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 2009-06-25, 15:53 #1 cipher     Feb 2007 211 Posts Calculation for Cunningham Chain 2nd Kind. Hello, I am planning to search Cunningham Chain 2nd for n=100,000. For more info. http://primes.utm.edu/top20/page.php?id=20 Can some one calculate the range of K i need to LLR/Sieve for me to have a chance of finding a CC 2nd prime for n=100k. For example LLR/Sieve k=10G to have 90% chance or each CC2nd prime occurs every 5G for n=100k. If you can also explain your step of calculation of occurrence of prime probability it will be great appreciated. Thanks Cipher. P.s: Here is a post where biwema calculated probabilities for Twin prime. http://www.mersenneforum.org/showpos...&postcount=146
 2009-09-01, 15:12 #2 biwema     Mar 2004 3×127 Posts The probability of constellations of primes (twins, CC etc.) is quite difficult to calculate. In this case I am sieving to some extent (for example 1 Million or 1 Billion). At this point all primes of the constellation are (almost) independent. So I can just square the probability of one number being prime. This way also prevents the influence of different weights etc. In your case we have for example CC: k*2^100000+1 and k*2^100001+1. Sieving a Range of k=1..1000000 (500000 candidates) leaves 705 candidates after sieving up to 35 Bit (34.5 billion). The probability of one number being prime is about 35/50000 (0.07%) (means no factor between 35 bit and sqrt (number). For a constellation of 2 we can square this probability: Expect 49 CC in 100000000 candidates (or about 1 in 2040816 candidates) If you test 2040816 candidates the chance of finding at least one CC is 1-1/e or 63.2% If you test 4081632 candidates the chance of finding at least one CC is 1-1/(e^2) or 86.5% A 90% Chance you get if you test 2040816* ln(10) candidates, that are about 4.7 Million. A 99% Chance you get if you test 2040816* ln(100) candidates, that are about 9.4 Million. A range 1000000 gives 705 Candidates. For 4.7 Million Candidates i recommend you sieving a range of 6.66 Billion. (4.7Million / 705) Note 1: when you sieve to a higher level, fewer candidates are remaining, but the probability of being prime also increases. The chance to find a CC in that range is still the same. Note 2: Some people recommend special k or n for testing because they are more dense. That means that after beginning of sieving more candidates remain. After all there is no advantage, you can just sieve a larger range which takes the same time (horizontal). After sieving you habe still to test (on average) the same number of candidates to find a prime. I hope i made no mistake in my calculations Good luck biwema

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