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 A214522 #8 Aug 30 2016 18:25:44
%S A214522 15,51,1155,1515,1551,5115,5151,5511,111555,115155,115515,115551,
%T A214522 151155,151515,151551,155115,155151,155511,511155,511515,511551,
%U A214522 515115,515151,515511,551115,551151,551511,555111
%N A214522 List of words over {1,5} with equal numbers of 1's and 5's.
%C A214522 This is to 5 as A214521 is to 4, as A214488 is to 3, and as A214218 is to 2. All of these, interpreted as decimal integers are divisible by 3, because each pair of "1" and "5" contributes a digital sum of 6, hence the total is divisible by 3.
%C A214522 The subsequence of semiprimes begins 15, 51, 15151551, 51115551, 51515511, 51555111, 55515111, 1115155551, 1115515551, 1151551551,..
%t A214522 Sort[FromDigits/@Flatten[Table[Permutations[PadRight[{},2n,{1,5}]],{n,3}],1]] (* _Harvey P. Dale_, Aug 30 2016 *)
%Y A214522 Cf. A214218, A214488, A214521.
%K A214522 nonn,base,easy
%O A214522 1,1
%A A214522 _Jonathan Vos Post_, Jul 19 2012