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Possible Duplicate:
Suppose $H$ is the only subgroup of order $o(H)$ in the finite group $G$. Prove that $H$ is a normal subgroup of $G$.

I have to solve the following problem. It's an exercise from Herstein's Topics in Algebra book.

Suppose $G$ is a finite group and let $H$ be a subgroup of $G$. Suppose that $H$ is the only subgroup of $G$ of order $o(H)$. Then prove that $H$ is normal in $G$.

Any hints?

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    @Mohamed: hmm, this raises possibly the question: what if $G$ is a finite group with the property that *all of its proper* subgroups $H$ have a unique order? Well, then $G$ must be *cyclic*. Proof (sketch): every p-Sylow subgroup of $G$ is normal and hence $G$ is nilpotent. So we can assume that $G$ is a p-group for some prime p. Note that $Z(G)$ is non-trivial and hence by induction $G/Z(G)$ is cyclic, whence $G$ is abelian. If $G$ would not be cyclic then we can spit of a non-trivial direct factor of $G$, giving rise to two different subgroups of order p. Contradiction.2012-06-09

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Hint. If $g\in G$, how many elements does $gHg^{-1}$ have?

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    @math-visitor: Both issues can be taken care of at the same time by noting that $x\mapsto gxg^{-1}$ is an invertible homomorphism from $G$ to itself: homomorphism since $(gxg^{-1})(gyg^{-1}) = g(xy)g^{-1}$, and invertible because $x\mapsto g^{-1}xg$ is the inverse map.2012-06-09
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For any $a∈G$, the set $a^{−1}Ha$ is also a subgroup of $G$ ($∵$ If $H$ is a subgroup of$G$, then $x^{−1}Hx$ is also a subgroup of $G $ for$x∈G$. Also if $H=\{e,h_{1},...,h_k\}$ then$ a^{−1}Ha=\{e,a^{−1}h_1a,...,a^{−1}h_ka\}$ and these are all distinct elements of $a^{−1}Ha$, i.e. $o(H)=o(a^{−1}Ha)$. But if $H$ is the only subgroup of a given order. Hence $a^{−1}Ha=H$. In other words $H$ is a normal subgroup of $G$.

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@theron: $H$ is even a characteristic subgroup: it is invariant under any automorphism of $G$. Can you see why? The normality comes from the so-called inner automorphisms, that is conjugation by any element $g \in G$.
Observe that the same holds true if your $H$ is the only subgroup of $index[G:H]$.