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Numbers k such that (10^k)/2 - 1 is prime.
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%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