# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a056693 Showing 1-1 of 1 %I A056693 #31 Feb 01 2023 18:07:06 %S A056693 1,65,85,89,101,385,623,7783,18535,113756,135878 %N A056693 Numbers k such that 70*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k. %C A056693 Also numbers k such that (7*10^(k+1)+11)/9 is prime. %C A056693 a(12) > 2*10^5. - _Tyler Busby_, Feb 01 2023 %H A056693 Makoto Kamada, Prime numbers of the form 77...779. %H A056693 Index entries for primes involving repunits %F A056693 a(n) = A098089(n+1) - 1. - _Robert Price_, Nov 22 2014 %t A056693 Do[ If[ PrimeQ[70*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}] %t A056693 Select[Range[700], PrimeQ[(7 10^(# + 1) + 11) / 9] &] (* _Vincenzo Librandi_, Nov 22 2014 *) %o A056693 (Magma) [n: n in [1..400] | IsPrime((7*10^(n+1)+11) div 9)]; _Vincenzo Librandi_, Nov 22 2014 %Y A056693 Cf. A002275, A093404, A098089. %K A056693 hard,nonn %O A056693 1,2 %A A056693 _Robert G. Wilson v_, Aug 10 2000 %E A056693 More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 %E A056693 a(9) derived from A098089 by _Robert Price_, Nov 22 2014 %E A056693 a(10)-a(11) from _Tyler Busby_, Feb 01 2023 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE