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I was reading through a proof that no group of order $400$ is simple which can be found here: https://math.stackexchange.com/a/79644/169389

Here is an outline for a solution.

First of all, $|G| = 400 = 2^4 \cdot 5^2\ $. By Sylow's theorem we know that the number of Sylow 5-subgroups must be a divisor of $2^4$ and that it is $1$ modulo $5$. Thus it is either $1$ or $2^4$. If there is only one Sylow 5-subgroup, it must be normal.

For the other case, suppose first that the intersections of different Sylow 5-subgroups are always trivial. By counting elements you can conclude that $G$ has exactly one Sylow 2-subgroup, which is then normal.

If we have Sylow 5-subgroups $P$ and $Q$ such that $P \cap Q \neq \{1\}$, then $|P \cap Q| = 5$. Therefore $P \cap Q$ is normal in $P$ and $Q$, and thus is normal in the subgroup $\langle P, Q \rangle$ generated by $P$ and $Q$. Finally, show that either $\langle P, Q \rangle$ is normal in $G$ or equals $G$.

I am trying to get to grips with general problems like this and feel that this is one of the best explanations, but I still feel that this argument goes too quickly and I still need some clarification on a few things.

First problem: In the second paragraph we consider $n_5$(number of sylow 5-subgroups)$=2^4$ and suppose that the intersections of different Sylow $5$-subgroups $=1$.

When he says "by counting the elements", I guess he means the intersections of different Sylow $5$-subgroups which would be $1$ which we have assumed, how can we now conclude that $n_2=1$?

Second problem: I am wondering why $|P \cap Q| = 5$ and not $5$ or $25$, I think that it may be because they are different their intersection would have to be $5$ since if the intersection is only of order $25$ when the groups are the same and of order $25$, but if this could be clarified, it would be greatly appreciated

Third problem: To show $\langle P, Q \rangle$ is normal in $G$ or equals $G$. How did a commenter discern that the possible orders for $\langle P, Q \rangle$ are $50,100$ and $200$, why not $40$ or $25$?

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(I) If $P_i, 1 \leq i \leq 2^4$ denote the distinct 5-Sylow subgroup of $G,$ then the number of non-identity element in $\cup_{1 \leq i \leq 2^4}P_i$ is $ 2^4 \cdot 24 = 384.$ The rest of the non-identity elements must belong to 2-Sylow subgroups and a 2-Sylow subgroup contains 15 non-identity elements. So it must be unique.

(II) $P \cap Q$ is a subgroup of $P$ and $Q.$ Since $|P| = |Q| = 25,$ the only choices for $|P \cap Q|$ is 1, 5, 25 (Lagrange's theorem). By assumption $|P \cap Q| \neq 1$ and $P, Q$ are distinct. So $|P \cap Q| = 5.$

(III) $\langle P, Q \rangle$ is a subgroup of $G$ and also contains both $P$ and $Q.$ So $|\langle P, Q \rangle|$ must divide $|G|$ and also must be divisible by $|P| = |Q|$ (Lagrange's theorem).

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  1. What he means by "counting the elements" is this: If there are $2^4 = 16$ subgroups of order $25$, all intersecting trivially, then they alone account for $16\cdot 24= 384$ elements of order $5$ or $25$ in $G$. That means there is only room for $399-384 = 15$ elements of order $2, 4, 8$ or $16$ ($399$ because the identity element must also be made room for). Together with the identity element these elements must be the unique Sylow-$2$-subgroup of order $16$.

  2. If $|P\cap Q| = 25$, with $|P| = |Q| = 25$, then yes, $P$ and $Q$ would share every element, and therefore be equal.

  3. If $|P\cap Q| = 5$, then $|P\cup Q| = 45$, and we must have $P\cup Q \subseteq \langle P, Q\rangle$, as sets. Therefore, $|\langle P, Q\rangle| \geq 45$, and the only options that statisfy Lagrange's theorem are $50$, $100$ and $200$.

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