%I #13 Jan 17 2019 13:44:07
%S 1,4,18,28,55,1728,1876,1884,6841,10782,25366,98446,137250
%N Numbers n such that 7*10^n + 8*R_n + 1 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (71*10^n+1)/9 is prime.
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/7/78889.htm#prime">Prime numbers of the form 788...889</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101153(n) + 1.
%t Do[ If[ PrimeQ[(71*10^n + 1)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A101153.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 19 2005
%E a(10)-a(13) from Kamada data by _Robert Price_, Dec 14 2010