This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A295988 #22 Sep 08 2021 21:14:11 %S A295988 3,4,5,7,15,55,211,391,595,3461,5029,5220,5333,8073,15797,16132,21457, %T A295988 29283,78791,85143,179973,211030,445774,464844,511057 %N A295988 Numbers k such that (10^k)/2 - 1 is prime. %F A295988 a(n) = 1 + A056712(n). - _Omar E. Pol_, Dec 02 2017 %e A295988 499, 4999, 49999, 4999999 are prime, while 4, 49, 499999, 49999999 are composite. %t A295988 Select[Range[10^3], PrimeQ[10^#/2 - 1] &] (* _Michael De Vlieger_, Dec 02 2017 *) %o A295988 (PARI) isok(n) = isprime(10^n/2 - 1); \\ _Michel Marcus_, Dec 02 2017 %Y A295988 Cf. A056712, A093945. %K A295988 nonn,more %O A295988 1,1 %A A295988 _Patrick A. Thomas_, Dec 02 2017 %E A295988 a(7)-a(25) from _Michel Marcus_ and _Omar E. Pol_, Dec 02 2017