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 A217942 #5 Oct 23 2012 15:37:41 %S A217942 1,3,7,16,33,70,141,287,578,1163,2327,4674,9349,18718,37452,74948, %T A217942 149897,299890,599781,1199724,2399498,4799278,9598557,19197792, %U A217942 38395600,76792313,153584811,307171904,614343809,1228692446,2457384893,4914778758,9829558241,19659134194,39318268464 %N A217942 A217941(n)/2. %Y A217942 A217941 is the main entry. %K A217942 nonn %O A217942 1,2 %A A217942 _N. J. A. Sloane_, Oct 23 2012