# Let $K$ be a normal subgroup of order 2 in group $G$, show that $K$ lies in the centre of $G$

Let $K$ be a normal subgroup of order 2 in group $G$, show that $K$ lies in the centre of $G$. Describe a surjective homomorphism of the orthogonal group $\mathrm{O}(3)$ onto $C_2$ and another onto the special orthogonal group $\mathrm{SO}(3)$.

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Too many questions in too short a lapse of time...and you hardly show any self work here. If this is homework you should add the corresponding tag. – DonAntonio Dec 30 '12 at 3:40
This is definitely homework. Just write it out and you are done for the first part. – Calvin Lin Dec 30 '12 at 3:44
@Calvin: I agree that Neptune's questions seem like homework. However, the general policy here is not to add the homework tag unless the OP says themselves it is homework. – Zev Chonoles Dec 30 '12 at 3:48
@ZevChonoles Thanks for informing me. New to MSE. – Calvin Lin Dec 30 '12 at 3:51
@ZevChonoles thanks for information. I don't know about the tag before. This one is from my homework but rest ones are from my revision material that answers are not available. I recently get to know this website and I posted the questions I got stuck in the past few weeks. – Neptune Dec 30 '12 at 4:14

Since $K$ is a normal subgroup of order $2$, there is only one nonidentity element, say $a\in K$. Then for all $g\in G$, we have that $gag^{-1}=e$ or $gag^{-1}=a$. If the latter occurs, then $K\subseteq Z(G)$ and we're done since $(gag^{-1})g=ga=ag$. Assume that $gag^{-1}=e$. Then $ga=g$, thus multiplying on the left by $g^{-1}$, we get $a=e$, a contradiction. Thus, $K\subseteq Z(G)$ as desired.
Hint: Prove that $[x,y]\in K$ for every $y\in G$, where $x$ is the generator of $K$.
$[x,y]=x^{-1}y^{-1}xy$. +1 – Babak S. Dec 30 '12 at 9:37