n => 2n+1 to get prime: seed = 73

[David C Terr]

This process should lead to a prime unless n is a Sierpinski number, the 
smallest known being 78557. For the example below, why not start with 36?

Dave






Don Reble <djr at nk.ca>
03/11/2005 05:47 AM

 
        To:     zakirs at yosh.ac.il, Seqfan <seqfan at ext.jussieu.fr>
        cc: 
        Subject:        Re: n => 2n+1 to get prime:  seed = 73

> Dear Seqfans,
> The operation n => 2n+1 quickly gives primes for most "seed" values of 
n.
> But for some seeds, the transformed numbers keep being composite.
> The first "tough" number is n=73.
> Can the n =>2n+1 transformation, in this particular case,
>  lead to prime number (and when?),

Starting from 73, the 2552nd number (74 * 2^2552 - 1) is prime. It's a
771-digit number which begins and ends 12525...16703.

See also A040081.

-- 
Don Reble  djr at nk.ca



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://list.seqfan.eu/pipermail/seqfan/attachments/20050314/abec9dea/attachment-0001.htm>