mersenneforum.org DONE - Polynomial selection for 2,2246M c221 and factoring
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 2021-11-26, 00:56 #1 charybdis     Apr 2020 2·19·23 Posts DONE - Polynomial selection for 2,2246M c221 and factoring This is one of only two remaining unambiguous GNFS numbers from the 1987 edition of the Cunningham book (base 2 <1200 bits), the other being 2,1109+ c225. Bob Silverman suggested that the forum run polynomial selection for this number, and Greg has agreed to sieve it with NFS@Home once a suitable poly is found. The composite is Code: 24023387191766184217094927222587025087468868508838209688355658860897738047963529318736265313199625192712082581481439831563094403250831892547179972207284600405234081309183045488404642574713121441527058807512178264038863337 This will be a degree-6 search. The obvious target for a good score is Gimarel's score of 3.36e-16 for 3,697+, which to the best of my knowledge is the only GNFS-221 that has ever been run before. The c220 record is 4.178e-16, so it's reasonable to believe that 3.5e-16 or better is possible. For those who prefer to use CADO, the parameters used to find the aforementioned c220 record polynomial might be a useful starting point: these are P=12M, incr=420, nq=46656, sopteffort=10, ropteffort=100. I am not claiming that these are optimal. The (pre-sizeopt) leading coefficient was in the 35M-40M range; unlike with degree-5, there did not appear to be a significant decrease in average polynomial quality with larger leading coefficients, so there was no need to move to higher values of incr. NFS@Home have a number of SNFS jobs queued up, so don't worry if you've been sieving 3,748+ for the last month and would like a break: there's no rush. It'll probably be about a week before I start any searching myself.
 2021-11-26, 22:46 #2 swellman     Jun 2012 EB116 Posts Sounds fun. Love your recipe for CADO, though I may increase ropteffort and sopteffort a bit. One question - is tasks.wutimeout = 24000 (default) sufficient? Reserving 1e3 < c6 < 2e6. ETA: adrange = 1680 Last fiddled with by swellman on 2021-11-27 at 00:55 Reason: Add adrange
2021-11-27, 02:01   #3
charybdis

Apr 2020

2·19·23 Posts

Quote:
 Originally Posted by swellman One question - is tasks.wutimeout = 24000 (default) sufficient?
Depends if you set ropteffort very high. With ropteffort=100 you'll be fine. 200 might potentially need more time on old/slow CPUs.
Your adrange is small enough that the length of the stage-1+sizeopt tasks won't be an issue.

 2021-11-27, 02:26 #4 swellman     Jun 2012 EB116 Posts FWIW, msieve is looking for an e-score of at least 2.71e-16 to > 3.12e-16. I’m running some msieve-GPU for c6 < 1M. Not expecting much from msieve on a C221 but blind squirrels etc. CADO is running on one of my old quad core laptops. It is SLOW but fingers crossed a flare pops up. Thank you for the advice @charybdis!
2021-11-27, 06:38   #5
VBCurtis

"Curtis"
Feb 2005
Riverside, CA

2·47·59 Posts

Quote:
 Originally Posted by charybdis For those who prefer to use CADO, the parameters used to find the aforementioned c220 record polynomial might be a useful starting point: these are P=12M, incr=420, nq=46656, sopteffort=10, ropteffort=100. I am not claiming that these are optimal. The (pre-sizeopt) leading coefficient was in the 35M-40M range; unlike with degree-5, there did not appear to be a significant decrease in average polynomial quality with larger leading coefficients, so there was no need to move to higher values of incr.
If there is little dropoff in score for larger leading coeff's, doesn't that argue for incr-4620 rather than 420? That is, we should take advantage of the slightly better typical poly that an extra small-factor of c6 provides, since the downside of searching large c6 range doesn't seem to be a problem?

2021-11-27, 14:38   #6
charybdis

Apr 2020

15528 Posts

Quote:
 Originally Posted by VBCurtis If there is little dropoff in score for larger leading coeff's, doesn't that argue for incr-4620 rather than 420? That is, we should take advantage of the slightly better typical poly that an extra small-factor of c6 provides, since the downside of searching large c6 range doesn't seem to be a problem?
Perhaps. While I didn't see much dropoff up to 40M, that doesn't mean there won't be by 400M. I suppose the prudent thing to do would be to run smallish ranges (100k?) at 40M and 400M and compare the "exp_E" scores of the polynomials at the bottom of the priority queue from each run.

 2021-11-29, 08:14 #7 Gimarel   Apr 2010 35 Posts 3.5e-16 is possible: Code: # norm 7.406551e-16 alpha -9.691472 e 3.501e-16 rroots 6 skew: 7000972.35 c0: 53635249950771487474963143079350712070525911980 c1: 34662949307835344131518242206719949955943 c2: -72498102600242091149685770153699749 c3: 3018341741417463336008367359 c4: 7813123654476017869721 c5: 45283110042666 c6: -25200 Y0: -1113552721024727138053628986496268142 Y1: 1434019800661087662503
 2021-11-29, 21:06 #8 swellman     Jun 2012 72618 Posts Very nice! A new record for C221, but can an even higher score be found? 3.7-3.8e-16 perhaps? I’ll keep searching but you’ve set the high water mark!
2021-11-29, 23:13   #9
charybdis

Apr 2020

2×19×23 Posts

Quote:
 Originally Posted by Gimarel 3.5e-16 is possible:
Very nice!

This is good enough that I'll stay away for longer than I was planning; there are lots of new base-2 Cunninghams to do...

 2021-11-29, 23:43 #10 VBCurtis     "Curtis" Feb 2005 Riverside, CA 2×47×59 Posts Would Greg like some test-sieving done? I can try to quantify the tradeoff between 34LP and 34/35 for this job, for instance. This poly is "good enough", but this is a very big job and saving even 1-2% of sieve time is quite a few workunits from the elves; a 2% savings is roughly a C190 sieve job in time saved! So, we should continue to look for a 3.6. There's some chance of "spin" still, too. Edit: I'm busy with ECM pretesting a couple jobs for 15e queue to meet the upcoming "challenge" demand, I'll work on CADO select on this job in a couple weeks once I get two jobs posted to 15e. Last fiddled with by VBCurtis on 2021-11-29 at 23:45
2021-12-06, 15:48   #11
swellman

Jun 2012

3,761 Posts

Quote:
 Originally Posted by swellman Very nice! A new record for C221, but can an even higher score be found? 3.7-3.8e-16 perhaps? I’ll keep searching but you’ve set the high water mark!
Done searching. Can’t find a poly even close to Gimarel’s gem.

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