%I #26 Apr 16 2024 16:00:29
%S 1,2,3,18,177,324,388,392,404,531,1083,12546,22443,54388,72916,185520
%N Numbers k such that 10^k + 6*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C Also numbers k such that (5*10^k - 11)/3 is prime.
%C a(16) > 10^5. - _Robert Price_, Nov 15 2014
%H Makoto Kamada, <a href="https://stdkmd.net/nrr/1/16663.htm#prime">Prime numbers of the form 166...663</a>.
%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F a(n) = A102023(n) + 1.
%p A102939:=n->`if`(isprime((5*10^n-11)/3), n, NULL): seq(A102939(n), n=1..1000); # _Wesley Ivan Hurt_, Nov 15 2014
%t Do[ If[ PrimeQ[(5*10^n - 11)/3], Print[n]], {n, 0, 10000}]
%Y Cf. A002275, A102023.
%K more,nonn
%O 1,2
%A _Robert G. Wilson v_, Dec 16 2004
%E Addition of a(12)-a(13) from Kamada data by _Robert Price_, Dec 12 2010
%E a(14)-a(15) from Kamada data by _Robert Price_, Nov 15 2014
%E a(16) from Kamada data by _Tyler Busby_, Apr 16 2024