Problem #5:
Question (a): Let ##\sigma \in \text{Aut} (S_n)## be an automorphism of the symmetric group ##S_n## ##(n \geq 4)## such that ##\sigma## sends transpositions to transpositions, then prove that ##\sigma## is an inner automorphism.
Question (b): Determine the inner automorphism groups of (i) the symmetric groups and (ii) the alternating groups for ##n \geq 4##.
Answer (a):
The transpositions ##(1,k)## are a generating set for ##S_n## for ##k = 2,3, \dots , n##. We have that ##\sigma (1,k) = (a_k , b_k)## for ##k = 2,3, \dots , n##, where ##a_k \not= b_k## and where ##a_k, b_k \in \{ 1,2, \dots , n \}##. As ##(1,2)## and ##(1,3)## don't commute, their images ##(a_2,b_2)## and ##(a_3,b_3)## don't commute. As such their intersection ##\{ a_2 , b_2 \} \cap \{ a_3 , b_3 \}## is a single number. WOLG we assume that ##a_2 = a_3 = a##.
We prove that ##a \in \{ a_k , b_k \}## for ##k >3##. Assume otherwise, that is assume that for some ##k > 3## we have that ##a_k \not= a \not= b_k##. As ##(1,k)## does not commute with ##(1,2)## we must have that ##b_2 \in \{ a_k , b_k \}##. Similarly, ##b_3 \in \{ a_k , b_k \}## as ##(1,k)## and ##(1,3)## don't commute. Therefore, ##(a_k , b_k) = (b_2 , b_3)##. But then we would have
\begin{align*}
\sigma (2,3) = \sigma ( (1,3) (1,2) (1,3)) = (a , b_3) (a , b_2) (a , b_3) = (b_2 , b_3) = \sigma (1,k)
\end{align*}
which is in contradiction with the fact that ##\sigma## is injective.
Thus the transpositions ##(a_k , b_k)## transforms the element ##a##, and WLOG we can assume ##a_k = a## for all ##k##. All the integers ##b_k \not= a## and are all distinct. So we have that ##\sigma (1 , k) (a) = (a , b_k) (a) = b_k##, ##\sigma (1 , k) (b_k) = (a , b_k) (b_k) = a##, and ##\sigma (1 , k) (b_{k'}) = (a , b_k) (b_{k'}) = b_{k'}## for ##k' \not= k##.
Consider the element ##\tau \in S_n## defined by ##\tau (1) = a## and ##\tau (k) = b_k##. We have
\begin{align*}
\tau (1,k) \tau^{-1} (a) & = \tau (1,k) (1)
\nonumber \\
& = \tau (k)
\nonumber \\
& = b_k
\end{align*}
and
\begin{align*}
\tau (1,k) \tau^{-1} (b_k) & = \tau (1,k) (k)
\nonumber \\
& = \tau (1)
\nonumber \\
& = a
\end{align*}
and when ##k' \not= k##:
\begin{align*}
\tau (1,k) \tau^{-1} (b_{k'}) & = \tau (1,k) (k')
\nonumber \\
& = \tau (k')
\nonumber \\
& = b_{k'}
\end{align*}
Thus we have shown that ##\sigma## agrees with the inner automorphism ##\tau t \tau^{-1}## on all the generators ##t = (1,k)## of the group ##S_n## and so it is an inner automorphism of ##S_n##.
QED
Answer (b) (i):
Lemma 1: Let ##G## be a group with centre ##Z (G)##. Then ##\text{Inn} (G) \simeq G / Z (G)##.
Proof: Let ##h \in G##. An inner automorphism ##\phi_h## of ##G## is given by ##\phi_h (g) = h g h^{-1}##
Define a group homomorphism:
\begin{align*}
\varphi : G \rightarrow \text{Inn} (G)
\end{align*}
by
\begin{align*}
\varphi (h) = \phi_h .
\end{align*}
##\varphi## is clearly surjective. We identify the kernel of ##\varphi##. We have that:
\begin{align*}
h \in \text{ker} \varphi & \Longleftrightarrow \phi_h (g) = g \quad \text{for all } g \in G
\nonumber \\
& \Longleftrightarrow h g h^{-1} = g \quad \text{for all } g \in G
\nonumber \\
& \Longleftrightarrow h g = g h \quad \text{for all } g \in G
\nonumber \\
& \Longleftrightarrow h \in Z (G) .
\end{align*}
We then apply the first isomorphism theorem.
QED
The centre of ##S_n## for ##n \geq 4## is the identity permutation ##e##.
Proof: Suppose that ##\rho## is an arbitrary element of ##S_n## which is not the identity. Choose ##i## such that ##j = \rho (i) \not= i##. Because ##n \geq 4## there is ##k \not= \{ i , j \}## and let ##\tau = (j,k)##. Then:
\begin{align*}
\tau \rho \tau^{-1} (i) = \tau \rho (i) = \tau (j) = k \not= j = \rho (i)
\end{align*}
so that ##\rho## does not belong to the centre.
QED
Using this result in lemma 1 we obtain that
\begin{align*}
\text{Inn} (S_n) \simeq S_n , \qquad \text{for } n \geq 4 .
\end{align*}
Answer (b) (ii):
The centre of ##A_n## for ##n \geq 4## is the identity permutation ##e##.
Proof: Suppose that ##\rho## is an arbitrary element of ##A_n## which is not the identity. Choose ##i## such that ##j = \rho (i) \not= i##. Because ##n \geq 4## we can choose distinct ##k## and ##l## such that ##k , l \not= \{ i , j \}## and let ##\tau = (j,k,l) = (j,l) (j,k) \in A_n##. Then:
\begin{align*}
\tau \rho \tau^{-1} (i) = \tau \rho (i) = \tau (j) = k \not= j = \rho (i)
\end{align*}
so that ##\rho## does not belong to the centre.
QED
Using this result in lemma 1 we obtain that
\begin{align*}
\text{Inn} (A_n) \simeq A_n , \qquad \text{for } n \geq 4 .
\end{align*}