%I A028491 M2643
%S A028491 3,7,13,71,103,541,1091,1367,1627,4177,9011,9551,36913,43063,49681,
%T A028491 57917
%N A028491 Numbers n such that (3^n - 1)/2 is prime.
%C A028491 If n is in the sequence and m=3^(n-1) then m is a term of A033632 (phi(sigma(m)) 
               = sigma(phi(m)), so 3^(A028491-1) is a subsequence of A033632. For 
               example since 9551 is in the sequence, phi(sigma(3^9550)) = sigma(phi(3^9550)). 
               - Farideh Firoozbakht (mymontain(AT)yahoo.com), Feb 09 2005
%C A028491 Salas shows that primes whose reciprocals are in the Cantor set are precisely 
               those of the form (3^a(n) - 1)/2. - Charles R Greathouse IV Jul 29 
               2009
%D A028491 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A028491 J. Brillhart et al., Factorizations of b^n +- 1. Contemporary Mathematics, 
               Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and 
               later supplements.
%D A028491 H. Dubner, Generalized repunit primes, Math. Comp., 61 (1993), 927-930.
%H A028491 J. Brillhart et al., <a href="http://www.ams.org/online_bks/conm22/">
               Factorizations of b^n +- 1</a>, Contemporary Mathematics, Vol. 22, 
               Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
%H A028491 S. S. Wagstaff, Jr., <a href="http://www.cerias.purdue.edu/homes/ssw/
               cun/index.html">The Cunningham Project</a>
%H A028491 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               Repunit.html">Repunit</a>
%H A028491 H. Lifchitz, <a href="http://www.primenumbers.net/Henri/us/MersFermus.htm">
               Mersenne and Fermat primes field</a>
%H A028491 Christian Salas, <a href="http://arxiv.org/abs/0906.0465">Base-3 repunit 
               primes and the Cantor set</a>
%t A028491 Do[If[PrimeQ[(3^n-1)/2], Print[n]], {n, 10000}] (Firoozbakht)
%Y A028491 Cf. A076481, A033632.
%Y A028491 Sequence in context: A103564 A083201 A004060 this_sequence A137474 A071087 
               A038691
%Y A028491 Adjacent sequences: A028488 A028489 A028490 this_sequence A028492 A028493 
               A028494
%K A028491 nonn
%O A028491 1,1
%A A028491 N. J. A. Sloane (njas(AT)research.att.com), Jean-Yves Perrier (nperrj(AT)ascom.ch)
%E A028491 36913 from Farideh Firoozbakht (mymontain(AT)yahoo.com), Mar 27 2005
%E A028491 a(14), a(15) & a(16) from Robert G. Wilson v (rgwv(AT)rgwv.com), Apr 
               11 2005