P-normal group

The article defines a property of groups, where the definition may be in terms of a particular prime that serves as parameter
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Definition

Suppose $G$ is a finite group and $p$ is a prime number. We say that $G$ is $p$-normal if it satisfies the following equivalent conditions:

1. There exists a $p$-Sylow subgroup $P$ of $G$ such that the center $Z(P)$ is a weakly closed subgroup of $P$ relative to $G$.
2. For every $p$-Sylow subgroup $P$ of $G$, the center $Z(P)$ is a weakly closed subgroup of $P$ relative to $G$.
3. There exists a $p$-Sylow subgroup $P$ of $G$ with center $Z(P)$ such that for any $p$-Sylow subgroup $Q$ of $G$ containing the center $Z(P)$, $Z(P)$ is a normal subgroup of $Q$.
4. For every $p$-Sylow subgroup $P$ of $G$ with center $Z(P)$, it is true that for any $p$-Sylow subgroup $Q$ of $G$ containing the center $Z(P)$, $Z(P)$ is a normal subgroup of $Q$.
5. There exists a $p$-Sylow subgroup $P$ such that, for every $p$-Sylow subgroup $Q$ containing the center $Z(P)$, $Z(P) = Z(Q)$.
6. For every $p$-Sylow subgroup $P$, and for every $p$-Sylow subgroup $Q$ containing the center $Z(P)$, $Z(P) = Z(Q)$.

Equivalence of definitions

The equivalence between definitions (1)-(2) and between definitions (3)-(4) follows from the fact that Sylow implies order-conjugate: any two $p$-Sylow subgroups are conjugate, and the conjugating automorphism preserves all properties including weak closure. The equivalence between (1) and (3) follows from the fact that characteristic subgroup of Sylow subgroup is weakly closed iff it is normal in every Sylow subgroup containing it.

Relation with other properties

Stronger properties

Property	Meaning	Proof of implication	Proof of strictness (reverse implication failure)	Intermediate notions
p-nilpotent group	there is a p'-Hall subgroup, i.e., a normal p-complement. The $p$-Sylow subgroup is thus a retract.	p-nilpotent implies p-normal	p-normal not implies p-nilpotent	|FULL LIST, MORE INFO
group in which the $p$-Sylow subgroup is abelian

Incomparable properties

• p-solvable group: See p-solvable not implies p-normal and p-normal not implies p-solvable

References

• The Theory of Groups by Marshall Hall, Jr., Page 205, Section 14.4, ^{More info}
• Finite Groups by Daniel Gorenstein, ISBN 0821843427, Page 256, Chapter 7 (Fusion, transfer and p-factor groups), Section 6(Elementary applications), ^{More info}