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 A216376 #26 Sep 08 2022 08:46:03
%S A216376 201,500001,130000000000001,280000000000000000000000000001,
%T A216376 340000000000000000000000000000000001,
%U A216376 36000000000000000000000000000000000001,39000000000000000000000000000000000000001
%N A216376 Semiprimes of the form n*10^n + 1.
%C A216376 This is to A216347 as semiprimes A001358 are to primes A000040. The corresponding n are 2, 5, 13, 28, 34, 36, 39, ... (A216378).
%C A216376 a(14) >= 414*10^414 + 1. - _Hugo Pfoertner_, Jul 28 2019
%H A216376 Hugo Pfoertner, <a href="/A216376/b216376.txt">Table of n, a(n) for n = 1..13</a>
%F A216376 semiprimes in A064748.
%e A216376 a(1) = 2 * 10^2 + 1 = 201 = 3 * 67.
%e A216376 a(2) = 5 * 10^5 + 1 = 500001 = 3 * 166667.
%e A216376 a(3) = 13*10^13 + 1 = 130000000000001 = 6529 * 19911165569.
%e A216376 a(4) = 28 * 10^28 + 1 = 29 * 9655172413793103448275862069.
%t A216376 SemiPrimeQ[n_Integer] := If[Abs[n] < 2, False, (2 == Plus @@ Transpose[FactorInteger[Abs[n]]][[2]])]; Select[Table[n*10^n + 1, {n, 50}], SemiPrimeQ[#] &] (* _T. D. Noe_, Sep 07 2012 *)
%t A216376 Select[Table[n*10^n + 1, {n, 50}], PrimeOmega[#] == 2&] (* _Vincenzo Librandi_, Sep 22 2012 *)
%o A216376 (Magma) IsSemiprime:= func<n | &+[d[2]: d in Factorization(n)] eq 2>; [s: n in [1..40] | IsSemiprime(s) where s is n*10^n + 1]; // _Vincenzo Librandi_, Sep 22 2012
%Y A216376 Cf. A001358, A064748, A216347, A216378.
%K A216376 nonn,hard
%O A216376 1,1
%A A216376 _Jonathan Vos Post_, Sep 06 2012