# Math Help - Show that 2k+1 cannot be a prime unless...

1. ## Show that 2k+1 cannot be a prime unless...

Show that 2k+1 cannot be a prime unless k is a power of 2.
[Hint: Find a (polynomial) factor of Xp+1 for every odd prime p.]

2. Setting $k=6$ we find that $2k+1= 13$ is prime... but 6 is not a power of 2 !...

Kind regards

$\chi$ $\sigma$

3. Originally Posted by NikoBellic
Show that 2k+1 cannot be a prime unless k is a power of 2.
[Hint: Find a (polynomial) factor of Xp+1 for every odd prime p.]
I suspect you mean $2^k+ 1$.

4. Originally Posted by HallsofIvy
I suspect you mean $2^k+ 1$.
Yeah, the formatting got lost for some reason.