This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A236128 #26 Jan 26 2014 14:25:30 %S A236128 2,3,5,7,11,13,29,53 %N A236128 Primes p such that gpf(gpf(2^p-1)-1) = p. %C A236128 No more terms found up to p = 1277, 1277 being the first prime for which the complete factorization of 2^p-1 is not currently known (see GIMPS link). - _Michel Marcus_, Jan 20 2014 %C A236128 Conjecture: gpf(gpf(2^p-1)-1) = p for finitely many p. %C A236128 Conjecture: gpf(lpf(2^p-1)-1) = p for infinitely many p. %C A236128 Namely, for p = 2, 3, 5, 7, 11, 13, 23, 29, 37, 43, 47, 53, ... - _Michael B. Porter_, Jan 26 2014 %C A236128 Note that gpf(lpf(2^p-1)-1) = gpf(gpf(2^p-1)-1) = p for p = 2, 3, 5, 7, 11, 13, 29, 53. See DATA. %H A236128 GIMPS, <a href="http://mersenne.org/report_exponent/">Exponent Status</a> %e A236128 For prime p=2, 2^p-1=3, gpf(3)=3, gpf(3-1)=2, so 2 is in the sequence. %e A236128 For prime p=3, 2^p-1=7, gpf(7)=7, gpf(7-1)=3, so 3 is in the sequence. %t A236128 Select[Prime[Range[25]], FactorInteger[FactorInteger[2^# - 1][[-1, 1]] - 1][[-1, 1]] == # &] (* _Alonso del Arte_, Jan 19 2014 *) %o A236128 (PARI) isok(p) = isprime(p) && (q = (vecmax(factor(2^p-1)[,1]))) && (vecmax(factor(q-1)[,1]) == p); \\ _Michel Marcus_, Jan 19 2014 %Y A236128 Cf. A003260, A006530. %K A236128 nonn,more %O A236128 1,1 %A A236128 _Thomas Ordowski_, Jan 19 2014