Commits on Jan 31, 2016

1. Add NormalHallSubgroups …

   This is the second installment in the series of enhancing
   StructureDescription. (The first being DirectFactorsOfGroup.)
   
   A new global function NormalHalSubgroupFromSylows is introduced, which
   computes normal Hall subgroups by computing normal closures of all Sylows.
   An optional second argument "any" is recognized, in which case the function
   returns the first nontrivial normal Hall subgroup, if exists, and returns
   fail, otherwise. This will be used for computing semidirect decompositions,
   because in this situation the Schur-Zassenhaus theorem guarantees a
   complement.
   
   NormalHallSubgroups is a new attribute, which consists of a list of all
   normal Hall subgroups of a group. This is currently defined only for groups
   which can compute size for all their subgroups. This check might not be
   needed, e.g. the function works for DihedralGroup(IsFpGroup, 60), as well.
   
   A test file is added.

   

   hungaborhorvath committed Jan 31, 2016
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