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 A133419 #21 Jul 08 2018 16:35:07
%S A133419 6,1,1,2,26,3,36,4,3,5,56,6,66,7,5,8,86,9,96,10,7,11,116,12,126,13,9,
%T A133419 14,146,15,156,16,11,17,176,18,186,19,13,20,206,21,216,22,15,23,236,
%U A133419 24,246,25,17,26,266,27,276,28,19,29,296,30,306,31,21,32,326,33,336,34,23,35
%N A133419 Image of n under one application of the "5x+1" map.
%C A133419 The 5x+1 map sends x to x/2 if x is even, x/3 if x is odd and divisible by 3, otherwise 5x+1.
%H A133419 Tomás Oliveira e Silva, <a href="http://sweet.ua.pt/tos/3x+1.html">The px+1 problem</a>
%H A133419 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a>
%F A133419 From _Chai Wah Wu_, Mar 04 2018: (Start)
%F A133419 a(n) = 2*a(n-6) - a(n-12) for n > 12.
%F A133419 G.f.: x*(4*x^10 + x^9 + x^8 + 2*x^7 + 24*x^6 + 3*x^5 + 26*x^4 + 2*x^3 + x^2 + x + 6)/(x^12 - 2*x^6 + 1). (End)
%t A133419 Table[If[EvenQ[n], n/2, If[Mod[n, 3] == 0, n/3, 5*n + 1]], {n, 1, 80}] (* _Stefan Steinerberger_, Feb 16 2008 *)
%t A133419 Table[Which[EvenQ[n],n/2,Divisible[n,3],n/3,True,5n+1],{n,70}] (* _Harvey P. Dale_, Jul 08 2018 *)
%o A133419 (PARI) a(n)=if(n%2,if(n%3,5*n+1,n/3),n/2) \\ _Charles R Greathouse IV_, Sep 02 2015
%Y A133419 Cf. A133420.
%K A133419 nonn,easy
%O A133419 1,1
%A A133419 _N. J. A. Sloane_, Nov 27 2007
%E A133419 More terms from _Stefan Steinerberger_, Feb 16 2008
%E A133419 Comment clarified by _Chai Wah Wu_, Mar 04 2018