A098930 - OEIS

%I #20 Jul 08 2021 03:14:33

%S 2,3,5,6,12,186,435,1746,3447,18798,70209

%N Numbers k such that 2*10^k + 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%C Also numbers k such that (23*10^k - 41)/9 is prime.

%C a(12) > 10^5. - _Robert Price_, Mar 04 2015

%H Makoto Kamada, <a href="https://stdkmd.net/nrr/2/25551.htm#prime">Prime numbers of the form 255...551</a>.

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

%F a(n) = A101961(n) + 1.

%e If n = 3 we get 2551, which is prime.

%t Do[ If[ PrimeQ[(23*10^n - 41)/9], Print[n]], {n, 0, 10000}]

%Y Cf. A002275, A101961.

%K more,nonn

%O 1,1

%A Julien Peter Benney (jpbenney(AT)ftml.net), Oct 20 2004

%E a(7) from _Ray Chandler_, Nov 04 2004

%E a(8) & a(9) from _Robert G. Wilson v_, Dec 17 2004

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

%E a(11) from _Robert Price_, Mar 04 2015