Number of automorphisms on alternating group $A_n$.

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How do I find the number of automorphisms of the alternating group on $n$ symbols $A_n$? I have found it for cyclic groups. But I have no idea how to do it here. In particular, I was asked to find it for $n=4 $. Please help me. I'll be thankful if to you if you suggest me how to approach and what I should read again.