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Old 2020-10-09, 10:41   #7
LaurV
Romulan Interpreter
 
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Jun 2011
Thailand

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Mod 120 doesn't bring any benefit compared with mod 60. The next values which bring benefits are 420, 4620, etc (what mfaktX use, they are primorials times 2).
The idea is that mersenne factors are 2kp+1 and 1 or 7 mod 8, so if you want to filter out whole classes, then the total number of classes must be multiple of 4 (so 2kp be multiple of 8). That is why we use 4, 12, 60, 420, 4620, etc. classes.

There are some posts on this forum where I put some pari/gp TF code with all those number of classes, shown how much faster is one compared with another, etc., and there were more explanations there.

Last fiddled with by LaurV on 2020-10-09 at 10:42
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