%I #18 Jan 17 2019 13:44:07
%S 1,2,5,6,7,11,17,29,42,47,96,108,166,210,331,1022,3593,4426,5704,
%T 13936,16486,85910
%N Numbers n such that 6*10^n-7 is prime.
%C Also numbers n such that 5*10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
%C a(23) > 2*10^5. - _Robert Price_, Aug 07 2015
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/5/59993.htm#prime">Prime numbers of the form 599...993</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F a(n) = A101590(n) + 1.
%t Do[ If[ PrimeQ[6*10^n - 7], Print[n]], {n, 0, 10000}]
%o (PARI) is(n)=ispseudoprime(6*10^n-7) \\ _Charles R Greathouse IV_, Jun 13 2017
%Y Cf. A002275, A101590.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Jan 18 2005
%E Addition of a(20)-a(22) from Kamada data by _Robert Price_, Dec 09 2010