%I A007505 M1395
%S A007505 2,5,11,23,47,191,383,6143,786431,51539607551,824633720831,26388279066623,
%T A007505 108086391056891903,55340232221128654847,226673591177742970257407,
%U A007505 59421121885698253195157962751,30423614405477505635920876929023
%N A007505 Primes of form 3*2^n -1.
%C A007505 a(1) = 2, define f(k) = 2k+1, then a(n+1) = least prime fff...(a(n)). 
               After 383 the next terem is 6143. We have f(383) = 767 (composite), 
               f(767) = 1535 (composite), f(1565)=3071(composite), f(3071) = 6143 
               (prime), hence the next term is 6143= ffff(383). - Amarnath Murthy 
               (amarnath_murthy(AT)yahoo.com), Jul 13 2005
%C A007505 If n is in the sequence and m=(n+1)/3 then m is a solution of the equation, 
               sigma(x+sigma(x))=3x (*). Is it true that there is no other solution 
               of (*)? - Farideh Firoozbakht (mymontain(AT)yahoo.com), Dec 05 2005
%D A007505 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A007505 H. Riesel, ``Prime numbers and computer methods for factorization,'' 
               Progress in Mathematics, Vol. 57, Birkhauser, Boston, 1985, Chap. 
               4, see pp. 381-384.
%H A007505 Wilfrid Keller, <a href="http://www.prothsearch.net/riesel2.html">List 
               of primes k.2^n - 1 for k < 300</a>
%H A007505 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               ThabitibnKurrahNumber.html">Thabit ibn Kurrah Number</a>
%H A007505 <a href="Sindx_Pri.html#riesel">Index entries for sequences of n such 
               that k*2^n-1 (or k*2^n+1) is prime</a>
%Y A007505 See A002235 for more terms.
%Y A007505 Sequence in context: A105120 A084403 A055011 this_sequence A059411 A126017 
               A034468
%Y A007505 Adjacent sequences: A007502 A007503 A007504 this_sequence A007506 A007507 
               A007508
%K A007505 nonn,easy,nice
%O A007505 1,1
%A A007505 N. J. A. Sloane (njas(AT)research.att.com), Robert G. Wilson v (rgwv(AT)rgwv.com)