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 A214532 #11 Aug 20 2014 08:46:11
%S A214532 19,91,1199,1919,1991,9119,9191,9911,111999,119199,119919,119991,
%T A214532 191199,191919,191991,199119,199191,199911,911199,911919,911991,
%U A214532 919119,919191,919911,991119,991191,991911,999111,11119999,11191999
%N A214532 List of words over {1,9} with equal numbers of 1's and 9's.
%C A214532 This is to 9 as A214531 is to 8, as A214530 is to 7, as A214530 is to 6, as A214522 is to 5, as A214521 is to 4, as A214488 is to 3, and as A214218 is to 2.
%C A214532 The subsequence of primes (when interpreted as decimal integers) is A087515.
%t A214532 FromDigits/@(Flatten[Permutations/@Table[Join[Table[1,{n}],Table[9,{n}]],{n,4}],1]) (* _Harvey P. Dale_, Aug 20 2014 *)
%Y A214532 Cf. A214218, A214488, A214521, A214522, A214524, A214530, A214531.
%K A214532 nonn,base,easy
%O A214532 1,1
%A A214532 _Jonathan Vos Post_, Jul 20 2012