mersenneforum.org Dumb question about missing srsieve factors
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 2008-04-20, 23:27 #1 robo_mojo     Mar 2008 25 Posts Dumb question about missing srsieve factors Greetings, I run a command such as: Code: $srsieve -f -N 500000 -P 10000000 "561179*2^n+1" It prints out a lot of factors and writes them to srfactors.txt. And at the end it prints: Code: Sieving 3 <= p <= 10000000 eliminated 484614 terms, 15387 remain. Wrote 15387 terms for 1 sequence to srsieve format file srsieve.out'. srsieve stopped: at p=10000000 because --pmax was reached. It says it eliminated 484614 terms, but there are only 6171 terms with factors in srfactors.txt: Code: $ wc -l srfactors.txt 6171 srfactors.txt I was rather expecting to have a srfactors.txt file with factors for 484614 terms. So my dumb question (I'm sure) is: What happened to the other terms? Last fiddled with by robo_mojo on 2008-04-20 at 23:28
2008-04-20, 23:48   #2
axn

Jun 2003

12B516 Posts

Quote:
 Originally Posted by robo_mojo What happened to the other terms?
HINT:- What is the smallest factor recorded in srfactors.txt?

Last fiddled with by axn on 2008-04-20 at 23:48 Reason: speeling

2008-04-21, 00:02   #3
robo_mojo

Mar 2008

408 Posts

Quote:
 Originally Posted by axn1 HINT:- What is the smallest factor recorded in srfactors.txt?
I told ya it was a dumb question :)
Code:
100069 | 561179*2^15071+1
So I take it the other terms have (trivially) small factors less than 10^5 and they don't get printed...

Well now that's cleared up.

2008-04-22, 03:51   #4
geoff

Mar 2003
New Zealand

13·89 Posts

Quote:
 Originally Posted by robo_mojo So I take it the other terms have (trivially) small factors less than 10^5 and they don't get printed...
You can use the -m switch if you really want the small factors: srsieve -m X ...' will cause factors less than X not to be recorded. X=100,000 is the default.

The reason for not recording small factors by default is that it usually takes much longer to write small factors to file (or even just print them to the screen) than it takes to find them in the first place.

Last fiddled with by geoff on 2008-04-22 at 03:52

2008-04-22, 04:37   #5
robo_mojo

Mar 2008

2016 Posts

Quote:
 Originally Posted by geoff You can use the -m switch if you really want the small factors: `srsieve -m X ...' will cause factors less than X not to be recorded. X=100,000 is the default. The reason for not recording small factors by default is that it usually takes much longer to write small factors to file (or even just print them to the screen) than it takes to find them in the first place.
Thanks for the info. It makes sense to only print large factors.

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