%I A056660
%S A056660 3,17,99,120,243,545,630,1493,2565,8087
%N A056660 Numbers n such that 20*R_n + 1 is a prime, where R_n = 11...1 is the 
               repunit (A002275) of length n.
%C A056660 Also numbers n such that (2*10^(n+1)-11)/9 is a prime.
%H A056660 <a href="Sindx_Pri.html#Pri_rep">Index entries for primes involving repunits</
               a>
%H A056660 Makoto Kamada, <a href="http://homepage2.nifty.com/m_kamada/math/22221.htm">
               Factorizations of 22...221</a>.
%t A056660 Do[ If[ PrimeQ[ 20*(10^n - 1)/9 + 1 ], Print[n]], {n, 7000}]
%Y A056660 Cf. A091189 (corresponding primes).
%Y A056660 Sequence in context: A151330 A010913 A142988 this_sequence A155610 A001541 
               A161940
%Y A056660 Adjacent sequences: A056657 A056658 A056659 this_sequence A056661 A056662 
               A056663
%K A056660 nonn
%O A056660 1,1
%A A056660 Robert G. Wilson v (rgwv(AT)rgwv.com), Aug 09 2000
%E A056660 a(8) and a(9) from Rick L. Shepherd (rshepherd2(AT)hotmail.com), Feb 
               22 2004
%E A056660 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008