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Continuing my work through Dummit & Foote's "Abstract Algebra", 3.1.36 asks the following (which is exactly the same as exercise 5 in this related MSE answer):

Prove that if $G/Z(G)$ is cyclic, then $G$ is abelian. [If $G/Z(G)$ is cyclic with generator $xZ(G)$, show that very element of $G$ can be written in the form $x^az$ for some $a \in \mathbb{Z}$ and some element $z \in Z(G)$]

The hint is actually the hardest part for me, as the quotient groups are somewhat abstract. But once I have the hint, I can write:
$g, h \in G$ implies that $g = x^{a_1}z_1$ and $h = x^{a_2}z_2$, so \begin{align*}gh &= (x^{a_1}z_1)(x^{a_2}z_2)\\ &= x^{a_1}x^{a_2}z_1z_2\\ & = x^{a_1 + a_2}z_2z_1\\ &= \ldots = (x^{a_2}z_2)(x^{a_1}z_1) = hg. \end{align*} Therefore, $G$ is abelian.
1) Is this right so far?
2) How can I prove the "hint"?

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That is correct. To prove the hint, just think that if $G/Z(G)$ is cyclic, then we can write $G/Z(G)=\langle xZ(G)\rangle$ for some $x\in G$. This means that for every $g\in G$, $gZ(G)=x^mZ(G)$ for some $m$, and thus $x^{-m}g\in Z(G)$. Do you see how to finish the proof? –  Robert Auffarth Sep 9 '11 at 12:16
    
@Robert: Yes, I think so. Where did the negative exponent come from? Would you want to make this comment a formal "answer"? –  Altar Ego Sep 9 '11 at 12:22
    
ok, I'll explain it better –  Robert Auffarth Sep 9 '11 at 12:24
3  
possible duplicate of Proof that if group $G/Z(G)$ is cyclic, then $G$ is commutative –  Jonas Meyer Sep 9 '11 at 15:33

1 Answer 1

up vote 8 down vote accepted

We have that $G/Z(G)$ is cyclic, and so there is an element $x\in G$ such that $G/Z(G)=\langle xZ(G)\rangle$, where $xZ(G)$ is the coset with representative $x$. Now let $g\in G$. We know that $gZ(G)=(xZ(G))^m$ for some $m$, and by definition $(xZ(G))^m=x^mZ(G)$. Now, in general, if $H\leq G$, we have by definition too that $aH=bH$ if and only if $b^{-1}a\in H$. In our case, we have that $gZ(G)=x^mZ(G)$, and this happens if and only if $(x^m)^{-1}g\in Z(G)$. There then exists a $z\in Z(G)$ such that $(x^{m})^{-1}g=z$, and so $g=x^mz$. The hint is then proved, and the rest is identical to the work you did.

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Many thanks! That was easy to follow. –  Altar Ego Sep 9 '11 at 12:31
    
@Robert: I am extremely confused about one thing. A group is abelian if and only its its center is the whole group. Then isn't $G/Z(G)$ the trivial group in this case? –  ramanujan_dirac Mar 15 '13 at 1:32
    
Yes, the proof shows then that $G/Z(G)$ is actually trivial. –  Robert Auffarth Mar 20 '13 at 19:07
    
Why "by defintion $(xZ(G))^m=x^mZ(G)$? That should be proven it, shouldn't it? –  user92324 Mar 7 at 2:57
    
Dear Twink, if $H$ is a normal subgroup of a group $G$ and $x,y\in G$, then by definition $(xH)(yH)=xyH$. –  Robert Auffarth Mar 7 at 12:24

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