%I #22 Jul 08 2021 03:14:41

%S 2,6,33,69,150,936,3135,5838,6990,20786,57138

%N Numbers k such that 2*10^k + 7*R_k - 6 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (25*10^k - 61)/9 is prime.

%C a(12) > 10^5. - _Robert Price_, Feb 27 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/27771.htm#prime">Prime numbers of the form 277...771</a>.

%H <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.

%F a(n) = A101966(n) + 1.

%e For n = 2 and 6, we get 271 and 2777771 which are primes.

%t Do[ If[ PrimeQ[(25*10^n - 61)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101966.

%K more,nonn

%O 1,1

%A Julien Peter Benney (jpbenney(AT)ftml.net), Oct 22 2004

%E a(6) from _Ray Chandler_, Nov 04 2004

%E a(7), a(8) & a(9) from Sam Handler (sam_5_5_5_0(AT)yahoo.com), Nov 03 2004

%E Addition of a(10) from Kamada data by _Robert Price_, Dec 13 2010

%E a(11) from _Robert Price_, Feb 27 2015