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 A105427 #19 Jan 17 2019 13:44:07
%S A105427 2,3,4,5,6,7,8,9,10,11,12,13,14,16,17,18,19,20,21,22,23,24,25,26,27,
%T A105427 28,29,30,31,32,33,34,35,36,37,38,39,40,42,43,44,45,46,47,48,49,50,51,
%U A105427 52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75
%N A105427 Numbers n such that the near-repdigit number consisting of a 1 followed by n 3's (i.e., of form 1333...33) is composite.
%C A105427 Complement of A056698.
%H A105427 FactorDB, <a href="http://factordb.com/index.php?query=(10%5En-1)%2F3%2B10%5En">Factorizations of (10^n-1)/3+10^n</a>
%H A105427 Makoto Kamada, <a href="https://stdkmd.net/nrr/1/13333.htm">Factorizations of 133...33</a>.
%t A105427 Select[Range[100],CompositeQ[FromDigits[PadRight[{1},#,3]]]&]-1 (* _Harvey P. Dale_, Jul 23 2014 *)
%o A105427 (PARI) isok(n) = ! isprime(10^n+(10^n-1)/3) \\ _Michel Marcus_, Jul 28 2013
%Y A105427 Cf. A051200, A089017, A093671.
%K A105427 nonn
%O A105427 1,1
%A A105427 _Lekraj Beedassy_, Apr 08 2005