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Sep 10, 2011 at 20:53 comment added Qayum Khan I believe that "$i \geq 0$" should be "$i \geq 1$" in the edit. Note: [ x_3^{-1} x_1 x_3 = x_3^{-1} x_3^4 x_3 = x_3^4 = x_1. ]
Sep 6, 2011 at 10:07 comment added Derek Holt The normalizer of a Klein 4-group in a dihedral group of order $16$ has order 8, so $H_1$ is not normal in $H_3$. I have added a more precise definition of the groups.
Sep 6, 2011 at 10:03 history edited Derek Holt CC BY-SA 3.0
added 169 characters in body; added 3 characters in body; added 3 characters in body
Sep 6, 2011 at 0:43 comment added Qayum Khan Maybe I'm misunderstanding, but I got $H_3 \subseteq N_G(H_1)$ ...
Sep 5, 2011 at 8:54 comment added Derek Holt But do you have a problem with the example I gave?
Sep 5, 2011 at 1:49 comment added Qayum Khan For your idea, it seems one can consider a slightly bigger group, $\hat{G} = O(2) = U(1) \rtimes_{c} C_2$, where $c$ is complex-conjugation, and the subgroup $H = D_2 = O(1) \rtimes_{c} C_2$. However $H \triangleleft \hat{G}$; hence $H \triangleleft G$.
Sep 2, 2011 at 9:11 history answered Derek Holt CC BY-SA 3.0