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So I'm proving that a group $G$ with order $112=2^4 \cdot 7$ is not simple. And I'm trying to do this in extreme detail :)

So, assume simple and reach contradiction. I've reached the point where I can conclude that $n_7=8$ and $n_2=7$.

I let $P, Q\in \mathrm{Syl}_2(G)$ and now dealing with cases that $|P\cap Q|=1, 2^2, 2^3$ or $2^4$.

I easily find contradiction when $|P\cap Q|=2^4$ and $2$.

Um, got stuck REAL bad on the case $|P\cap Q|=2^3$ and $2^2$.

If $|P \cap Q |=2^3= 8$ and $|P|=|Q|=16$, is there any relationship between $P,Q$ and their intersection that can help me?

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2 Answers

up vote 5 down vote accepted

If $G$ is a simple group, it must have exactly $7$ Sylow $2$-subgroups. Thus $G$ embeds into $S_7$, and in particular into $A_7$ since $G$ does not have a subgroup of index $2$. But the order $A_7$ is not divisible by $112$.

If you want to go along the lines of your original idea, you can rule out the case $|P \cap Q| = 2^3$ by noticing that then $P \cap Q$ is normal in $P$ and $Q$ (as a subgroup of index $2$), so $N_G(P \cap Q)$ contains $P$ and $Q$, which implies that $N_G(P \cap Q) = G$.

ADDED: I'm not sure if there is an easy way to deal with rest of the cases. However, there is a nice argument which also works for proving that every group of order $p^n q$ ($p$, $q$ distinct primes) is nonsimple. I believe the idea of the proof goes back to G. A. Miller (around 1900-1910). Here's an illustration of it in this case.

Suppose that $G$ is a simple group of order $112$. Then $G$ has exactly $7$ Sylow $2$-subgroups. Let $P, Q \in Syl_2(G)$ be such that $P \neq Q$ and that $D = P \cap Q$ has largest possible order. Steps for the proof:

  1. Using the fact that $D < N_P(D)$ and $D < N_Q(D)$ (proper inclusion), prove that $N_G(D)$ cannot be a $2$-group.

  2. Thus $D$ is normalized by an element $g \in G$ of order $7$. Prove that $P, gPg^{-1}, \ldots, g^6Pg^{-6}$ are distinct. Conclude that $D$ is contained in every Sylow $2$-subgroup.

  3. Since the intersection of all Sylow $2$-subgroups is normal, $D$ is trivial.

  4. By counting elements in Sylow $2$-subgroups, prove that $G$ contains exactly one Sylow $7$-subgroup.

This same argument works for proving the statement for groups of order $p^n q$.

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Could you expand on your statement "Thus G embeds into S7". Thanks! –  Ziggy Apr 24 '13 at 0:50
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@Ziggy: If $G$ is a simple group and $[G:H] = n$, then the left coset action gives us an injective homomorphism $f: G \rightarrow S_n$ (in this case $H = N_G(P)$ for $P$ some Sylow $2$-subgroup). With Sylow subgroups you could apply the conjugation action as well. –  Mikko Korhonen Apr 24 '13 at 8:07
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Sylow's theorems require that $n_2=1$ or 7, $n_7=1$ or 8, so $G$ has a chance to be simple only if $n_2=7$ and $n_7=8$. Note that the Sylow 7-subgroups can only intersect at the identity, any two Sylow 2-subgroups can share a subgroup of order at most 8, and a Sylow 7-subgroup and a Sylow 2-subgroup can share only the identity.

Hence the union of Sylow 7-subgroups has $1+8\cdot(7-1)=49$ elements, and the union of the Sylow 2-subgroups has at least $8+7\cdot(16-8)=64$ elements, which happens precisely when they all share a subgroup $H$ of order 8. In this case, the union of all Sylow 7-subgroups and 2-subgroups has $64+49-1=112$ elements, so we learn that no other scenario (one with a greater union of the Sylow 2-subgroups) is allowed.

Now notice that $H$ is a normal subgroup of $G$ since conjugation by $g\in G$ permutes the Sylow 2-subgroups and so preserves their intersection. Hence $G$ is not simple.

Edit: This answer is wrong because the union of the Sylow 2-sbgps can be smaller than 112, see the comments below.

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how exactly do you know that the intersection of 2 sylow 2 subgp cannot be 8 though? –  Akaichan Apr 5 '13 at 3:14
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If $n_2=7$, $n_7=8$, the intersection of all Sylow 2-subgroups (and hence any two) MUST have order 8, otherwise the elements wouldn't fit in $G$. The point is that when this happens, the intersection is a normal subgroup of $G$. –  Dalimil Mazáč Apr 5 '13 at 3:19
    
I'm sorry that I'm having a hard time with this but how is the intersection is a normal subgp of $G$? –  Akaichan Apr 5 '13 at 3:33
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Let $H_j$, $j=1,\ldots,7$ be the Sylow 2-sbgps and let $H$ be their intersection. Let $g\in G$, then $gH_jg^{-1}=H_{\sigma(j)}$, where $\sigma\in S_7$. If $h\in H_j$, then $g h g^{-1}\in H_{\sigma(j)}$ and so if $h\in H$, then $ghg^{-1}\in H$ since $\sigma$ is a bijection. –  Dalimil Mazáč Apr 5 '13 at 3:47
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I'm not sure I follow your argument in "the union of the Sylow $2$-subgroups has at least..". For example, let $G = S_3 \times S_3$. Then $G$ has exactly $9$ Sylow $2$-subgroups. Intersections of distinct Sylow $2$-subgroups have order at most $2$. So if I understand your claim, you're saying that $G$ would have at least $2 + 9 \cdot (4 - 2) = 20$ elements in the union of Sylow $2$-subgroups. But this isn't true, there are only $16$ elements in the union of Sylow $2$-subgroups of $G$. –  Mikko Korhonen Apr 5 '13 at 7:28
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