Who can find a necessary and suficient condition on an positive integer $n$, making $Aut\mathbb{Z}_{n+1}=\mathbb{Z}_{n}$?

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Who can find a necessary and suficient condition on a positive integer $n$, making $Aut\mathbb{Z}_{n+1}=\mathbb{Z}_{n}$? Aut means Automorphism group. This statement is true for n=1,2. However it is not true for all positive integer $n$.