%I A002957 M0680
%S A002957 1,2,3,5,7,26,27,53,147,236,248,386,401,546,785,1325,1755,2906,3020,
%T A002957 5407,5697,5969,7517,15749,19233,38232,55347
%N A002957 Numbers n such that 2*10^n - 1 is prime.
%C A002957 Also numbers n such that 10^n + 9*R_n is prime, where R_n = 11...1 is 
               the repunit (A002275) of length n.
%D A002957 H. Riesel, "Prime numbers and computer methods for factorization," Progress 
               in Mathematics, Vol. 57, Birkhauser, Boston, 1985, Page 162.
%D A002957 N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, 
               Academic Press, 1995 (includes this sequence).
%D A002957 C. R. Zarnke and H. C. Williams, Computer determination of some large 
               primes, pp. 563-570 in Proceedings of the Louisiana Conference on 
               Combinatorics, Graph Theory and Computer Science. Vol. 2, edited 
               R. C. Mullin et al., 1971.
%H A002957 Makoto Kamada, <a href="http://homepage2.nifty.com/m_kamada/math/19999.htm">
               Factorizations of 199...99</a>
%H A002957 <a href="Sindx_Pri.html#Pri_rep">Index entries for primes involving repunits</
               a>
%t A002957 Do[ If[ PrimeQ[ 2*10^n - 1], Print[n] ], {n, 1, 15000} ]
%Y A002957 Sequence in context: A088054 A085907 A024777 this_sequence A117135 A019372 
               A117299
%Y A002957 Adjacent sequences: A002954 A002955 A002956 this_sequence A002958 A002959 
               A002960
%K A002957 hard,nonn
%O A002957 1,2
%A A002957 N. J. A. Sloane (njas(AT)research.att.com), Simon Plouffe (simon.plouffe(AT)gmail.com)
%E A002957 Corrected and extended by Robert G. Wilson v (rgwv(AT)rgwv.com), Feb 
               02 2001.
%E A002957 Other known primes: 15749, 19233, 38232, 55347.
%E A002957 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008