%I
%S 2,9,15,32,38,65,123,173,257,320,326,639,719,774,902,15210,69999
%N Numbers n such that 8*10^n + 7*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C Also numbers n such that (79*10^n7)/9 is prime.
%C a(18) > 10^5.  _Robert Price_, Jan 11 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/8/87777.htm#prime">Prime numbers of the form 877...77</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%t Do[ If[ PrimeQ[ 8*10^n + 7*(10^n1)/9], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A093943.
%K hard,nonn
%O 1,1
%A _Robert G. Wilson v_, Aug 11 2000
%E a(16)a(17) from _Robert Price_, Jan 11 2015
