Comparable with all normal subgroups implies normal in nilpotent group
This article gives the statement and possibly, proof, of an implication relation between two subgroup properties, when the big group is a nilpotent group. That is, it states that in a Nilpotent group (?), every subgroup satisfying the first subgroup property (i.e., Subgroup comparable with all normal subgroups (?)) must also satisfy the second subgroup property (i.e., Normal subgroup (?)). In other words, every subgroup comparable with all normal subgroups of nilpotent group is a normal subgroup of nilpotent group.
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Proof of normality
Given: A nilpotent group , a subgroup of such that for every normal subgroup of , either or .
To prove: is a normal subgroup of
Proof: Consider a central series of :
Then, each is normal in , so we can find some such that . We have is central in , so is central in . In particular, is normal in . Since is normal in , and normality is quotient-transitive, we obtain that is normal in .