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.
Collatz[n_] := NestWhileList[If[EvenQ[#], #/2, 3 # + 1] &, n, # > 1 &]; t = Table[c = Collatz[n]; co = Select[c, OddQ]; If[Length[co] == 1, 0, co[[-2]]], {n, 10000}]; Flatten[Table[Position[t, (4^n - 1)/3, 1, 1], {n, 2, 7}]]
{0}~Join~Table[If[AllTrue[#, EvenQ], 0, SelectFirst[Reverse@ #, OddQ]] &@ Most@ Rest@ NestWhileList[If[EvenQ@ #, #/2, (3 # + 1)/2] &, n, # > 1 &], {n, 2, 113}] (* Michael De Vlieger, Aug 14 2017 *)
a(n)=my(k); if(n%2, n=3*n+1); n>>=valuation(n,2); if(n==1, return(0)); k=n; while(1, n+=(n+1)>>1; n>>=valuation(n,2); if(n==1, return(k), k=n)) \\ Charles R Greathouse IV, Aug 14 2017
(define (A237660 n) (let loop ((n (A014682 n)) (last-odd 0)) (if (= 1 n) last-odd (loop (A014682 n) (if (odd? n) n last-odd))))) (define (A014682 n) (if (even? n) (/ n 2) (/ (+ n n n 1) 2))) ;; Antti Karttunen, Aug 13 2017
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