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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?

Martin Sleziak
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Rob
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1 Answers1

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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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anon
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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 08:01
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    @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 08:10
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    @TylerHilton Every positive integer can be expressed as a power of 2 times an odd number. There is a law $(a^b)^c = a^(b \times c)$. Take any positive integer n. Now, $2^n + 1$ can be expressed as $(2^b)^c + 1$ where $b$ is a power of 2 and $c$ is an odd number. Since $c$ is odd, $(2^b) + 1$ is a factor of $(2^b)^c + 1$. – Timothy Apr 04 '20 at 21:55