%I
%S 3,17,99,120,243,545,630,1493,2565,8087
%N Numbers n such that 20*R_n + 1 is a prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (2*10^(n+1)11)/9 is a prime.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/22221.htm#prime">Prime numbers of the form 22...221</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A084832(n)  1.  _Robert Price_, Jan 30 2015
%t Do[ If[ PrimeQ[ 20*(10^n  1)/9 + 1 ], Print[n]], {n, 7000}]
%Y Cf. A002275, A084832, A091189 (corresponding primes).
%K nonn
%O 1,1
%A _Robert G. Wilson v_, Aug 09 2000
%E a(8) and a(9) from _Rick L. Shepherd_, Feb 22 2004
%E More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
