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 2010-12-01, 04:01 #1 sean     Aug 2004 New Zealand 22·3·19 Posts Numbers wanted for OEIS sequences The OEIS has a number of sequences needing more terms but which are currently blocked on finding factors of various numbers. In some cases, it would suffice to find a single factor, in other cases complete factorization is required. This list should not be considered exhaustive. A046461 Is Smarandache(691) a semiprime. A063684 Factorization of 105!+2. A078781 Is 151!-1 a semiprime. A080802 Is 151!-1 a semiprime. A081715 Is 3^514+2 a semiprime. A085745 Is 2^1239+1239 a semiprime. A085747 Is 90!+97 a semiprime. A099954 Is Fibonacci(1801) a semiprime. A115101 Factorization of Lucas(2602). A115973 Factorization of 101^101+1 A165767 Is 2^669-669 a semiprime. A167937 Is 114!+1 a semiprime.
2010-12-01, 04:28   #2
CRGreathouse

Aug 2006

3·1,993 Posts

Quote:
 Originally Posted by sean A078781 Is 151!-1 a semiprime. A080802 Is 151!-1 a semiprime.
A factorization of 154!-1 would also be nice. Note that 157 / 37272934189201737869016720929 are also members.

 2010-12-01, 06:39 #3 kar_bon     Mar 2006 Germany 32×7×47 Posts Here's another one: Euclid-Mullin-Sequence: A000945 and an overview here (needs first factor of a C256 of index 47). Or the smallest open Sequence of Home Prime Base 10 for n=49 (Factorization of index 103 of C178).
2010-12-01, 13:35   #4
R.D. Silverman

"Bob Silverman"
Nov 2003
North of Boston

11101001001002 Posts

Quote:
 Originally Posted by sean The OEIS has a number of sequences needing more terms but which are currently blocked on finding factors of various numbers. In some cases, it would suffice to find a single factor, in other cases complete factorization is required. This list should not be considered exhaustive. A046461 Is Smarandache(691) a semiprime. A063684 Factorization of 105!+2. A078781 Is 151!-1 a semiprime. A080802 Is 151!-1 a semiprime. A081715 Is 3^514+2 a semiprime. A085745 Is 2^1239+1239 a semiprime. A085747 Is 90!+97 a semiprime. A099954 Is Fibonacci(1801) a semiprime. A115101 Factorization of Lucas(2602). A115973 Factorization of 101^101+1 A165767 Is 2^669-669 a semiprime. A167937 Is 114!+1 a semiprime.
What is this "fetish" that people seem to have for semi-primes? ("p2's")???
Some of the numbers mentioned above are well within current capabilities.
Some are just out of reach (e.g. 2^1239 + 1239, 151!-1).
Some are well beyond them. (e.g. F1801, L2602)

2010-12-01, 14:17   #5
CRGreathouse

Aug 2006

175B16 Posts

Quote:
 Originally Posted by R.D. Silverman What is this "fetish" that people seem to have for semi-primes? ("p2's")?
It's selection bias: detecting semiprimes requires factorization of a 'pure' composite, and this thread is looking for sequences requiring hard factorizations. Similarly, to be large enough to be difficult but also small enough to be doable, most of these sequences have exponential growth; that's also an artifact of our selection, since most OEIS sequences don't share that feature.

 2010-12-10, 19:21 #6 CRGreathouse     Aug 2006 3·1,993 Posts Also someone might consider submitting a b-file for sequences such as http://oeis.org/A078604
2010-12-11, 17:03   #7
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

26·131 Posts

Quote:
 Originally Posted by sean The OEIS has a number of sequences needing more terms but which are currently blocked on finding factors of various numbers. In some cases, it would suffice to find a single factor, in other cases complete factorization is required. This list should not be considered exhaustive. A046461 Is Smarandache(691) a semiprime. A063684 Factorization of 105!+2. A078781 Is 151!-1 a semiprime. A080802 Is 151!-1 a semiprime. A081715 Is 3^514+2 a semiprime. A085745 Is 2^1239+1239 a semiprime. A085747 Is 90!+97 a semiprime. A099954 Is Fibonacci(1801) a semiprime. A115101 Factorization of Lucas(2602). A115973 Factorization of 101^101+1 A165767 Is 2^669-669 a semiprime. A167937 Is 114!+1 a semiprime.
If CRG can help me with 1 or 2 things I may have a PARI code to search for numbers in A046461 , pretty much what I need is to count the factors to make sure it's only 2 and check to see only 1's in the exponents place. I solve that and I can come up with a script to test numbers.

2010-12-11, 17:24   #8
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

838410 Posts

Quote:
 Originally Posted by science_man_88 If CRG can help me with 1 or 2 things I may have a PARI code to search for numbers in A046461 , pretty much what I need is to count the factors to make sure it's only 2 and check to see only 1's in the exponents place. I solve that and I can come up with a script to test numbers.
never mind:

Code:
if([isprime(factor(6)),1] && !isprime(6) ,print(factor(6)))
seems to work. now i replace 6 with eval(c) where c is a string from numbers 1 - x and i think it should work, put a for loop around it and change print(factor(6)) to print(x) and it should work fine for smaller ones.

2010-12-11, 17:26   #9
science_man_88

"Forget I exist"
Jul 2009
Dumbassville

20C016 Posts

Quote:
 Originally Posted by science_man_88 never mind: Code: if([isprime(factor(6)),1] && !isprime(6) ,print(factor(6))) seems to work. now i replace 6 with eval(c) where c is a string from numbers 1 - x and i think it should work, put a for loop around it and change print(factor(6)) to print(x) and it should work fine for smaller ones.
slight problem it doesn't just find the ones already in the sequence it finds a lot more because of a faulty check.

 2010-12-11, 17:38 #10 science_man_88     "Forget I exist" Jul 2009 Dumbassville 100000110000002 Posts never mind the [] gives a fault anyways. Code: checking(y)= c="";for(z=1,y,c=concat(c,z));if(!isprime(eval(c)) ,print(y)) this is what I have now. Last fiddled with by science_man_88 on 2010-12-11 at 18:07
 2010-12-11, 18:33 #11 science_man_88     "Forget I exist" Jul 2009 Dumbassville 26×131 Posts http://oeis.org/A006881 has a small error in the PARI code ( a missing semicolon) I know this because i was thinking of going off this next, and then just using eval(c) in the place of n and checking if a integer sqrt was present I do that and we have a code for http://oeis.org/A046461 as well.

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