20070716, 11:56  #1 
Jul 2007
2·3 Posts 
GNFS  Factor Base
Hi!
i started to factor a c138 a few days ago, and i've finished the polynomial selection with GGNFs now. How can i choose the factor base parameters? Kind regards 
20070716, 13:05  #2 
Jun 2007
Moscow,Russia
205_{8} Posts 
Use factlat.pl perl script, which automatically makes all steps of factorization from creating factorbase file to square rooting.

20070716, 13:10  #3 
(loop (#_fork))
Feb 2006
Cambridge, England
7·911 Posts 
c138 you'll want LP bound either 2^28 or 2^29, SP bound probably around 10^7.
I tend to sieve an appropriatelychosen range (for your size of number probably Q=15 million) of length 1000 with a variety of LP and SP bounds, and see how long it takes per relation, then assume that you need 16M relations for LP bound 2^28 and 32M for 2^29 and see how long the whole thing will take. If you want, sieve around 15M, around 20M, around 25M to see if the rate of generation is dropping off too fast. Don't use factLat.pl: it doesn't have sensible parameters for numbers this big. 
20070716, 16:39  #4 
Jul 2007
2·3 Posts 
Thanks, I made such a factorbase and I'm in the sieving step now. But i have another question:
I have another c138 (C138_147_75 for XYYXF) and I have 853779 relations and 1869192 primes in my relation set. ggnfs.log says: [...] minimum number of FF's: 838743 [...] But when I run matbuild, there are only 9935 (full) relations and I need 748910. What can I do to get more full relation sets (sieving with gnfslasieve4I13e only brings less then 50 new full relation sets)? Kind regards 
20070716, 16:46  #5 
Nov 2003
2^{2}·5·373 Posts 

20070716, 18:01  #6 
Jul 2007
2·3 Posts 
So I have to do more sieving?
Kind regards 
20070716, 18:16  #7 
Nov 2003
2^{2}·5·373 Posts 

20070716, 20:18  #8  
"Sander"
Oct 2002
52.345322,5.52471
29×41 Posts 
Quote:
I extended my own parameter file a bit so i can run numbers in batches, but i agree, when doing a factorization this large you should spend some time testing. For a c110 or c120 i won't take the trouble. 

20070716, 20:56  #9 
Jul 2007
2×3 Posts 
@Silverman:
Oh no, i think this will take a while! @smh: I think factLat.pl is good if you use SNFS, but although its parameters seem to be better than mine. Kind regards 
20070717, 06:01  #10 
"Sander"
Oct 2002
52.345322,5.52471
29×41 Posts 

20070717, 14:16  #11 
Jul 2007
2×3 Posts 
I think if you use SNFS you have a better polynomial which results in a faster factorisation. This speed increase compensates the bad choices of the script.
Kind regards 
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