# Are there infinitely many primes of the form $(3^{n} - 1)/2$?

I've been trying to show that there are infinitely many primes in the form of $$\frac {3^n - 1}{2}$$

Up until now I've tried a modified version Euclid's way of proving the infinity of primes but didn't find a solid answer. Do you have a particular way of approaching this problem? Any help will be appreciated.

• Given that we don't know the same for $2^n-1$, it is unlikely that we know it for $(3^n-1)/2$, unless it is in the negative. We also don't know it for $(10^n-1)/9$ - the repunit primes. Basically, this is a hard sort of problem. – Thomas Andrews Nov 30 '16 at 19:44
• Possible duplicate of Prime numbers of the form $(3^n-1)/2$ – CIJ Nov 30 '16 at 19:48
• Not at all a duplicate. This is asking if there are infinite primes in the form. The other is to show some of the non-primes in the form. – fleablood Nov 30 '16 at 20:22