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 A156321 #5 May 23 2012 15:13:42
%S A156321 0,1,4,9,20,42,85,171,343,687,1375,2751,5504,11009,22019,44039,88080,
%T A156321 176162,352325,704651,1409303,2818607,5637215,11274431,22548863,
%U A156321 45097727,90195455,180390911,360781823,721563647,1443127295,2886254591
%N A156321 a(1)=0, a(n+1) is smallest nonprime > 2*a(n).
%H A156321 Harvey P. Dale, <a href="/A156321/b156321.txt">Table of n, a(n) for n = 1..1000</a>
%e A156321 a(5) is the smallest nonprime > 2*a(4)=2*9=18, hence a(5)=20.
%t A156321 np[n_]:=Module[{k=2n+1},If[PrimeQ[k],k+1,k]]; NestList[np,0,40] (* _Harvey P. Dale_, May 23 2012 *)
%Y A156321 Cf. A000027, A055496, A141468.
%K A156321 nonn
%O A156321 1,3
%A A156321 _Juri-Stepan Gerasimov_, Feb 08 2009
%E A156321 More terms from _R. J. Mathar_, Feb 10 2009