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 A008874 #32 Jul 08 2025 01:21:51 %S A008874 63,190,95,286,143,430,215,646,323,970,485,1456,728,364,182,91,274, %T A008874 137,412,206,103,310,155,466,233,700,350,175,526,263,790,395,1186,593, %U A008874 1780,890,445,1336,668,334,167,502,251,754,377,1132,566,283,850,425,1276,638,319,958,479,1438,719,2158,1079 %N A008874 3x+1 sequence starting at 63. %C A008874 a(n) = A008875(n - 2) for n > 1. %C A008874 Doesn't reach a power of 2 until a(103) = 16. - _Alonso del Arte_, May 18 2015 %D A008874 Richard K. Guy, Unsolved Problems in Number Theory, E16. %H A008874 T. D. Noe, <a href="/A008874/b008874.txt">Table of n, a(n) for n=0..107</a> %H A008874 <a href="/index/3#3x1">Index entries for sequences related to 3x+1 (or Collatz) problem</a> %p A008874 f := proc(n) option remember; if n = 0 then 63; elif f(n-1) mod 2 = 0 then f(n-1)/2 else 3*f(n-1)+1; fi; end; %t A008874 NestList[If[EvenQ[#], #/2, 3# + 1] &, 63, 100] (* _Vincenzo Librandi_, Jul 29 2014 *) %o A008874 (Haskell) %o A008874 a008874 n = a008874_list !! n %o A008874 a008874_list = 63 : iterate a006370 63 %o A008874 -- _Reinhard Zumkeller_, Aug 30 2012 %o A008874 (Magma) [n eq 1 select 63 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 A008874 Cf. A006370. %Y A008874 Row 63 of A347270. %K A008874 nonn,easy %O A008874 0,1 %A A008874 _N. J. A. Sloane_ %E A008874 More terms from Larry Reeves (larryr(AT)acm.org), Apr 27 2001