|
%I #22 Sep 08 2021 21:14:11
%S 3,4,5,7,15,55,211,391,595,3461,5029,5220,5333,8073,15797,16132,21457,
%T 29283,78791,85143,179973,211030,445774,464844,511057
%N Numbers k such that (10^k)/2 - 1 is prime.
%F a(n) = 1 + A056712(n). - _Omar E. Pol_, Dec 02 2017
%e 499, 4999, 49999, 4999999 are prime, while 4, 49, 499999, 49999999 are composite.
%t Select[Range[10^3], PrimeQ[10^#/2 - 1] &] (* _Michael De Vlieger_, Dec 02 2017 *)
%o (PARI) isok(n) = isprime(10^n/2 - 1); \\ _Michel Marcus_, Dec 02 2017
%Y Cf. A056712, A093945.
%K nonn,more
%O 1,1
%A _Patrick A. Thomas_, Dec 02 2017
%E a(7)-a(25) from _Michel Marcus_ and _Omar E. Pol_, Dec 02 2017
|
