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 2019-04-11, 13:14 #1 lukerichards     "Luke Richards" Jan 2018 Birmingham, UK 25·32 Posts Known primes We know all primes, p such that p < 10: 2, 3, 5 7 We do not know all primes, r, such that r < 282,589,933-1 Therefore there is a prime, q, such that all primes less than q are known and proven, but there are primes greater than q not known. What estimates can you make for the size of q?
2019-04-11, 13:51   #2
Dr Sardonicus

Feb 2017
Nowhere

2·32·199 Posts

Quote:
 Originally Posted by lukerichards We know all primes, p such that p < 10: 2, 3, 5 7 We do not know all primes, r, such that r < 282,589,933-1 Therefore there is a prime, q, such that all primes less than q are known and proven, but there are primes greater than q not known. What estimates can you make for the size of q?
If memory serves, the number of primes less than 1027 is known, though I doubt they are all listed.

Pari-GP offers values of primelimit, the upper bound on the list of precomputed primes, of 232 or 264, according to whether you have a 32-bit or 64-bit machine.

I'm guessing the precomputation of all primes up to 264 might take a while, and the resulting vector of primes might use up a bit of RAM...

2019-04-11, 13:56   #3
rudy235

Jun 2015
Vallejo, CA/.

3C316 Posts

Quote:
 Originally Posted by lukerichards We know all primes, p such that p < 10: 2, 3, 5 7 We do not know all primes, r, such that r < 282,589,933-1 Therefore there is a prime, q, such that all primes less than q are known and proven, but there are primes greater than q not known. What estimates can you make for the size of q?
That is an interesting question. I believe it is a number "slightly" over 4*1018

Tomas Oliveira e Silva computed all prime numbers up to that limit exactly 7 years ago in April 2012. See http://sweet.ua.pt/tos/gaps.html

Perhaps –because it is relatively easy to do– someone else with the right equipment has computed 1015 over that limit.

Last fiddled with by rudy235 on 2019-04-11 at 14:07 Reason: spelling

2019-04-12, 14:31   #4
uau

Jan 2017

79 Posts

Quote:
 Originally Posted by lukerichards What estimates can you make for the size of q?
I'm not sure this is a meaningful question. People don't actually use such lists of primes. What does it mean for a prime to be "known"? That someone has in principle run a probabilistic primality test on it, then thrown away the result?

For comparison, what do you think is the largest number such that someone has counted up to it, but no one has counted further?

 2019-04-12, 15:05 #5 firejuggler     Apr 2010 Over the rainbow 244110 Posts Remeber that the universe has *only * 10^78 to 10^82 atoms. Last fiddled with by firejuggler on 2019-04-12 at 15:06
2019-04-12, 18:21   #6
lukerichards

"Luke Richards"
Jan 2018
Birmingham, UK

25×32 Posts

Quote:
 Originally Posted by uau I'm not sure this is a meaningful question. People don't actually use such lists of primes. What does it mean for a prime to be "known"? That someone has in principle run a probabilistic primality test on it, then thrown away the result? For comparison, what do you think is the largest number such that someone has counted up to it, but no one has counted further?
It wasn't supposed to be a meaningful question, that's why it was posted to "Fun Stuff 》Puzzles".

2019-04-12, 18:22   #7
lukerichards

"Luke Richards"
Jan 2018
Birmingham, UK

25×32 Posts

Quote:
 Originally Posted by firejuggler Remeber that the universe has *only * 10^78 to 10^82 atoms.
Struggling to identify the significance of this point, re the original question.

2019-04-12, 18:58   #8
CRGreathouse

Aug 2006

10111001011002 Posts

Quote:
 Originally Posted by firejuggler Remeber that the universe has *only * 10^78 to 10^82 atoms.
Quote:
 Originally Posted by lukerichards Struggling to identify the significance of this point, re the original question.
It gives a hard upper bound on the number of "known" primes, depending on the representation used of course. Say an atom represents a bit and the primes are written in binary, then you can't know all the primes higher than ~ 7e81 because you run out of space.

2019-04-12, 19:21   #9
lukerichards

"Luke Richards"
Jan 2018
Birmingham, UK

25·32 Posts

Quote:
 Originally Posted by CRGreathouse It gives a hard upper bound on the number of "known" primes, depending on the representation used of course. Say an atom represents a bit and the primes are written in binary, then you can't know all the primes higher than ~ 7e81 because you run out of space.
Now that is a fascinating way to look at it.

2019-04-12, 20:22   #10
rudy235

Jun 2015
Vallejo, CA/.

32·107 Posts

Quote:
 Originally Posted by CRGreathouse It gives a hard upper bound on the number of "known" primes, depending on the representation used of course. Say an atom represents a bit and the primes are written in binary, then you can't know all the primes higher than ~ 7e81 because you run out of space.
That is a really high number but the practical limit is way less than that.

I don't have any special predictive ability but I would say that the actual limit which is 4*1018 (which was reached on April 2012) can be at most be squared to 1.6*1037 and I don't expect anyone who is alive now will be alive if and when this happens.

2019-04-13, 15:02   #11
Dr Sardonicus

Feb 2017
Nowhere

2·32·199 Posts

Quote:
 Originally Posted by rudy235 That is an interesting question. I believe it is a number "slightly" over 4*1018 Tomas Oliveira e Silva computed all prime numbers up to that limit exactly 7 years ago in April 2012. See http://sweet.ua.pt/tos/gaps.html Perhaps –because it is relatively easy to do– someone else with the right equipment has computed 1015 over that limit.
Curiously, this is still less than 2^64, the maximum value for primelimit offered by Pari-GP for 64-bit machines.

This leads me to wonder -- what is the largest value for primelimit people actually use commonly? (The default is 500k.)

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