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 A214521 #17 Aug 30 2016 18:24:29 %S A214521 14,41,1144,1414,1441,4114,4141,4411,111444,114144,114414,114441, %T A214521 141144,141414,141441,144114,144141,144411,411144,411414,411441, %U A214521 414114,414141,414411,441114,441141,441411,444111,11114444,11141444 %N A214521 List of words over {1,4} with equal numbers of 1's and 4's. %C A214521 This is to 4 as A214488 is to 3, and as A214218 is to 2. The subsequence of primes (when interpreted as decimal integers) is A087512. %t A214521 Sort[FromDigits/@Flatten[Table[Permutations[PadRight[{},2n,{1,4}]],{n,4}],1]] (* _Harvey P. Dale_, Aug 30 2016 *) %Y A214521 Cf. A214218, A214488. %K A214521 nonn,base %O A214521 1,1 %A A214521 _Jonathan Vos Post_, Jul 19 2012