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 A016027 Indices of prime Mersenne numbers (A001348). 11
 1, 2, 3, 4, 6, 7, 8, 11, 18, 24, 28, 31, 98, 111, 207, 328, 339, 455, 583, 602, 1196, 1226, 1357, 2254, 2435, 2591, 4624, 8384, 10489, 12331, 19292, 60745, 68301, 97017, 106991, 215208, 218239, 474908, 877615, 1329726, 1509263, 1622441, 1881339, 2007537, 2270720, 2584328, 2610944 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The following are also members of the sequence: 3443958, 4350601, 4517402. Numbers n such that A001348(n) is a Mersenne prime A000668. - Omar E. Pol, Jul 14 2012 Numbers n such that A060286(n) is a perfect number A000396.  Assuming there are no odd perfect numbers, A060286(a(n)) = A000396(n). - Omar E. Pol, Dec 13 2012 REFERENCES R. K. Guy, Unsolved Problems in Number Theory, 3rd ed., Springer-Verlag, NY, 2004, Sec. A3. G. H. Hardy and E. M. Wright, An Introduction to the Theory of Numbers, 3rd ed., Oxford Univ. Press, 1954, p. 16. P. Ribenboim, The New Book of Prime Number Records, Springer-Verlag, NY, 1996, Chap. 2, Sec. VII. LINKS Andrew R. Booker, The Nth Prime Page Chris K. Caldwell, Mersenne Primes Will Edgington, List of Mersenne primes [from Internet Archive Wayback Machine] Great Internet Mersenne Prime Search (GIMPS), Distributed Computing Projects Paulo Ribenboim, Galimatias arithmeticae, Mathematics Magazine, vol. 71, no. 5, page 337, Dec. 1998. Wikipedia, Mersenne Primes. FORMULA a(n) = pi(A000043(n)). a(n) = A000720(A000043(n)). EXAMPLE The first four Mersenne numbers 2^2 - 1 = 3, 2^3 - 1 = 7, 2^5 - 1 = 31 and 2^7 - 1 = 127 are prime, so 1, 2, 3, 4 are members. But the fifth Mersenne number 2^11 - 1 = 2047 = 23*89 is composite, so 5 is not a member. MATHEMATICA a = {}; Do[If[PrimeQ[2^Prime[n] - 1], AppendTo[a, n]], {n, 1, 100}]; a (* Artur Jasinski *) PrimePi[{* copy the terms from A000043 *}] (* Robert G. Wilson v, Jan 20 2014 *) Position[Array[2^Prime[#] - 1 &, 640], _?PrimeQ][[All, 1]] (* Michael De Vlieger, Jan 31 2018 *) Array[PrimePi@ MersennePrimeExponent@# &, 45] (* Robert G. Wilson v, Feb 12 2018 *) PROG (PARI) i=0; for(n=1, 1e3, if(isprime(n), i++; if(ispseudoprime(2^n-1), print1(i, ", ")))) \\ Felix Fröhlich, Aug 12 2014 CROSSREFS Cf. A000043, A001348, A059305 (index of the n-th Mersenne prime). Sequence in context: A163866 A027206 A198034 * A265347 A205591 A191282 Adjacent sequences:  A016024 A016025 A016026 * A016028 A016029 A016030 KEYWORD nonn,nice,hard AUTHOR EXTENSIONS Corrected by Vasiliy Danilov (danilovv(AT)usa.net), Jun 15 1998 Further corrections from Reto Keiser (rkeiser(AT)stud.ee.ethz.ch), Jan 10 2001 a(39) from Robert G. Wilson v, Mar 20 2006 a(40) from Robert G. Wilson v, May 29 2011 a(41) from Robert G. Wilson v, Jul 07 2012 a(42) from Robert G. Wilson v, Jan 20 2014 a(43)-a(44) from Robert G. Wilson v, Aug 20 2015 a(45) from Patrick J. McNab, Dec 18 2017 a(46)-a(47) from Ivan Panchenko, Apr 09 2018 STATUS approved

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Last modified December 5 14:32 EST 2021. Contains 349557 sequences. (Running on oeis4.)