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A little bird told me that if $2^n+1$ is prime, then $n$ is a power of $2$. I tend not to trust talking birds, so I'm trying to verify that statement independently.

Suppose $n$ is not a power of $2$. Then $n = a \cdot 2^m$ for some $a$ not a power of $2$ and some integer $m$. This gives $2^n+1 = 2^{a \cdot 2^m}+1$. Now I suspect there's a way to factor that, but I don't see how. Can someone give me a hint?

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A proof of this fact is also given in Wikipedia article on Fermat primes. –  Martin Sleziak May 4 '12 at 10:06
    
Neat. I didn't know it had a name. –  Rob May 4 '12 at 10:10

1 Answer 1

up vote 10 down vote accepted

Hint. For any odd natural number $a$, the polynomial $x+1$ divides $x^a+1$ evenly.

In particular, we have $$ \frac{x^a+1}{x+1}=\frac{(-x)^a-1}{(-x)-1}=1-x+x^2-\cdots+ (-x)^{a-1}$$ by the geometric sum formula. In this case, specialize to $x=2^{2^{\large m}}$ and we have a nontrivial divisor. (Also, $x^a+1\equiv(-1)^a+1\equiv-1+1\equiv0\mod x+1$ inside $\Bbb Z[ x]$ is pretty slick.)

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Hi, sorry for a late post, but could you explain your answer a bit more for me. Did you mean to say, set $x = 2^m$ instead of $x = 2^{2^m}$ –  Tyler Hilton Jan 28 '13 at 8:01
    
@TylerHilton No, I meant $x=2^{2^m}$ as I wrote. If $n=a2^m$ with $a$ odd, then this shows that $2^{2^m}+1$ divides $2^{a2^m}+1=2^n+1$, and hence $2^n+1$ is composite when $n$ is not a power of $2$. –  anon Jan 28 '13 at 8:10

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