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 A121540 #2 Mar 30 2012 17:26:21
%S A121540 3,4,5,6,8,10,12,14,15,16,18,19,20,22,23,24,26,27,28,30,32,34,35,36,
%T A121540 38,40,42,43,44,46,48,50,51,52,54,56,58,59,60,62,63,64,66,67,68,70,72,
%U A121540 74,75,76,78,79,80,82,83,84,86,88,90,91,92,94,95,96,98,99,100
%N A121540 Increasing sequence: "if n appears a*n+b does not", case a(1)=3, a=2, b=1.
%t A121540 s={3};With[{a=2,b=1},Do[If[FreeQ[s,(n-b)/a],AppendTo[s,n]],{n,4,100}]];s
%Y A121540 Cf. A121538, A121539, A121541, A121542.
%K A121540 nonn
%O A121540 1,1
%A A121540 _Zak Seidov_, Aug 08 2006