|
%I #29 Apr 25 2018 12:02:19
%S 4,7,16,43,58,106,160,229,628,1579,3805,16153,31420
%N Numbers k such that (10^k/8 + 1)/9 is prime.
%C Equivalently, numbers k such that (10^k + 8)/72 is prime.
%C a(12) > 15000. - _Robert G. Wilson v_, Dec 09 2017
%C a(14) > 10^5. - _Robert Price_, Apr 24 2018
%e 139, 138889, and 138888888888889 are prime, so 4, 7, and 16 are in the sequence.
%t Select[Range[2000], PrimeQ[(10^#/8 + 1)/9] &] (* _Michael De Vlieger_, Dec 06 2017 *)
%t ParallelMap[ If[ PrimeQ[(10^#/8 +1)/9], #, Nothing] &, Range@ 10000] (* _Robert G. Wilson v_, Dec 08 2017 *)
%o (PARI) isok(k) = (k>2) && isprime((10^k/8 + 1)/9); \\ _Michel Marcus_, Dec 05 2017
%Y Cf. A296029.
%K nonn,more
%O 1,1
%A _Patrick A. Thomas_, Dec 04 2017
%E a(8)-a(11) from _Jon E. Schoenfield_, Dec 04 2017
%E a(12)-a(13) from _Robert Price_, Apr 24 2018
|
