[seqfan] Re: (3^n+1)/2 is prime, A171381

[Georgi Guninski]

On Mon, Jun 14, 2010 at 04:31:56PM -0400, Max Alekseyev wrote:
> Btw, the exponent 20 is listed there as (primality) "character unknown".
> So, the claim that (3^(2^20)+1)/2 is not a prime should probably be
> submitted to reflect an updated status of the exponent 20.
>

According to PFGW, 2^20 is really not in the sequence (less than an hour):

PFGW Version 3.3.4.20100405.x86_Stable [GWNUM 25.14]
(3^1048576+1)/2 is composite: RES64: [9EE4CA1AABB9A816] (3229.8733s+0.0424s)

Base 5. Base 3 is ugly for this thread.

According to prognosis of prime hunting pros at:

http://mersenneforum.org/showthread.php?s=4b8f00aa5494ae17bac4342a4cf7719c&p=218705#post218705

2^22 will take "a couple of days" (well not with gp/pari).