# Normal subgroups In a p-group [Reference?]

Dear Experts,

I'm a graduate student, dealing with group-theory.

In my current research, I used the bound "Alexander Gruber" wrote about in this post: See Here (Actually, I have just found out that the bound is known and already mentioned online).

I was able to give a detailed proof for this claim myself of course , but since it seems like this bound is well known, I want to add a reference .

So... does anyone know about a reference for the bound Alexander Gruber gives in the post above? I want to add it to the Bibliography section.

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For general Hall Algebra,