This site is supported by donations to The OEIS Foundation.

 Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing. Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A078419 Numbers n such that h(n) = 2 h(n-1) where h(n) is the length of the sequence {n, f(n), f(f(n)), ...., 1} in the Collatz (or 3x + 1) problem. (The earliest "1" is meant.) 2
 2, 5, 22, 495, 559, 2972, 3092, 3124, 3147, 3153, 3184, 3367, 3711, 3748, 3857, 3921, 3982, 4450, 4767, 17019, 17708, 17769, 17771, 17782, 17796, 17825, 17835, 17857, 17863, 17892, 18079, 18082, 18139, 18298, 18422, 18580, 18644, 18688, 18784 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Recall that f(n) = n/2 if n is even; = 3n + 1 if n is odd. LINKS EXAMPLE n, f(n), f(f(n)), ...., 1 for n = 22, 21, respectively, are: 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1; 21, 64, 32, 16, 8, 4, 2, 1. Hence h(22) = 16 = 2 * 8 = h(21) and 22 belongs to the sequence. MATHEMATICA f[n_] := If[EvenQ[n], n/2, 3n+1]; h[n_] := Module[{a, i}, i=n; a=1; While[i>1, a++; i=f[i]]; a]; Select[Range[2, 18800], 2h[ #-1]==h[ # ]&] CROSSREFS Cf. A078418, A078420. Sequence in context: A050994 A326959 A034384 * A241428 A070281 A198444 Adjacent sequences:  A078416 A078417 A078418 * A078420 A078421 A078422 KEYWORD nonn AUTHOR Joseph L. Pe, Dec 29 2002 EXTENSIONS Extended by Robert G. Wilson v, Dec 30 2002 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 14 03:31 EST 2019. Contains 329978 sequences. (Running on oeis4.)