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 A121541 #2 Mar 30 2012 17:26:21
%S A121541 4,5,6,7,8,10,12,14,16,18,19,20,22,23,24,26,27,28,30,31,32,34,35,36,
%T A121541 38,40,42,43,44,46,48,50,51,52,54,56,58,59,60,62,64,66,67,68,70,72,74,
%U A121541 75,76,78,79,80,82,83,84,86,88,90,91,92,94,95,96,98,99,100
%N A121541 Increasing sequence: "if n appears a*n+b does not", case a(1)=4, a=2, b=1.
%t A121541 s={4};With[{a=2,b=1},Do[If[FreeQ[s,(n-b)/a],AppendTo[s,n]],{n,5,100}]];s
%Y A121541 Cf. A121538, A121539, A121540, A121542.
%K A121541 nonn
%O A121541 1,1
%A A121541 _Zak Seidov_, Aug 08 2006