NormalizerTom( tom, u )
NormalizerTom tries to find conjugacy class of the normalizer of a
subgroup with class number u. It will return the list of class numbers
of those subgroups which have the right size and contain the subgroup and
all subgroups which clearly contain it as a normal subgroup. If the
normalizer is uniquely determined by these conditions then only its class
number will be returned. NormalizerTom should never return an empty
list.
gap> NormalizerTom( a5, 4 );
8
The example shows that a subgroup with class number 4 in A_5 (which is a Kleinan four group) is normalized by a subgroup in class 8. This class contains the subgroups of A_5 which are isomorphic to A_4.
GAP 3.4.4