cp's OEIS Frontend

A102940 Numbers k such that 10^k + 6*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.

%I A102940 #30 Jul 08 2021 03:09:34
%S A102940 0,1,2,3,5,9,11,14,32,54,55,60,153,200,461,569,840,847,1296,1356,2007,
%T A102940 2627,2847,3110,6876,9161,17765,33555,59142,65773,280710
%N A102940 Numbers k such that 10^k + 6*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
%C A102940 Also numbers k such that (5*10^k + 1)/3 is prime.
%C A102940 Numbers k such that A126109(k) is prime.
%C A102940 a(32) > 3*10^5. - _Robert Price_, Nov 15 2014
%H A102940 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/16667.htm#prime">Prime numbers of the form 166...667</a>.
%H A102940 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>
%F A102940 a(n) = A102024(n-1) + 1.
%p A102940 A102940:=n->`if`(isprime((5*10^n+1)/3),n,NULL): seq(A102940(n),n=0..1000); # _Wesley Ivan Hurt_, Nov 15 2014
%t A102940 Do[ If[ PrimeQ[(5*10^n + 1)/3], Print[n]], {n, 0, 10000}]
%Y A102940 Cf. A000040, A002275, A102024, A126109.
%K A102940 more,nonn
%O A102940 1,3
%A A102940 _Robert G. Wilson v_, Dec 16 2004
%E A102940 Edited by _N. J. A. Sloane_, Mar 16 2007
%E A102940 Addition of a(27) from Kamada data by _Robert Price_, Dec 08 2010
%E A102940 a(28) from Erik Branger May 01 2013 by _Ray Chandler_, Aug 16 2013
%E A102940 a(29)-a(31) from Kamada data by _Robert Price_, Nov 15 2014