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 A309358 #27 Sep 08 2022 08:46:21 %S A309358 4,5,6,7,8,19,31,53,67,293,586,641,922,2137,3011 %N A309358 Numbers k such that 10^k + 1 is a semiprime. %C A309358 a(16) > 12000. %C A309358 10^k + 1 is composite unless k is a power of 2, and it can be conjectured that it is composite for all k > 2, cf. A038371 and A185121. - _M. F. Hasler_, Jul 30 2019 %C A309358 Suppose k is odd. Then k is a term if and only if (10^k+1)/11 is prime. - _Chai Wah Wu_, Jul 31 2019 %e A309358 a(1) = 4 because 10^4 + 1 = 10001 = 73*137. %t A309358 Select[Range[200], Plus@@Last/@FactorInteger[10^# + 1] == 2 &] (* _Vincenzo Librandi_, Jul 31 2019 *) %o A309358 (Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..200] | IsSemiprime(s) where s is 10^n+1]; // _Vincenzo Librandi_, Jul 31 2019 %Y A309358 Cf. A003021, A038371, A046413, A057934, A062397, A092559. %Y A309358 Odd terms in sequence: A001562. %K A309358 nonn,more,hard %O A309358 1,1 %A A309358 _Hugo Pfoertner_, Jul 29 2019