Fitting subgroup is normal-isomorph-free in finite

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This article gives the statement, and possibly proof, of the fact that in any finite group, the subgroup obtained by applying a given subgroup-defining function (i.e., Fitting subgroup) always satisfies a particular subgroup property (i.e., normal-isomorph-free subgroup)
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Statement

In a finite group, the Fitting subgroup is normal-isomorph-free: there is no other normal subgroup of the whole group isomorphic to it.

Related facts

Proof

Proof idea

In a finite group, the Fitting subgroup is the unique largest nilpotent normal subgroup. Any normal subgroup isomorphic to it would also be a nilpotent normal subgroup, and hence, in particular, would be contained in it. Since the group is finite, this forces that any normal subgroup isomorphic to the Fitting subgroup must be equal to it.