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 A008878 #28 Jul 08 2025 01:22:08 %S A008878 39,118,59,178,89,268,134,67,202,101,304,152,76,38,19,58,29,88,44,22, %T A008878 11,34,17,52,26,13,40,20,10,5,16,8,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2, %U A008878 1,4,2,1,4,2,1,4,2,1,4,2,1,4 %N A008878 3x+1 sequence starting at 39. %D A008878 R. K. Guy, Unsolved Problems in Number Theory, E16. %H A008878 Vincenzo Librandi, <a href="/A008878/b008878.txt">Table of n, a(n) for n = 0..200</a> %H A008878 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a> %H A008878 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,1). %p A008878 f := proc(n) option remember; if n = 0 then 39; elif f(n-1) mod 2 = 0 then f(n-1)/2 else 3*f(n-1)+1; fi; end; %t A008878 NestList[If[EvenQ[#], #/2, 3# + 1]&, 39, 100] (* _Vincenzo Librandi_, Jul 29 2014 *) %o A008878 (Magma) [n eq 1 select 39 else IsOdd(Self(n-1)) select 3*Self(n-1)+1 else Self(n-1) div 2: n in [1..70]]; // _Vincenzo Librandi_, Jul 29 2014 %Y A008878 Row 39 of A347270. %K A008878 nonn %O A008878 0,1 %A A008878 _N. J. A. Sloane_