# 17 Unknowns

Sometimes the result of an operation does not allow further computations with it. In many cases, then an error is signalled, and the computation is stopped.

This is not appropriate for some applications in character theory. For example, if a character shall be induced up (see Induced) but Maps and Parametrized Maps), there are positions where the value of the induced character are not known, and other values which are determined by the fusion map:

gap> m11:= CharTable( "M11" );; m12:= CharTable( "M12" );;
gap> fus:= InitFusion( m11, m12 );
[ 1, [ 2, 3 ], [ 4, 5 ], [ 6, 7 ], 8, [ 9, 10 ], [ 11, 12 ],
[ 11, 12 ], [ 14, 15 ], [ 14, 15 ] ]
gap> Induced(m11,m12,Sublist(m11.irreducibles,[ 6 .. 9 ]),fus);
#I Induced: subgroup order not dividing sum in character 1 at class 4
#I Induced: subgroup order not dividing sum in character 1 at class 5
#I Induced: subgroup order not dividing sum in character 1 at class 14
#I Induced: subgroup order not dividing sum in character 1 at class 15
#I Induced: subgroup order not dividing sum in character 2 at class 4
#I Induced: subgroup order not dividing sum in character 2 at class 5
#I Induced: subgroup order not dividing sum in character 2 at class 14
#I Induced: subgroup order not dividing sum in character 2 at class 15
#I Induced: subgroup order not dividing sum in character 3 at class 2
#I Induced: subgroup order not dividing sum in character 3 at class 3
#I Induced: subgroup order not dividing sum in character 3 at class 4
#I Induced: subgroup order not dividing sum in character 3 at class 5
#I Induced: subgroup order not dividing sum in character 3 at class 9
#I Induced: subgroup order not dividing sum in character 3 at class 10
#I Induced: subgroup order not dividing sum in character 4 at class 2
#I Induced: subgroup order not dividing sum in character 4 at class 3
#I Induced: subgroup order not dividing sum in character 4 at class 6
#I Induced: subgroup order not dividing sum in character 4 at class 7
#I Induced: subgroup order not dividing sum in character 4 at class 11
#I Induced: subgroup order not dividing sum in character 4 at class 12
#I Induced: subgroup order not dividing sum in character 4 at class 14
#I Induced: subgroup order not dividing sum in character 4 at class 15
[ [ 192, 0, 0, Unknown(9), Unknown(12), 0, 0, 2, 0, 0, 0, 0, 0,
Unknown(15), Unknown(18) ],
[ 192, 0, 0, Unknown(27), Unknown(30), 0, 0, 2, 0, 0, 0, 0, 0,
Unknown(33), Unknown(36) ],
[ 528, Unknown(45), Unknown(48), Unknown(51), Unknown(54), 0, 0,
-2, Unknown(57), Unknown(60), 0, 0, 0, 0, 0 ],
[ 540, Unknown(75), Unknown(78), 0, 0, Unknown(81), Unknown(84), 0,
0, 0, Unknown(87), Unknown(90), 0, Unknown(93), Unknown(96) ] ]

For this and other situations, in GAP there is the data type unknown. Objects of this type, further on called unknowns, may stand for any cyclotomic (see Cyclotomics).

Unknowns are parametrized by positive integers. When a GAP session is started, no unknowns do exist.

The only ways to create unknowns are to call Unknown Unknown or a function that calls it, or to do arithmetical operations with unknowns (see Operations for Unknowns).

Two properties should be noted:

Lists of cyclotomics and unknowns are no vectors, so cannot be added or multiplied like vectors; as a consequence, unknowns never occur in matrices.

GAP objects which are printed to files will contain fixed unknowns, i.e., function calls Unknown( n ) instead of Unknown(), so be careful to read files printed in different sessions, since there may be the same unknown at different places.

The rest of this chapter contains informations about the unknown constructor (see Unknown), the characteristic function (see IsUnknown), and comparison of and arithmetical operations for unknowns (see Comparisons of Unknowns, Operations for Unknowns); more is not yet known about unknowns.

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GAP 3.4.4
April 1997