Normal p-complement


In group theory, a branch of mathematics, a normal p-complement of a finite group for a prime p is a normal subgroup of order coprime to p and index a power of p. In other words the group is a semidirect product of the normal p-complement and any Sylow p-subgroup. A group is called p-nilpotent if it has a normal.

Cayley normal 2-complement theorem

showed that if the Sylow 2-subgroup of a group G is cyclic then the group has a normal, which shows that the Sylow of a simple group of even order cannot be cyclic.

Burnside normal ''p''-complement theorem

showed that if a Sylow p-subgroup of a group G is in the center of its normalizer then G has a normal. This implies that if p is the smallest prime dividing the order of a group G and the Sylow is cyclic, then G has a normal.

Frobenius normal ''p''-complement theorem

The Frobenius normal p-complement theorem is a strengthening of the Burnside normal theorem, which states that if the normalizer of every non-trivial subgroup of a Sylow of G has a normal, then so does G. More precisely, the following conditions are equivalent:
  • G has a normal p-complement
  • The normalizer of every non-trivial p-subgroup has a normal p-complement
  • For every p-subgroup Q, the group NG/CG is a p-group.

    Thompson normal ''p''-complement theorem

The Frobenius normal p-complement theorem shows that if every normalizer of a non-trivial subgroup of a Sylow has a normal then so does G. For applications it is often useful to have a stronger version where instead of using all non-trivial subgroups of a Sylow, one uses only the non-trivial characteristic subgroups. For odd primes p Thompson found such a strengthened criterion: in fact he did not need all characteristic subgroups, but only two special ones.
showed that if p is an odd prime and the groups N and C both have normal for a Sylow of G, then G has a normal.
In particular if the normalizer of every nontrivial characteristic subgroup of P has a normal, then so does G. This consequence is sufficient for many applications.
The result fails for p = 2 as the simple group PSL2(F7) of order 168 is a counterexample.
gave a weaker version of this theorem.

Glauberman normal ''p''-complement theorem

Thompson's normal p-complement theorem used conditions on two particular characteristic subgroups of a Sylow. Glauberman improved this further by showing that one only needs to use one characteristic subgroup: the center of the Thompson subgroup.
used his ZJ theorem to prove a normal theorem, that if p is an odd prime and the normalizer of Z has a normal, for P a Sylow of G, then so does G. Here Z stands for the center of a group and J for the Thompson subgroup.
The result fails for p = 2 as the simple group PSL2(F7) of order 168 is a counterexample.