%I A098088
%S A098088 2,3,4,10,18,21,22,28,43,66,121,133,178,241,454,553,1600,2175,2978,3649,
%T A098088 7708,8316
%N A098088 Numbers n such that 6*R_n - 5 is prime, where R_n = 11...1 is the repunit 
               (A002275) of length n.
%C A098088 Also numbers n such that (2*10^n-17)/3 is prime.
%C A098088 No others less than 7000.
%C A098088 The terms 1600, 2175, 2978 and 3649 correspond to primes. - Joao da Silva 
               (zxawyh66(AT)yahoo.com), Oct 3 2005
%H A098088 Makoto Kamada, <a href="http://homepage2.nifty.com/m_kamada/math/66661.htm">
               Factorizations of 66...661</a>.
%H A098088 <a href="Sindx_Pri.html#Pri_rep">Index entries for primes involving repunits</
               a>
%F A098088 a(n) = A056658(n) + 1. - Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 
               01 2008
%e A098088 If n = 4 we get ((2*10^4)-17)/3 = 19983/3 = 6661, which is prime.
%t A098088 Do[ If[ PrimeQ[ 2(10^n - 1)/3 - 5], Print[n]], {n, 0, 7000}]
%Y A098088 Sequence in context: A085934 A056701 A055506 this_sequence A080500 A007661 
               A049891
%Y A098088 Adjacent sequences: A098085 A098086 A098087 this_sequence A098089 A098090 
               A098091
%K A098088 more,nonn
%O A098088 1,1
%A A098088 Julien Peter Benney (jpbenney(AT)ftml.net), Sep 14 2004
%E A098088 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008