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 A066861 For x > 0 let f(x) = x/2 if x is even, f(x) = (3*x+1)/2 if x is odd (3x+1 or Collatz problem). Sequence gives numbers such that k-1 applications of f lead to k for some k > 0. 1
 1, 3, 4, 10, 11, 12, 18, 19, 24, 26, 32, 34, 35, 43, 49, 56, 58, 60, 61, 65, 66, 67, 80, 96, 104, 106, 113, 121, 130, 131, 132, 133, 134, 144, 145, 146, 147, 148, 149, 153, 156, 157, 158, 167, 169, 176, 180, 181, 184, 186, 192, 196, 197, 198, 200, 202, 204, 205, 206, 207, 222, 223, 246, 247, 249, 254, 255 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Conjecture: a(n) = n*log(n) asymptotically. Numbers n such that A258769(n)> 0. - Derek Orr, Jun 11 2015 LINKS FORMULA f^(k-1)(n) = k. EXAMPLE 11 is in the sequence since seven applications of f lead to 8: 11 -> 17 -> 26 -> 13 -> 20 -> 10 -> 5 -> 8; 145 is in the sequence since 60 applications of f lead to 61. PROG (PARI) {for(n=1, 205, k=1; x=n; while(x!=1&&x!=k, x=if(x%2==0, x/2, (3*x+1)/2); k++); if(x==k, print1(n, ", ")))} CROSSREFS Cf. A014682, A258769. Sequence in context: A075100 A288660 A212440 * A292564 A191192 A139063 Adjacent sequences:  A066858 A066859 A066860 * A066862 A066863 A066864 KEYWORD nonn,easy AUTHOR Benoit Cloitre, Jan 22 2002 EXTENSIONS Edited by Klaus Brockhaus, May 26 2003 STATUS approved

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Last modified March 31 17:36 EDT 2020. Contains 333151 sequences. (Running on oeis4.)