, 8 ] ) ]

- `ExtensionInfoCharacterTable(` tbl`) A`

  Let tbl be the ordinary character table of a group G, say. If this attribute is set for tbl then the value is a list of length two, the first entry being a string M that describes the Schur multiplier of G and the second entry being a string A that describes the outer automorphism group of G. Trivial multiplier or outer automorphism group are denoted by an empty string.

  If tbl is a table from the GAP Character Table Library and G is (nonabelian and) simple then the value is set. In this case, an admissible name for the character table of the Darstellungsgruppe of G (if this table is available and different from tbl) is given by the concatenation of M, `"."`, and the `Identifier` value of tbl. Analogously, an admissible name for the character table of the automorphism group of G (if this table is available and different from tbl) is given by the concatenation of the `Identifier` value of tbl, `"."`, and A.

      gap> ExtensionInfoCharacterTable( CharacterTable( "A5" ) );
      [ "2", "2" ]

- `AllCharacterTableNames( [`func`,` val`, ... ] ) F`
- `AllCharacterTableNames(` func`,` val`, ...[, OfThose,` func`] ) F`

  Similar to group libraries (see Chapter Group Libraries in the GAP Reference Manual), the GAP character table library can be used to search for ordinary character tables with prescribed properties.

  A specific library table can be selected by an admissible name (see CharacterTableFromLibrary).

  The selection function for character tables from the GAP Character Table Library that have certain abstract properties is `AllCharacterTableNames`. Contrary to the situation in the case of group libraries, the selection function returns a list not of library character tables but of their names; using `CharacterTable` one can then access the tables themselves.

  `AllCharacterTableNames` takes an arbitrary even number of arguments. The argument at each odd position must be a function, and the argument at the subsequent even position must be a value that this function must return when called for the character table in question, in order to have the name of the table included in the selection, or a list of such values. For example,

      gap> names:= AllCharacterTableNames();;

  returns a list containing one admissible name of each ordinary character table in the GAP library, and

      gap> simpnames:= AllCharacterTableNames( IsSimple, true );;
      gap> AllCharacterTableNames( IsSimple, true, Size, [ 1 .. 100 ] );
      [ "A5" ]

  return lists containing an admissible name of each ordinary character table in the GAP library whose groups are simple or are simple and have order at most 100, respectively.

  For the sake of efficiency, the arguments `IsSimple` and `IsSporadicSimple` followed by `true` are handled in a special way, GAP need not read all files of the table library in these cases in order to find the desired names.

  If the function `OfThose` is an argument at an odd position then the following argument func must be a function that takes a character table and returns a name of a character table or a list of names; this is interpreted as replacement of the names computed up to this position by the union of names returned by func. For example, func may be `Maxes` (see Maxes) or `NamesOfFusionSources` (see NamesOfFusionSources in the GAP Reference Manual).

      gap> maxesnames:= AllCharacterTableNames( IsSporadicSimple, true,
      >                                         HasMaxes, true,
      >                                         OfThose, Maxes );;

  returns the union of names of ordinary tables of those maximal subgroups of sporadic simple groups that are contained in the table library in the sense that the attribute `Maxes` is set.

  For the sake of efficiency, `OfThose` followed by one of the arguments `AutomorphismGroup`, `SchurCover`, `CompleteGroup` is handled in a special way.

- `OneCharacterTableName(` func`,` val`) F`
- `OneCharacterTableName(` func`,` val`, ..., OfThose,` func`) F`

  The example function for character tables from the GAP Character Table Library that have certain abstract properties is `OneCharacterTableName`. It is analogous to the selection function `AllCharacterTableNames` (see AllCharacterTableNames), the difference is that it returns one `Identifier` value of a character table with the properties in question instead of the list of all such values. If no table with the required properties is contained in the GAP Character Table Library then `fail` is returned.

      gap> OneCharacterTableName( IsSimple, true, Size, 60 );
      "A5"
      gap> OneCharacterTableName( IsSimple, true, Size, 20 );
      fail

- `CTblLibSetUnload(` value`) F`

  If value is `false` then the call to `CTblLibSetUnload` has the effect that data files from the GAP Character TableLibrary are read only once in the current session. By default, the contents of only one data file is kept in memory, in order to keep the space small. This behaviour can be achieved also by calling `CTblLibSetUnload` with `true`.

  ## 2.3 Generic Character Tables

  Generic character tables provide a means for writing down the character tables of all groups in a (usually infinite) series of similar groups, e.g., cyclic groups, or symmetric groups, or the general linear groups GL(2,q) where q ranges over certain prime powers.

  Let { Gq || q Î I } be such a series, where I is an index set. The character table of one fixed member Gq could be computed using a function that takes q as only argument and constructs the table of Gq. It is, however, often desirable to compute not only the whole table but to access just one specific character, or to compute just one character value, without computing the whole character table.

  For example, both the conjugacy classes and the irreducible characters of the symmetric group Sn are in bijection with the partitions of n. Thus for given n it makes sense to ask for the character corresponding to a particular partition, or just for its character value at another partition.

  A generic character table in GAP allows one such local evaluations. In this sense, GAP can deal also with character tables that are too big to be computed and stored as a whole.

  Currently the only operations for generic tables supported by GAP are the specialisation of the parameter q in order to compute the whole character table of Gq, and local evaluation (see ClassParameters for an example). GAP does not support the computation of, e.g., generic scalar products.

  Currently, generic tables of the following groups --in alphabetical order-- are available in GAP. (A list of the names of generic tables known to GAP is `LIBTABLE.GENERIC.firstnames`.) We list the function calls needed to get a specialized table, the generic table itself can be accessed by calling `CharacterTable` with the first argument only; for example, `CharacterTable( "Cyclic" )` yields the generic table of cyclic groups.

    `CharacterTable( "Alternating",` n`)`, the table of the alternating group on n letters,
    `CharacterTable( "Cyclic",` n`)`, the table of the cyclic group of order n,
    `CharacterTable( "Dihedral",` 2n`)`, the table of the dihedral group of order 2n,
    `CharacterTable( "DoubleCoverAlternating",` n`)`, the table of the Schur double cover of the alternating group on n letters (see Noe02),
    `CharacterTable( "DoubleCoverSymmetric",` n`)`, the table of the standard Schur double cover of the symmetric group on n letters (see Noe02),
    `CharacterTable( "GL", 2,` q`)`, the table of the general linear group GL(2,q), for a prime power q,
    `CharacterTable( "GU", 3,` q`)`, the table of the general unitary group GU(3,q), for a prime power q,
    `CharacterTable( "P:Q", [` p`,` q`] )` and `CharacterTable( "P:Q", [` p`,` q`,` k`] )`, the table of the Frobenius extension of the cyclic group of order p by a cyclic group of order q where q divides p-1; if p is a prime integer then q determines the group uniquely and thus the first version can be used, otherwise the action of the residue class of k modulo p is taken for forming orbits of length q each on the nonidentity elements of the group of order p,
    `CharacterTable( "PSL", 2,` q`)`, the table of the projective special linear group PSL(2,q), for a prime power q,
    `CharacterTable( "SL", 2,` q`)`, the table of the special linear group SL(2,q), for a prime power q,
    `CharacterTable( "SU", 3,` q`)`, the table of the special unitary group SU(3,q), for a prime power q,
    `CharacterTable( "Suzuki",` q`)`, the table of the Suzuki group Sz(q) = 2B2(q), for q an odd power of 2,
    `CharacterTable( "Symmetric",` n`)`, the table of the symmetric group on n letters,
    `CharacterTable( "WeylB",` n`)`, the table of the Weyl group of type Bn,
    `CharacterTable( "WeylD",` n`)`, the table of the Weyl group of type Dn.
  In addition to the above calls that really use generic tables, the following calls to `CharacterTable` are to some extent ``generic'' constructions. But note that no local evaluation is possible in these cases, as no generic table object exists in GAP that can be asked for local information.
    `CharacterTable( "Quaternionic",` 4n`)`, the table of the quaternionic (dicyclic) group of order 4n,
    `CharacterTableWreathSymmetric(` tbl`,` n`)`, the character table of the wreath product of the group whose table is tbl with the symmetric group on n letters (see CharacterTableWreathSymmetric in the GAP Reference Manual).

- `CharacterTableSpecialized(` generic_table`,` q`) F`

  For a record generic_table representing a generic character table, and a parameter value q, `CharacterTableSpecialized` returns a character table object computed by evaluating generic_table at q.

      gap> c5:= CharacterTableSpecialized( CharacterTable( "Cyclic" ), 5 );
      CharacterTable( "C5" )
      gap> Display( c5 );
      C5

           5  1  1  1  1  1

             1a 5a 5b 5c 5d
          5P 1a 1a 1a 1a 1a

      X.1     1  1  1  1  1
      X.2     1  A  B /B /A
      X.3     1  B /A  A /B
      X.4     1 /B  A /A  B
      X.5     1 /A /B  B  A

      A = E(5)
      B = E(5)^2

  (Also `CharacterTable( "Cyclic", 5 )` could have been used to construct the above table.)

  While the numbers of conjugacy classes for the members of a series of groups are usually not bounded, there is always a fixed finite number of types (equivalence classes) of conjugacy classes; very often the equivalence relation is isomorphism of the centralizers of the representatives.

  For each type t of classes and a fixed q Î I, a parametrisation of the classes in t is a function that assigns to each conjugacy class of Gq in t a parameter by which it is uniquely determined. Thus the classes are indexed by pairs [t,pt] consisting of a type t and a parameter pt for that type.

  For any generic table, there has to be a fixed number of types of irreducible characters of Gq, too. Like the classes, the characters of each type are parametrised.

  In GAP, the parametrisations of classes and characters for tables computed from generic tables is stored using the attributes `ClassParameters` and `CharacterParameters`.

- `ClassParameters(` tbl`) A`
- `CharacterParameters(` tbl`) A`

  are lists containing a parameter for each conjugacy class or irreducible character, respectively, of the character table tbl.

  It depends on tbl what these parameters are, so there is no default to compute class and character parameters.

  For example, the classes of symmetric groups can be parametrized by partitions, corresponding to the cycle structures of permutations. Character tables constructed from generic character tables (see Generic Character Tables) usually have class and character parameters stored.

  If tbl is a p-modular Brauer table such that class parameters are stored in the underlying ordinary table (see OrdinaryCharacterTable in the GAP Reference Manual) of tbl then `ClassParameters` returns the sublist of class parameters of the ordinary table, for p-regular classes.

      gap> HasClassParameters( c5 );  HasCharacterParameters( c5 );
      true
      true
      gap> ClassParameters( c5 );  CharacterParameters( c5 );
      [ [ 1, 0 ], [ 1, 1 ], [ 1, 2 ], [ 1, 3 ], [ 1, 4 ] ]
      [ [ 1, 0 ], [ 1, 1 ], [ 1, 2 ], [ 1, 3 ], [ 1, 4 ] ]
      gap> ClassParameters( CharacterTable( "Symmetric", 3 ) );
      [ [ 1, [ 1, 1, 1 ] ], [ 1, [ 2, 1 ] ], [ 1, [ 3 ] ] ]

  Here are examples for ``local evaluation'' of generic character tables, first a character value of the cyclic group shown above, then a character value and a representative order of a symmetric group.

      gap> CharacterTable( "Cyclic" ).irreducibles[1][1]( 5, 2, 3 );
      E(5)
      gap> tbl:= CharacterTable( "Symmetric" );;
      gap> tbl.irreducibles[1][1]( 5, [ 3, 2 ], [ 2, 2, 1 ] );
      1
      gap> tbl.orders[1]( 5, [ 2, 1, 1, 1 ] );
      2

  Any generic table in GAP is represented by a record. The following components are supported for generic character table records.

  `centralizers`
    list of functions, one for each class type t, with arguments q and pt, returning the centralizer order of the class [t,pt],

  `charparam`
    list of functions, one for each character type t, with argument q, returning the list of character parameters of type t,

  `classparam`
    list of functions, one for each class type t, with argument q, returning the list of class parameters of type t,

  `classtext`
    list of functions, one for each class type t, with arguments q and pt, returning a representative of the class with parameter [t,pt],

  `domain`
    function of q returning `true` if q is a valid parameter, and `false` otherwise,

  `identifier`
    identifier string of the generic table,

  `irreducibles`
    list of list of functions, in row i and column j the function of three arguments, namely q and the parameters pt and ps of the class type t and the character type s,

  `isGenericTable`
    always `true`

  `libinfo`
    record with components `firstname` (`Identifier` value of the table) and `othernames` (list of other admissible names)

  `matrix`
    function of q returning the matrix of irreducibles of Gq,

  `orders`
    list of functions, one for each class type t, with arguments q and pt, returning the representative order of elements of type t and parameter pt,

  `powermap`
    list of functions, one for each class type t, each with three arguments q, pt, and k, returning the pair [s,ps] of type and parameter for the k-th power of the class with parameter [t,pt],

  `size`
    function of q returning the order of Gq,

  `specializedname`
    function of q returning the `Identifier` value of the table of Gq,

  `text`
    string informing about the generic table

  In the specialized table, the `ClassParameters` and `CharacterParameters` values are the lists of parameters [t,pt] of classes and characters, respectively.

  If the `matrix` component is present then its value implements a method to compute the complete table of small members Gq more efficiently than via local evaluation; this method will be called when the generic table is used to compute the whole character table for a given q (see CharacterTableSpecialized).

  ## 2.4 Examples of Generic Character Tables

  1. The generic table of cyclic groups.

  For the cyclic group Cq = á x ñ of order q, there is one type of classes. The class parameters are integers k Î { 0, ¼, q-1 }, the class with parameter k consists of the group element xk. Group order and centralizer orders are the identity function q ® q, independent of the parameter k. The representative order function maps the parameter pair [q,k] to [(q)/(gcd(q,k))], which is the order of xk in Cq; the p-th power map is the function mapping the triple (q,k,p) to the parameter [1,(kp mod q)].

  There is one type of characters, with parameters l Î { 0, ¼, q-1 }; for eq a primitive complex q-th root of unity, the character values are cl(xk) = eqkl.

  The library file contains the following generic table.

      rec(
      identifier := "Cyclic",
      specializedname := ( q -> Concatenation( "C", String(q) ) ),
      size := ( n -> n ),
      text := "generic character table for cyclic groups",
      centralizers := [ function( n, k ) return n; end ],
      classparam := [ ( n -> [ 0 .. n-1 ] ) ],
      charparam := [ ( n -> [ 0 .. n-1 ] ) ],
      powermap := [ function( n, k, pow ) return [ 1, k*pow mod n ]; end ],
      orders := [ function( n, k ) return n / Gcd( n, k ); end ],
      irreducibles := [ [ function( n, k, l ) return E(n)^(k*l); end ] ],
      domain := IsPosInt,
      libinfo := rec( firstname:= "Cyclic", othernames:= [] ),
      isGenericTable := true )

  2. The generic table of the general linear group GL(2,q).

  We have four types t1, t2, t3, t4 of classes, according to the rational canonical form of the elements. t1 describes scalar matrices, t2 nonscalar diagonal matrices, t3 companion matrices of (X-r)2 for elements r Î Fq*, and t4 companion matrices of irreducible polynomials of degree 2 over Fq.

  The sets of class parameters of the types are in bijection with Fq* for t1 and t3, with the set {{r,t}; r, t Î Fq*, r ¹ t} for t2, and with the set {{e,eq}; e Î Fq2\Fq} for t4.

  The centralizer order functions are q ® (q2-1)(q2-q) for type t1, q ® (q-1)2 for type t2, q ® q(q-1) for type t3, and q ® q2-1 for type t4.

  The representative order function of t1 maps (q,r) to the order of r in Fq, that of t2 maps (q,{r,t}) to the least common multiple of the orders of r and t.

  The file contains something similar to the following table.

      rec(
      identifier := "GL2",
      specializedname := ( q -> Concatenation( "GL(2,", String(q), ")" ) ),
      size := ( q -> (q^2-1)*(q^2-q) ),
      text := "generic character table of GL(2,q), see Robert Steinberg: ...",
      centralizers := [ function( q, k ) return (q^2-1) * (q^2-q); end,
                        ..., ..., ... ],
      classparam := [ ( q -> [ 0 .. q-2 ] ), ..., ..., ... ],
      charparam := [ ( q -> [ 0 .. q-2 ] ), ..., ..., ... ],
      powermap := [ function( q, k, pow ) return [ 1, (k*pow) mod (q-1) ]; end,
                    ..., ..., ... ],
      orders:= [ function( q, k ) return (q-1)/Gcd( q-1, k ); end,
                 ..., ..., ... ],
      irreducibles := [ [ function( q, k, l ) return E(q-1)^(2*k*l); end,
                          ..., ..., ... ],
                        [ ..., ..., ..., ... ],
                        [ ..., ..., ..., ... ],
                        [ ..., ..., ..., ... ] ],
      classtext := [ ..., ..., ..., ... ],
      domain := IsPrimePowerInt,
      isGenericTable := true )

  ## 2.5 ATLAS Tables

  The GAP character table library contains all character tables of bicyclic extensions of simple groups that are included in the ATLAS of Finite Groups (CCN85, from now on called ATLAS), and the Brauer tables contained in the ATLAS of Brauer Characters (JLPW95).

  These tables have the information

      origin: ATLAS of finite groups

  or

      origin: modular ATLAS of finite groups

  in their `InfoText` value (see InfoText in the GAP Reference Manual), they are simply called ATLAS tables further on.

  For displaying ATLAS tables with the row labels used in the ATLAS, or for displaying decomposition matrices, see LaTeXStringDecompositionMatrix in the GAP Reference Manual and AtlasLabelsOfIrreducibles.

  In addition to the information given in Chapters 6--8 of the ATLAS which tell you how to read the printed tables, there are some rules relating these to the corresponding GAP tables.

  Improvements

  For the GAP Character Table Library not the printed versions of the ATLAS of Finite Groups and the ATLAS of Brauer Characters are relevant but the revised versions given by the currently three lists of improvements that are maintained by Simon Norton. The first such list is contained in BN95, and is printed in the Appendix of JLPW95; it contains the improvements that had been known until the ``ATLAS of Brauer Characters'' was published. The second list contains the improvements to the ATLAS of Finite Groups that were found since the publication of JLPW95. It can be found in the internet, namely, an HTML version at http://web.mat.bham.ac.uk/atlas/html/atlasmods.html

  and a DVI version at http://web.mat.bham.ac.uk/atlas/html/atlasmods.dvi

  The third list contains the improvements to the ATLAS of Brauer Characters, HTML and PDF versions can be found in the internet at http://www.math.rwth-aachen.de/~MOC/ABCerr.html

  and http://www.math.rwth-aachen.de/~MOC/ABCerr.pdf

  respectively.

  Also some tables are regarded as ATLAS tables which are not printed in the ATLAS but available in ATLAS format from Cambridge, according to the lists of improvements mentioned above. Currently these are the tables related to L2(49), L2(81), L6(2), O8-(3), O8+(3), S10(2), and 2E6(2).3.

  Power Maps

  For the tables of 3.McL, 32.U4(3) and its covers, and 32.U4(3).23 and its covers, the power maps are not uniquely determined by the information from the ATLAS but determined only up to matrix automorphisms (see MatrixAutomorphisms in the GAP Reference Manual) of the irreducible characters. In these cases, the first possible map according to lexicographical ordering was chosen, and the automorphisms are listed in the `InfoText` strings of the tables.

  Projective Characters and Projections

  If G (or G.a) has a nontrivial Schur multiplier then the attribute `ProjectivesInfo` of the GAP table object of G (or G.a) is set (see ProjectivesInfo); the `chars` component of the record in question is the list of values lists of those faithful projective irreducibles that are printed in the ATLAS (so--called proxy characters), and the `map` component lists the positions of columns in the covering for which the column is printed in the ATLAS (a so--called proxy class, this preimage is denoted by g0 in Chapter 7, Section 14 of the ATLAS).

  Tables of Isoclinic Groups

  As described in Chapter 6, Section 7 and in Chapter 7, Section 18 of the ATLAS, there exist two (often nonisomorphic) groups of structure 2.G.2 for a simple group G, which are isoclinic. The table in the GAP Character Table Library is the one printed in the ATLAS, the table of the other isoclinic variant can be constructed using `CharacterTableIsoclinic` (see CharacterTableIsoclinic in the GAP Reference Manual).

  Ordering of Characters and Classes

  (Throughout this paragraph, G always means the simple group involved.)

  1.
    For G itself, the ordering of classes and characters in the GAP table coincides with the one in the ATLAS.
  2.
    For an automorphic extension G.a, there are three types of characters.

    If a character c of G extends to G.a then the different extensions c0, c1, ¼, ca-1 are consecutive in the table of G.a (see Chapter 7, Section 16 of the ATLAS).

    If some characters of G fuse to give a single character of G.a then the position of that character in the table of G.a is given by the position of the first involved character of G.

    If both extension and fusion occur for a character then the resulting characters are consecutive in the table of G.a, and each replaces the first involved character of G.

  3.
    Similarly, there are different types of classes for an automorphic extension G.a, as follows.

    If some classes collapse then the resulting class replaces the first involved class of G.

    For a > 2, any proxy class and its algebraic conjugates that are not printed in the ATLAS are consecutive in the table of G.a; if more than two classes of G.a have the same proxy class (the only case that actually occurs is for a = 5) then the ordering of non-printed classes is the natural one of corresponding Galois conjugacy operators *k (see Chapter 7, Section 19 in the ATLAS).

    For a1, a2 dividing a such that a1 < a2, the classes of G.a1 in G.a precede the classes of G.a2 not contained in G.a1. This ordering is the same as in the ATLAS, with the only exception U3(8).6.

  4.
    For a central extension M.G, there are two different types of characters, as follows.

    Each character can be regarded as a faithful character of a factor group m.G, where m divides M. Characters with the same kernel are consecutive as in the ATLAS, the ordering of characters with different kernels is given by the order of precedence 1, 2, 4, 3, 6, 12 for the different values of m.

    If m > 2, a faithful character of m.G that is printed in the ATLAS (a so-called proxy character) represents two or more Galois conjugates. In each ATLAS table in GAP, a proxy character always precedes the non-printed characters with this proxy. The case m = 12 is the only one that actually occurs where more than one character for a proxy is not printed. In this case, the non-printed characters are ordered according to the corresponding Galois conjugacy operators *5, *7, *11 (in that succession).

  5.
    For the classes of a central extension we have the following.

    The preimages of a G-class in M.G are subsequent, the ordering is the same as that of the lifting order rows in the ATLAS (see Chapter 7, Section 7 there).

    The primitive roots of unity chosen to represent the generating central element (i.e., the element in the second class of the GAP table) are `E(3)`, `E(4)`, `E(6)^5` (`= E(2) * E(3)`), and `E(12)^7` (`= E(3) * E(4)`), for m = 3, 4, 6, and 12, respectively.

  6.
    For tables of bicyclic extensions m.G.a, both the rules for automorphic and central extensions hold. Additionally we have the following three rules.

    Whenever classes of the subgroup m.G collapse in m.G.a then the resulting class replaces the first involved class.

    Whenever characters of the subgroup m.G collapse fuse in m.G.a then the result character replaces the first involved character.

    Extensions of a character are subsequent, and the extensions of a proxy character precede the extensions of characters with this proxy that are not printed.

    Preimages of a class of G.a in m.G.a are subsequent, and the preimages of a proxy class precede the preimages of non-printed classes with this proxy.

- `AtlasLabelsOfIrreducibles(` tbl`[, "short"] ) F`

  Let tbl be the (ordinary or Brauer) character table of a bicyclic extension of a simple group that occurs in the ATLAS of Finite Groups CCN85 or the ATLAS of Brauer Characters JLPW95. `AtlasLabelsOfIrreducibles` returns a list of strings, the i-th entry being a label for the i-th irreducible character of tbl.

  The labels have the following form. We state the rules only for ordinary characters, the rules for Brauer characters are obtained by replacing c by j.

  First consider only downward extensions m.G of a simple group G. If m £ 2 then only labels of the form ci occur, which denotes the i-th ordinary character shown in the ATLAS.

  The labels of faithful ordinary characters of groups m.G with m ³ 3 are of the form ci, ci*, or ci*k, which means the i-th character printed in the ATLAS, the unique character that is not printed and for which ci acts as proxy (see Sections 8 and 19 of Chapter 7 in the ATLAS of Finite Groups), and the image of the printed character ci under the algebraic conjugacy operator *k, respectively.

  For groups m.G.a with a > 1, the labels of the irreducible characters are derived from the labels of the irreducible constituents of their restrictions to m.G, as follows.

  1.
    If the ordinary irreducible character ci of m.G extends to m.G.a then the a¢ extensions are denoted by ci,0, ci,1, ¼, ci,a¢, where ci,0 is the character whose values are printed in the ATLAS.
  2.
    The label ci1 + i2 + ¼+ ia means that a different characters ci1, ci2, ¼, cia of m.G induce to an irreducible character of m.G.a with this label.

    If the string `"short"` was entered as the second argument then the label has the short form ci1+. Note that i2, i3, ¼, ia can be read off from the fusion signs in the ATLAS.

  3.
    Finally, the label ci1,j1 + i2,j2 + ¼+ ia¢,ja¢ means that the characters ci1, ci2, ¼, cia¢ of m.G extend to a group that lies properly between m.G and m.G.a, and the extensions ci1,j1, ci2,j2, ¼cia¢,ja¢ induce to an irreducible character of m.G.a with this label.

    Again, if the string `"short"` was entered as the second argument then the label has a short form, namely ci,j+.

      gap> AtlasLabelsOfIrreducibles( CharacterTable( "3.A7.2" ) );
      [ "\\chi_{1,0}", "\\chi_{1,1}", "\\chi_{2,0}", "\\chi_{2,1}", "\\chi_{3+4}",
        "\\chi_{5,0}", "\\chi_{5,1}", "\\chi_{6,0}", "\\chi_{6,1}", "\\chi_{7,0}",
        "\\chi_{7,1}", "\\chi_{8,0}", "\\chi_{8,1}", "\\chi_{9,0}", "\\chi_{9,1}",
        "\\chi_{17+17\\ast 2}", "\\chi_{18+18\\ast 2}", "\\chi_{19+19\\ast 2}",
        "\\chi_{20+20\\ast 2}", "\\chi_{21+21\\ast 2}", "\\chi_{22+23\\ast 8}",
        "\\chi_{22\\ast 8+23}" ]
      gap> AtlasLabelsOfIrreducibles( CharacterTable( "3.A7.2" ), "short" );
      [ "\\chi_{1,0}", "\\chi_{1,1}", "\\chi_{2,0}", "\\chi_{2,1}", "\\chi_{3+}",
        "\\chi_{5,0}", "\\chi_{5,1}", "\\chi_{6,0}", "\\chi_{6,1}", "\\chi_{7,0}",
        "\\chi_{7,1}", "\\chi_{8,0}", "\\chi_{8,1}", "\\chi_{9,0}", "\\chi_{9,1}",
        "\\chi_{17+}", "\\chi_{18+}", "\\chi_{19+}", "\\chi_{20+}", "\\chi_{21+}",
        "\\chi_{22+}", "\\chi_{23+}" ]

  ## 2.6 Examples of the ATLAS Format for GAP Tables

  We give three little examples for the conventions stated in ATLAS Tables, listing both the ATLAS format and the table displayed by GAP.

  First, let G be the trivial group. We consider the cyclic group C6 of order 6. It can be viewed in several ways, namely

  1.
    as a downward extension of the factor group C2 which contains G as a subgroup, or equivalently, as an upward extension of the subgroup C3 which has a factor group isomorphic to G,
  2.
    as a downward extension of the factor group C3 which contains G as a subgroup, or equivalently, as an upward extension of the subgroup C2 which has a factor group isomorphic to G,
  3.
    as a downward extension of the factor groups C3 and C2 which have G as a factor group, or
  4.
    as an upward extension of the subgroups C3 or C2 which both contain a subgroup isomorphic to G.

  Situation 1. is shown here.

       -------   -------         ;   @   ;   ;   @      2   1   1   1   1   1   1
      |       | |       |            1           1      3   1   1   1   1   1   1
      |   G   | |  G.2  |      p power           A
      |       | |       |      p' part           A         1a  3a  3b  2a  6a  6b
       -------   -------       ind  1A fus ind  2A     2P  1a  3b  3a  1a  3b  3a
       -------   -------                               3P  1a  1a  1a  2a  2a  2a
      |       | |       |  chi1  +   1   :  ++   1
      |  3.G  | | 3.G.2 |                            X.1    1   1   1   1   1   1
      |       | |       |      ind   1 fus ind   2   X.2    1   1   1  -1  -1  -1
       -------   -------             3           6   X.3    1   A  /A   1   A  /A
                                     3           6   X.4    1   A  /A  -1  -A -/A
                                                     X.5    1  /A   A   1  /A   A
                           chi2 o2   1   : oo2   1   X.6    1  /A   A  -1 -/A  -A

                                                     A = E(3)
                                                       = (-1+ER(-3))/2 = b3

  `X.1`, `X.2` extend c1. `X.3`, `X.4` extend the proxy character c2. `X.5`, `X.6` extend the not printed character with proxy c2. The classes `1a`, `3a`, `3b` are preimages of `1A`, and `2a`, `6a`, `6b` are preimages of `2A`.

  Situation 2. is shown here.

       -------   -------         ;   @   ;   ;   @      2   1   1   1   1   1   1
      |       | |       |            1           1      3   1   1   1   1   1   1
      |   G   | |  G.3  |      p power           A
      |       | |       |      p' part           A         1a  2a  3a  6a  3b  6b
       -------   -------       ind  1A fus ind  3A     2P  1a  1a  3b  3b  3a  3a
       -------   -------                               3P  1a  2a  1a  2a  1a  2a
      |       | |       |  chi1  +   1   : +oo   1
      |  2.G  | | 2.G.3 |                            X.1    1   1   1   1   1   1
      |       | |       |      ind   1 fus ind   3   X.2    1   1   A   A  /A  /A
       -------   -------             2           6   X.3    1   1  /A  /A   A   A
                                                     X.4    1  -1   1  -1   1  -1
                           chi2  +   1   : +oo   1   X.5    1  -1   A  -A  /A -/A
                                                     X.6    1  -1  /A -/A   A  -A

                                                     A = E(3)
                                                       = (-1+ER(-3))/2 = b3

  `X.1`--`X.3` extend c1, `X.4`--`X.6` extend c2. The classes `1a` and `2a` are preimages of `1A`, `3a` and `6a` are preimages of the proxy class `3A`, and `3b` and `6b` are preimages of the not printed class with proxy `3A`.

  Situation 3. is shown here.

       -------             ;   @        2   1   1   1   1   1   1
      |       |                1        3   1   1   1   1   1   1
      |   G   |          p power
      |       |          p' part           1a  6a  3a  2a  3b  6b
       -------           ind  1A       2P  1a  3a  3b  1a  3a  3b
       -------                         3P  1a  2a  1a  2a  1a  2a
      |       |      chi1  +   1
      |  2.G  |                      X.1    1   1   1   1   1   1
      |       |          ind   1     X.2    1  -1   1  -1   1  -1
       -------                 2     X.3    1   A  /A   1   A  /A
       -------                       X.4    1  /A   A   1  /A   A
      |       |      chi2  +   1     X.5    1  -A  /A  -1   A -/A
      |  3.G  |                      X.6    1 -/A   A  -1  /A  -A
      |       |          ind   1
       -------                 3     A = E(3)
       -------                 3       = (-1+ER(-3))/2 = b3
      |       |
      |  6.G  |      chi3 o2   1
      |       |
       -------           ind   1
                               6
                               3
                               2
                               3
                               6

                     chi4 o2   1

  `X.1`, `X.2` correspond to c1, c2, respectively; `X.3`, `X.5` correspond to the proxies c3, c4, and `X.4`, `X.6` to the not printed characters with these proxies. followers. The factor fusion onto 3.G is given by `[ 1, 2, 3, 1, 2, 3 ]`, that onto G.2 by `[ 1, 2, 1, 2, 1, 2 ]`.

  Finally, situation 4. is shown here.

       -------   -------   -------   -------
      |       | |       | |       | |       |
      |   G   | |  G.2  | |  G.3  | |  G.6  |
      |       | |       | |       | |       |
       -------   -------   -------   -------

            ;   @   ;   ;   @   ;   ;   @   ;     ;   @

                1           1           1             1
          p power           A           A            AA
          p' part           A           A            AA
          ind  1A fus ind  2A fus ind  3A fus   ind  6A

      chi1  +   1   :  ++   1   : +oo   1   :+oo+oo   1

          2   1   1   1   1   1   1
          3   1   1   1   1   1   1

             1a  2a  3a  3b  6a  6b
         2P  1a  1a  3b  3a  3b  3a
         3P  1a  2a  1a  1a  2a  2a
       X.1    1   1   1   1   1   1
       X.2    1  -1   A  /A  -A -/A
       X.3    1   1  /A   A  /A   A
       X.4    1  -1   1   1  -1  -1
       X.5    1   1   A  /A   A  /A
       X.6    1  -1  /A   A -/A  -A

       A = E(3)
         = (-1+ER(-3))/2 = b3

  The classes `1a`, `2a` correspond to 1A, 2A, respectively. `3a`, `6a` correspond to the proxies 3A, 6A, and `3b`, `6b` to the not printed classes with these proxies.

  The second example explains the fusion case; again, G is the trivial group.

       -------   -------        ;   @   ;   ;  @      3.G.2
      |       | |       |           1          1
      |   G   | |  G.2  |     p power          A         2   1   .   1
      |       | |       |     p' part          A         3   1   1   .
       -------   -------      ind  1A fus ind 2A
       -------   -------                                    1a 3a 2a
      |       | |       |   X1  +   1   :  ++  1        2P  1a 3a 1a
      |  2.G  | | 2.G.2 |                               3P  1a 1a 2a
      |       | |       |     ind   1 fus ind  2
       -------   -------            2          2      X.1    1  1  1
       -------   -------                              X.2    1  1 -1
      |       | |           X2  +   1   :  ++  1      X.3    2 -1  .
      |  3.G  | | 3.G.2
      |       | |             ind   1 fus ind  2
       -------                      3                 6.G.2
       -------   -------            3
      |       | |                                        2   2  1  1  2  2  2
      |  6.G  | | 6.G.2     X3 o2   1   *   +            3   1  1  1  1  .  .
      |       | |
       -------                ind   1 fus ind  2            1a 6a 3a 2a 2b 2c
                                    6          2        2P  1a 3a 3a 1a 1a 1a
                                    3                   3P  1a 2a 1a 2a 2b 2c
                                    2
                                    3                 Y.1    1  1  1  1  1  1
                                    6                 Y.2    1  1  1  1 -1 -1
                                                      Y.3    1 -1  1 -1  1 -1
                            X4 o2   1   *   +         Y.4    1 -1  1 -1 -1  1
                                                      Y.5    2 -1 -1  2  .  .
                                                      Y.6    2  1 -1 -2  .  .

  The tables of G, 2.G, 3.G, 6.G and G.2 are known from the first example, that of 2.G.2 will be given in the next one. So here we print only the GAP tables of 3.G.2 @ D6 and 6.G.2 @ D12.

  In 3.G.2, the characters `X.1`, `X.2` extend c1; c3 and its non-printed partner fuse to give `X.3`, and the two preimages of `1A` of order 3 collapse.

  In 6.G.2, `Y.1`--`Y.4` are extensions of c1, c2, so these characters are the inflated characters from 2.G.2 (with respect to the factor fusion `[ 1, 2, 1, 2, 3, 4 ]`). `Y.5` is inflated from 3.G.2 (with respect to the factor fusion `[ 1, 2, 2, 1, 3, 3 ]`), and `Y.6` is the result of the fusion of c4 and its non-printed partner.

  For the last example, let G be the elementary abelian group 22 of order 4. Consider the following tables.

       -------   -------            ;   @   @   @   @   ;   ;   @
      |       | |       |               4   4   4   4           1
      |   G   | |  G.3  |         p power   A   A   A           A
      |       | |       |         p' part   A   A   A           A
       -------   -------          ind  1A  2A  2B  2C fus ind  3A
       -------   -------
      |       | |       |     chi1  +   1   1   1   1   : +oo   1
      |  2.G  | | 2.G.3 |     chi2  +   1   1  -1  -1   .   +   0
      |       | |       |     chi3  +   1  -1   1  -1   |
       -------   -------      chi4  +   1  -1  -1   1   |

                                  ind   1   4   4   4 fus ind   3
                                        2                       6

                              chi5  -   2   0   0   0   : -oo   1

        G.3

           2   2   2   .   .
           3   1   .   1   1

              1a  2a  3a  3b
          2P  1a  1a  3b  3a
          3P  1a  2a  1a  1a

        X.1    1   1   1   1
        X.2    1   1   A  /A
        X.3    1   1  /A   A
        X.4    3  -1   .   .

        A = E(3)
          = (-1+ER(-3))/2 = b3

        2.G                          2.G.3

           2   3   3   2   2   2        2   3   3   2   1   1   1   1
                                        3   1   1   .   1   1   1   1
              1a  2a  4a  4b  4c
          2P  1a  1a  2a  1a  1a           1a  2a  4a  3a  6a  3b  6b
          3P  1a  2a  4a  4b  4c       2P  1a  1a  2a  3b  3b  3a  3a
                                       3P  1a  2a  4a  1a  2a  1a  2a
        X.1    1   1   1   1   1
        X.2    1   1   1  -1  -1     X.1    1   1   1   1   1   1   1
        X.3    1   1  -1   1  -1     X.2    1   1   1   A   A  /A  /A
        X.4    1   1  -1  -1   1     X.3    1   1   1  /A  /A   A   A
        X.5    2  -2   .   .   .     X.4    3   3  -1   .   .   .   .
                                     X.5    2  -2   .   1   1   1   1
                                     X.6    2  -2   .   A  -A  /A -/A
                                     X.7    2  -2   .  /A -/A   A  -A

                                     A = E(3)
                                       = (-1+ER(-3))/2 = b3

  In the table of G.3 @ A4, the characters c2, c3, and c4 fuse, and the classes `2A`, `2B` and `2C` collapse. For getting the table of 2.G @ Q8, one just has to split the class `2A` and adjust the representative orders. Finally, the table of 2.G.3 @ SL2(3) is given; the class fusion corresponding to the injection 2.G ®2.G.3 is `[ 1, 2, 3, 3, 3 ]`, and the factor fusion corresponding to the epimorphism 2.G.3 ® G.3 is `[ 1, 1, 2, 3, 3, 4, 4 ]`.

  (The beautiful LaTeX pictures that were part of the GAP 3 manual will be reintroduced as soon as the bad decision to use TeX for the manual will be revised.)

  ## 2.7 CAS Tables

  All character tables of the CAS table library (see NPP84) are available in GAP except if stated otherwise in the file `doc/ctbldiff.pdf`. This sublibrary has been completely revised before it was included in GAP, for example, errors have been corrected and power maps have been completed.

  Any CAS table is accessible by each of its CAS names (except if stated otherwise in `doc/ctbldiff.pdf`), that is, the table name or the filename used in CAS.

      gap> tbl:= CharacterTable( "m10" );
      CharacterTable( "A6.2_3" )

- `CASInfo(` tbl`) A`

  Let tbl be an ordinary character table tbl in the GAP library that was (up to permutations of classes and characters) contained already in the CAS table library. When one fetches tbl from the library, one does in general not get the original CAS table. Namely, in many cases (mostly ATLAS tables, see ATLAS Tables) the identifier of the table (see Identifier!for character tables in the GAP Reference Manual) as well as the ordering of classes and characters are different for the CAS table and its GAP version.

  Note that in several cases, the CAS library contains different tables of the same group, in particular these tables may have different names and orderings of classes and characters.

  The `CASInfo` value of tbl, if stored, is a list of records, each describing the relation between tbl and a character table in the CAS library. The records have the components

  `name`
    the name of the CAS table,

  `permchars` and `permclasses`
    permutations of the `Irr` values and the classes of tbl, respectively, that must be applied in order to get the orderings in the original CAS table, and

  `text`
    the text that was stored on the CAS table (which may contain incorrect statements).

      gap> HasCASInfo( tbl );
      true
      gap> CASInfo( tbl );
      [ rec( name := "m10", permchars := (3,5)(4,8,7,6), permclasses := (),
            text := "names:     m10\norder:     2^4.3^2.5 = 720\nnumber of classes: \
      8\nsource:    cambridge atlas\ncomments:  point stabilizer of mathieu-group m1\
      1\ntest:      orth, min, sym[3]\n" ) ]

  The class fusions stored on tables from the CAS library have been computed anew; the `text` component of such a fusion record tells if the fusion map is equal to that in the CAS library --of course modulo the permutation of classes between the table in CAS and its GAP version.

      gap> First( ComputedClassFusions( tbl ), x -> x.name = "M11" );
      rec( name := "M11", map := [ 1, 2, 3, 4, 5, 4, 7, 8 ],
        text := "fusion is unique up to table automorphisms,\nthe representative is \
      equal to the fusion map on the CAS table" )

  ## 2.8 Organization of the Character Table Library

  The data files of the GAP Character Table Library reside in the `data` directory of the package `ctbllib`.

  The filenames start with `ct` (for ``character table''), followed by either `o` (for ``ordinary''), `b` (for ``Brauer''), or `g` (for ``generic''), then a description of the contents (up to 5 characters, e.g., `alter` for the tables of alternating and related groups), and the suffix `.tbl`.

  The file `ctb` descr`.tbl` contains the known Brauer tables corresponding to the ordinary tables in the file `cto` descr`.tbl`.

  Each data file of the table library is supposed to consist of

  1.
    comment lines, starting with `` in the first column,
  2.
    assignments to `ALN` (short for ``add library name'', see NotifyNameOfCharacterTable) and to a component of `Revision`, at the beginning of the file, for example in the file with name `ctoalter.tbl` a value is assigned to `Revision.ctoalter_tbl`,
  3.
    assignments to `ALN` and to a component of `LIBTABLE.LOADSTATUS`, at the end of the file, and
  4.
    function calls of the form `SET_TABLEFILENAME(` filename`)`, `MBT(` name`,` data`)` (``make Brauer table''), `MOT(` name`,` data`)` (``make ordinary table''), `ALF(` from`,` to`,` map`)`, `ALF(` from`,` to`,` map`,` textlines`)` (``add library fusion''), `ALN(` name`,` listofnames`)`, and `ARC(` name`,` component`,` compdata`)` (``add record component'').

    Here filename must be a string corresponding to the filename but without suffix, for example `"ctoalter"` if the file has the name `ctoalter.tbl`; name must be the identifier value of the ordinary character table corresponding to the table to which the command refers; data must be a comma separated sequence of GAP objects; from and to must be identifier values of ordinary character tables, map a list of positive itegers, textlines and listofnames lists list of strings, component a string, and compdata any GAP object.

    `MOT`, `ALF`, `ALN`, and `ARC` occur only in files containing ordinary character tables, and `MBT` occurs only in files containing Brauer tables.

  Besides the above calls, the data in files containing ordinary and Brauer tables may contain only the following GAP functions. (Files containing generic character tables may contain calls to arbitrary GAP library functions.)

  `ACM`, `Concatenation`, `E`, `EvalChars`, `GALOIS`, `Length`, `NotifyCharTableName`, `ShallowCopy`, `TENSOR`, and `TransposedMat`.

  The `awk` script `maketbl` in the `etc` directory of the `ctbllib` package expects the file format described above, and to some extent this format is checked by this script.

  The function calls may be continued over several lines of a file. A semicolon is assumed to be the last character in its line if and only if it terminates a function call.

  Names of character tables are strings (see Chapter Strings and Characters in the GAP Reference Manual), i.e., they are enclosed in double quotes; strings in table library files must not be split over several lines, because otherwise the `awk` script may get confused. Additionally, no character table name is allowed to contain double quotes.

  GAP's knowledge about the ordinary tables in the table library is given by the file `ctprimar.tbl` (the ``primary file'' of the table library). This file can be produced from the library files by the script `maketbl` in the `etc` directory of the `ctbllib` package. The information is stored in the global variable `LIBLIST`, which is a record with the following components.

  `firstnames`
    the list of `Identifier` (see Identifier!for character tables in the GAP Reference Manual) values of the ordinary tables,

  `files`
    the list of filenames containing the data of ordinary tables,

  `filenames`
    a list of positive integers, value j at position i means that the table whose identifier is the i--th in the `firstnames` list is contained in the j-th file of the `files` component,

  `fusionsource`
    a list containing at position i the list of names of tables that store a fusion into the table whose identifier is the i--th in the `firstnames` list,

  `allnames`
    a list of all admissible names of ordinary library tables,

  `position`
    a list that stores at position i the position in `firstnames` of the identifier of the table with the i--th admissible name in `allnames`,

  `projections`
    a list of triples [ name , factname , map ] describing a factor fusion map from the table with identifier name to the table with identifier factname (this is used to construct the table of name using the data of the table of factname),

  `simpleinfo`
    a list of triples [ m , name , a ] describing the tables of simple groups in the library; name is the identifier of the table, ``m`.`name`` and ``name`.`a`` are admissible names for its Schur multiplier and automorphism group, respectively,

  `sporadicSimple`
    a list of identifiers of the tables of the 26 sporadic simple groups, and

  `GENERIC`
    a record with information about generic tables (see Generic Character Tables).

  There are three different ways how the table data can be stored in the file.

  Full ordinary tables are encoded by a call to the function `MOT`, where the arguments correspond to the relevant attribute values; each fusion into another library table is added by a call to `ALF`, values to be stored in components of the table object are added with `ARC`, and admissible names are notified with `ALN`. The argument of `MOT` that encodes the irreducible characters is abbreviated as follows. For each subset of characters that differ just by multiplication with a linear character or by Galois conjugacy, only the first one is given by its values, the others are replaced by `[TENSOR,[`i`,`j`]]` (which means that the character is the tensor product of the i-th and the j-th character in the list) or `[GALOIS,[`i`,`j`]]` (which means that the character is obtained from the i-th character by applying `GaloisCyc( .,` j`)` to it.

  Brauer tables are stored relative to the corresponding ordinary tables; attribute values that can be got by restriction from the ordinary table to p--regular classes are not stored, and instead of the irreducible characters the files contain (inverses of) decomposition matrices or Brauer trees for the blocks of nonzero defects.

  Ordinary construction tables have the attribute `ConstructionInfoCharacterTable` (see ConstructionInfoCharacterTable) set, with value a list that contains the name of the construction function used and the arguments for a call to this function; The function call is performed by `CharacterTable` when the table is constructed (not when the file containing the table is read). The aim of this mechanism is to store structured character tables such as tables of direct products and tables of central extensions of other tables in a compact way.

- `LibInfoCharacterTable(` tblname`) F`

  is a record with components

  `firstName`
    the `Identifier` value (see Identifier!for character tables in the GAP Reference Manual) of the library table for which tblname is an admissible name, and

  `fileName`
    the name of the file in which the table data is stored.
  If no such table exists in the GAP library then `fail` is returned.

  If tblname contains the substring `"mod"` then it is regarded as the name of a Brauer table. In this case the result is computed from that for the corresponding ordinary table and the characteristic. So if the ordinary table exists then the result is a record although the Brauer table in question need not be contained in the GAP library.

  ## 2.9 How to Extend the Character Table Library

  GAP users may want to extend the character table library in different respects. Probably the easiest change is to add new admissible names to library tables, in order to use these names in calls of `CharacterTable` (see CharacterTable in the GAP Reference Manual, and CharacterTableFromLibrary). This can be done as follows.

- `NotifyNameOfCharacterTable(` firstname`,` newnames`) F`
- `ALN(` firstname`,` newnames`) F`

  notifies the strings in the list newnames as new admissible names for the library table with `Identifier` value firstname, see Identifier!for character tables in the GAP Reference Manual. If there is already another library table for which some of these names are admissible then an error is signaled.

  `NotifyNameOfCharacterTable` modifies the global variable `LIBLIST`.

  `ALN` is a shorthand for `NotifyNameOfCharacterTable`. In those library files for which the `maketbl` script has produced the necessary information for `LIBLIST`, `ALN` is set to `Ignore` in the beginning and back to `NotifyNameOfCharacterTable` in the end.

      gap> CharacterTable( "private" );
      fail
      gap> NotifyNameOfCharacterTable( "A5", [ "private" ] );
      gap> a5:= CharacterTable( "private" );
      CharacterTable( "A5" )

  The next kind of changes is the addition of new fusions between library tables. Once a fusion map is known, it can be added to the library file containing the table of the subgroup, using the format produced by `LibraryFusion`.

- `ALF(` from`,` to`,` map`[,` text`,` spec`] ) F`

  `ALF` stores the fusion map map between the ordinary character tables with identifier strings from and to in the record encoding the table with identifier from. If the string text is given then it is added as `text` component of the fusion. If the argument spec is given then it is added as `specification` component of the fusion.

  `ALF` changes the global list `LIBLIST.fusionsource`.

  Note that the `ALF` statement should be placed in the file containing the data for the table with identifier from.

- `LibraryFusion(` name`,` fus`) F`

  For a string name that is an `Identifier` value (see Identifier!for character tables in the GAP Reference Manual) of an ordinary character table in the GAP library, and a record fus with the components `name` (the identifier of the destination table, or this table itself), `map` (the fusion map, a list of image positions), and optionally `text` (a string containing information about the fusion) and `specification` (a string or an integer), `LibraryFusion` returns a string whose printed value can be used to add the fusion in question to the library file containing the data for the table with identifier name.

  name may also be a character table, in this case its `Identifier` value is used as string.

      gap> s5:= CharacterTable( "S5" );
      CharacterTable( "A5.2" )
      gap> fus:= PossibleClassFusions( a5, s5 );
      [ [ 1, 2, 3, 4, 4 ] ]
      gap> fusion:= rec( name:= Identifier( s5 ), map:= fus[1], text:= "unique" );;
      gap> Print( LibraryFusion( "A5", fusion ) );
      ALF("A5","A5.2",[1,2,3,4,4],[
      "unique"
      ]);

  The last kind of changes is the addition of new character tables to the GAP character table library. Data files containing tables in library format (i.e., in the form of calls to `MOT` or `MBT`) can be produced using `PrintToLib`.

- `PrintToLib(` file`,` tbl`) F`

  prints the (ordinary or Brauer) character table tbl in library format to the file ``file`.tbl` or file (if this has already the suffix `.tbl`), respectively.

  If tbl is an ordinary table then the value of the attribute `NamesOfFusionSources` is ignored by `PrintToLib`, since for library tables this information is extracted from the source files by the `maketbl` script.

      gap> PrintToLib( "private", a5 );

  The above command appends the data of the table `a5` to the file `private.tbl`; the first lines printed to this file are

      SET_TABLEFILENAME("private");
      MOT("A5",
      [
      "origin: ATLAS of finite groups, tests: 1.o.r., pow[2,3,5]"
      ],
      [60,4,3,5,5],
      [,[1,1,3,5,4],[1,2,1,5,4],,[1,2,3,1,1]],
      [[1,1,1,1,1],[3,-1,0,-E(5)-E(5)^4,-E(5)^2-E(5)^3],
      [GALOIS,[2,2]],[4,0,1,-1,-1],[5,1,-1,0,0]],
      [(4,5)]);
      ARC("A5","projectives",["2.A5",[[2,0,-1,E(5)+E(5)^4,E(5)^2+E(5)^3],
      [GALOIS,[1,2]],[4,0,1,-1,-1],[6,0,0,1,1]],]);
      ARC("A5","extInfo",["2","2"]);

  If you have an ordinary character table in library format which you want to add to the table library, for example because it shall be accessible via `CharacterTable` (see CharacterTableFromLibrary), you must notify this table, i.e., tell GAP in which file it can be found, and which names shall be admissible for it.

- `NotifyCharacterTable(` firstname`,` filename`,` othernames`) F`

  notifies a new ordinary table to the library. This table has `Identifier` value firstname, it is contained (in library format, see PrintToLib) in the file with name filename (without suffix `.tbl`), and the names contained in the list othernames are admissible for it.

  If the initial part of filename is one of `/`, `/` or `./` then it is interpreted as an absolute path. Otherwise it is interpreted relative to the `data` directory of the `ctbllib` package.

  `NotifyCharacterTable` modifies the global variable `LIBLIST` for the current GAP session, after having checked that there is no other library table yet with an admissible name equal to firstname or contained in othernames.

  For example, let us change the name `A5` to `icos` wherever it occurs in the file `private.tbl` that was produced above, and then notify the ``new'' table in this file as follows. (The name change is needed because GAP knows already a table with name `A5` and would not accept to add another table with this name.)

      gap> NotifyCharacterTable( "icos", "private", [] );
      gap> icos:= CharacterTable( "icos" );
      CharacterTable( "icos" )
      gap> Display( icos );
      icos

           2  2  2  .  .  .
           3  1  .  1  .  .
           5  1  .  .  1  1

             1a 2a 3a 5a 5b
          2P 1a 1a 3a 5b 5a
          3P 1a 2a 1a 5b 5a
          5P 1a 2a 3a 1a 1a

      X.1     1  1  1  1  1
      X.2     3 -1  .  A *A
      X.3     3 -1  . *A  A
      X.4     4  .  1 -1 -1
      X.5     5  1 -1  .  .

      A = -E(5)-E(5)^4
        = (1-ER(5))/2 = -b5

  So the private table is treated as a library table. Note that the table can be accessed only if it has been notified in the current GAP session. For frequently used private tables, it may be reasonable to put the `NotifyCharacterTable` statements into your `.gaprc` file (see The .gaprc File in the GAP Reference Manual), or into a file that is read via the `.gaprc` file. For adding interesting character tables to the GAP distribution, please send the tables to the e-mail address mentioned in the first paragraph of this chapter.

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