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 A118521 #14 Oct 03 2015 16:35:45
%S A118521 6,9,12,24,45,57,78,90,12,24,45,57,78,90,12,24,45,57,78,90,12,24,45,
%T A118521 57,78,90,12,24,45,57,78,90,12,24,45,57,78,90,12,24,45,57,78,90,12,24,
%U A118521 45,57,78,90,12,24,45,57,78,90,12,24,45,57,78,90,12,24,45,57,78,90
%N A118521 Define sequence S_m by: initial term = m, reverse digits and add 3 to get next term. Entry shows S_6. This reaches a cycle of length 6 in 2 steps.
%H A118521 N. J. A. Sloane and others, <a href="/wiki/Sequences_of_RADD_type">Sequences of RADD type</a>, OEIS wiki.
%H A118521 <a href="/index/Rec#order_06">Index entries for linear recurrences with constant coefficients</a>, signature (0, 0, 0, 0, 0, 1).
%F A118521 a(n)=A118519(n+1). [From _R. J. Mathar_, Aug 08 2008]
%t A118521 NestList[FromDigits[Reverse[IntegerDigits[#]]]+3&,6,70] (* _Harvey P. Dale_, Sep 25 2012 *)
%t A118521 Join[{6,9},LinearRecurrence[{0,0,0,0,0,1},{12,24,45,57,78,90},66]] (* _Ray Chandler_, Jul 18 2015 *)
%t A118521 PadRight[{6,9},100,{78,90,12,24,45,57}] (* _Harvey P. Dale_, Oct 03 2015 *)
%Y A118521 Cf. A117831, A118517 on.
%K A118521 nonn,base
%O A118521 1,1
%A A118521 _N. J. A. Sloane_, May 06 2006