Quote:
Originally Posted by bsquared
In my tinyecm code (< 64 bits) where there are just a few fixed and very small B1 values, I found that in a few cases the cost of PRAC(p1*p2) is cheaper than PRAC(p1) + PRAC(p2). It isn't practical to test combinations of primes like this in real time for arbitrary (and larger) B1. But I wonder if it is worth compiling a list of prime pairs that are cheaper when combined below bounds like 1M, 3M, 11M, etc.

Ran a quick test attempting to pair primes < 1M. There are ~5k pairs that are cheaper this way, sometimes dramatically so
^{1}. But there are ~78k primes below this bound so the total cost savings is a fraction of a percent.
edit: nevermind  this is not valid. Have to check that the chain actually works! conclusion is unchanged  finding cheaper pairs doesn't seem worthwhile.
Code:
prime 9769 * 9781 cost = 103.5
prime 9769 cost = 109.0
prime 9781 cost = 103.5
(sum 212.5, savings 109.0 @ prac v=0.620181980807)