20140211, 16:57  #1 
Oct 2007
3^{2}×11 Posts 
51*2^37741291 is not prime, but 51*2^50851421 is
The first find since several years:
51*2^37741291 
20140211, 17:39  #2 
Dec 2011
After milion nines:)
3·5·83 Posts 

20140212, 01:49  #3 
Dec 2011
After milion nines:)
1245_{10} Posts 
it looks like your "prime" is not prime"
51*2^37741291 is not prime. LLR Res64: 992EC623E6652D30 Time : 30452.098 sec. [Elapsed time: 8.46 hours] Warning: This number is composite and will be deleted soon! 
20140212, 02:42  #4 
Nov 2003
2×1,811 Posts 
Hi Retep,
Can you check your log files for possible typos? I just started a double check on my machine. By the way, "51 3774129" is in one of the latest sieve files. Note that your submission was deleted and can be found here. Also, please check your lresults.txt and your working directory for any traces of errors. My double check is over, I got the same result like the machine on Top5000: Code:
51*2^37741291 is not prime. LLR Res64: 992EC623E6652D30 Time : 5196.177 sec. Last fiddled with by Kosmaj on 20140212 at 03:50 
20140212, 09:44  #5 
"Carlos Pinho"
Oct 2011
Milton Keynes, UK
3^{3}×13^{2} Posts 
Do we need a double check effort for k=51? I see Retep reserved k=51 from 25003700k (completed) and from 3700k4M (in progress).

20140212, 12:13  #6 
Feb 2003
3555_{8} Posts 
Retep, could you post your lresults.txt file(s)? At least the part containing the "false" prime.
At first we should run double checks on the candidates immediately before and after the problematic case (e.g. exponents 3773995 and 3774142) and see if we get matching residues. 
20140212, 16:45  #7 
Oct 2007
3^{2}×11 Posts 
I'm using a PRPNet server since n=3.7M to get some help from outside. I run it with double checks, since I thought this would be a good idea at such an high level.
For 51*2^3774129 there was one database entry with residue=992EC623E6652D30 and one entry (which was not submitted by one of my computers, by the way) with residue=PRIME. The server displayed it to be prime, which seems to be a bug of PRPNet, I'm in contact with the developer at the moment. Last fiddled with by Retep on 20140212 at 16:45 
20140212, 23:05  #8 
Nov 2003
2×1,811 Posts 
All right then, there is no prime found. The machine which returned "prime" is obviously not suitable for prime search (unstable, or with poor cooling) and it's a good idea to remove it from your search. Also, in spite of all double cheching by PRP net etc, it's recommended to double check potential primes on another trusted machine, and if possible using another software like pfgw (PRP confirmation is enough), or another version of LLR (32 vs. 64 bit). You can also post your candidate here and somobody will double check it for you.
By the way, as Thomas suggested, can you kindly attach your k=51 results, all the way from n=2.5M if you have them handy, or at least the most recent ones. Thanks. 
20140213, 08:11  #9 
Feb 2003
1,901 Posts 

20140215, 11:56  #10 
Oct 2007
99_{10} Posts 
I got ill at the same day I came to know 51*2^37741291 is composite, I am not at home so I don't have access to all the data at the moment.
The results for n=3.5M up to n=3.7M can be found here: http://www.mersenneforum.org/showpos...&postcount=735 http://www.mersenneforum.org/showpos...&postcount=736 We have already eliminated one error in PRPNet, I will keep you informed about that. I remember I checked some residues at random and they were matching. I will check all of them via SQL when I am at home again 
20140804, 21:28  #11 
"Carlos Pinho"
Oct 2011
Milton Keynes, UK
10723_{8} Posts 
Peter Riesen "found" a prime for this k. Let's wait for its validation.
http://primes.utm.edu/primes/page.php?id=118295 
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