$a^n+1$ prime $\Rightarrow n = 2^k$ is a power of $2$

Question

I'm trying to prove that $a^n+1$ can only be prime if $n$ is a power of $2$. Is there a general factorization of $a^n+1$?

Answer 1

Hint: $\ a^k+1\mid a^n+1\,$ if $\,k\mid n\ $ & $ $ quotient $\,m = n/k\,$ is odd, by

$\bmod\, a^k+1\!:\,\ \color{#c00}{a^k\equiv -1} \Rightarrow\,\ a^n =\, (\color{#c00}{a^k})^m\equiv (\color{#c00}{-1})^m\equiv -1$

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Or $ $ put $\ x = a^k\ $ in $\ x+1\mid x^m + 1,\, $ via Factor Theorem: $\ x-c\, \mid\, f(x)-f(c)\ $ in $\ \mathbb Z[x].$

Note the this is simply the case $\,m\,$ odd, $\ x\to -x\ $ in this prior question today.

Answer 2

For some non-negative integers $b$ and $k$, we have $n=2^b(2k+1)$, in which case

$$a^n+1 = (a^{2^b}+1)(a^{2k\cdot2^b}-a^{(2k-1)2^b}+a^{(2k-2)2^b}-\cdots -a^{3\cdot2^b}+a^{2\cdot2^b}-a^{2^b}+1)$$

If $k>0$ and $a>1$ then this is a factorisation (if $a=1$ then $a^n+1$ is prime); but if $k=0$, i.e. if $n$ is a power of 2, then it leaves you where you started so it is still possible that $a^n+1$ could be prime.