%I A003307 M0823
%S A003307 1,2,3,7,8,12,20,23,27,35,56,62,68,131,222,384,387,579,644,1772,3751,
%T A003307 5270,6335,8544,9204,12312,18806,21114,49340,75551,90012,128295,143552,
%U A003307 147488
%N A003307 Numbers n such that 2*3^n - 1 is prime.
%D A003307 R. K. Guy, Unsolved Problems in Theory of Numbers, Section A3.
%D A003307 Wilfrid Keller and Jorg Richstein, Solutions of the congruence a^(p-1) 
               = 1 (mod p^r), Math. Comp., Vol. 74 (2005), 927-936.
%D A003307 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A003307 H. C. Williams, The primality of certain integers of the form 2Ar^n - 
               1, Acta Arith. 39 (1981), 7-17.
%D A003307 H. C. Williams and C. R. Zarnke, Some prime numbers of the forms 2*3^n+1 
               and 2*3^n-1, Math. Comp., 26 (1972), 995-998.
%H A003307 Steven Harvey, <a href="http://www.geocities.com/harvey563/wills.txt">
               Williams Primes</a>
%t A003307 lst={};Do[p=2*3^n-1;If[PrimeQ[p], AppendTo[lst, n]], {n, 0, 7!}];lst 
               [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Dec 05 2008]
%Y A003307 Cf. A002235, A046865, A079906, A046866, A001771, A005541, A056725, A046867, 
               A079907.
%Y A003307 Cf. A079363 (primes of the form 2*3^n-1), A003306 (n such that 2*3^n+1 
               is prime).
%Y A003307 Sequence in context: A047221 A032967 A111101 this_sequence A105601 A033082 
               A084406
%Y A003307 Adjacent sequences: A003304 A003305 A003306 this_sequence A003308 A003309 
               A003310
%K A003307 nonn,hard,nice
%O A003307 1,2
%A A003307 N. J. A. Sloane (njas(AT)research.att.com).
%E A003307 More terms from Douglas Burke (dburke(AT)nevada.edu)
%E A003307 More terms from T. D. Noe (noe(AT)sspectra.com), Aug 24 2005
%E A003307 Corrected and extended by Herman Jamke (hermanjamke(AT)fastmail.fm), 
               Jan 05 2008