20090718, 00:49  #1 
P90 years forever!
Aug 2002
Yeehaw, FL
2^{2}·43·47 Posts 
GIMPS' first Fermat factor!
User Buckle turned in this gem recently!
F19 has a factor: 37590055514133754286524446080499713 Anyone want to test if this completes the factorization of F19? 
20090718, 01:02  #2 
"Nancy"
Aug 2002
Alexandria
2,467 Posts 
Congratulations! This is truly a rare discovery. It's the first factor of a Fermat number of index < 30 since the turn of the century, and only the fourth of index < 80. For comparison, GIMPS found 9 Mersenne primes in the same time frame.
How would one test the primality with Prime95/mprime? With a PRP line in worktodo? Can you specify the known factors like you can for ECM? Alex 
20090718, 01:02  #3  
A Sunny Moo
Aug 2007
USA (GMT5)
1869_{16} Posts 
Quote:
Edit: Regarding the primality of the factor, that has also been automatically proven by the abovelinked website upon submission of the factor. Edit2: scratch all that, I just remembered that when a number shows up in bright red on factordb.com, it means that it was too big to be tested for primality or even PRPed. Last fiddled with by mdettweiler on 20090718 at 01:04 

20090718, 01:02  #4  
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
2·3·23·31 Posts 
Quote:
I'll run the PRP real quick. Looks like it'll only take 10 minutes on one core of my semislow CPU. Quote:
Edit 2: Ah, I missed that the known factors had to be inside quotes. I put that in the other worker, and they both resumed from the save point of the first one. I suppose there's no reason that's wrong...anyway, both will finish soon (>75%) Edit 3: Code:
[Fri Jul 17 20:12:38 2009] UID: tim_s/tim, F19 is not prime. RES64: 449FBCA640B4FA27. Wd1: 75E095DF,00000000 UID: tim_s/tim, F19/known_factors is not prime. RES64: 03B53E7A627BF963. Wd1: 75E095DF,00000000 Known factors used for PRP test were: 70525124609,646730219521,37590055514133754286524446080499713 Last fiddled with by MiniGeek on 20090718 at 01:14 

20090718, 01:15  #5  
May 2008
3×5×73 Posts 
Quote:
Code:
PRP=1,2,524288,+1,"70525124609,646730219521,37590055514133754286524446080499713" Code:
[Work thread Jul 17 20:16] F19/known_factors is not prime. RES64: C61CCF4C8ABEF914. Wd8: 75E095DF,00000000 [Work thread Jul 17 20:16] Known factors used for PRP test were: 70525124609,646730219521,37590055514133754286524446080499713 

20090718, 01:15  #6 
"Nancy"
Aug 2002
Alexandria
2,467 Posts 
I ran it with
Code:
PRP=1,2,524288,1,"70525124609,646730219521,37590055514133754286524446080499713" Code:
[Work thread Jul 18 03:29] F19/known_factors is not prime. RES64: C61CCF4C8ABEF914. Wd8: 75E095DF,00000000 [Work thread Jul 18 03:29] Known factors used for PRP test were: 70525124609,646730219521,37590055514133754286524446080499713 Edit: MiniGeek, your RES64 differs. I used mprime 25.11 64bit. Last fiddled with by akruppa on 20090718 at 01:18 Reason: How's that for a fast doublecheck? :) 
20090718, 01:16  #7 
May 2008
1095_{10} Posts 
Hmm, my RES64 is different from yours (MiniGeek).
EDIT: It matches akruppa's though. Last fiddled with by jrk on 20090718 at 01:16 
20090718, 01:22  #8  
May 2008
3×5×73 Posts 
Quote:


20090718, 01:24  #9 
Sep 2004
2·5·283 Posts 
Prime95 version 25.11 64bit
Code:
[Sat Jul 18 02:22:47 2009] F19/known_factors is not prime. RES64: C61CCF4C8ABEF914. Wd4: 75E095DF,00000000 Known factors used for PRP test were: 70525124609,646730219521,37590055514133754286524446080499713 
20090718, 01:25  #10 
Account Deleted
"Tim Sorbera"
Aug 2006
San Antonio, TX USA
2·3·23·31 Posts 
I think it's quite safe to say my result was incorrect and akruppa's and jrk's were correct.
Now the question is why was mine incorrect. Are the save files between F19 and F19/known_factors incompatible? It was used automatically. If they're not compatible and that's the cause of the difference, George ought to fix that (I'm sure it rarely comes up, but it's a problem nonetheless). Perhaps mark the save files with a short hash of the known factors, if any? If they are supposed to be compatible, then there's a bug (we could run another test or two to double check, but it seems pretty clear to me) related to resuming from a save file of the noknownfactors version. I used Prime95 25.11 build 2, 32bit, on Windows XP, Athlon. I can post any log files that are wanted (and not autodeleted already). I began the PRP of F19 on Worker #2, then stopped at about bit 330,000, added the F19/known_factors to Worker #1 (keeping F19 in #2) and restarted Prime95. They both resumed from the save file left by F19. Last fiddled with by MiniGeek on 20090718 at 01:29 
20090718, 01:32  #11 
"Nancy"
Aug 2002
Alexandria
2,467 Posts 
When I run the test with the new factor omitted, I get RES: AD48BE5F089980AC. Does not match MiniGeek's, either. Strange.
Alex 
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