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A008908 (1 + number of halving and tripling steps to reach 1 in the Collatz (3x+1) problem), or -1 if 1 is never reached. 41
1, 2, 8, 3, 6, 9, 17, 4, 20, 7, 15, 10, 10, 18, 18, 5, 13, 21, 21, 8, 8, 16, 16, 11, 24, 11, 112, 19, 19, 19, 107, 6, 27, 14, 14, 22, 22, 22, 35, 9, 110, 9, 30, 17, 17, 17, 105, 12, 25, 25, 25, 12, 12, 113, 113, 20, 33, 20, 33, 20, 20, 108, 108, 7, 28, 28, 28, 15, 15, 15, 103 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,2
COMMENTS
The number of steps (iterations of the map A006370) to reach 1 is given by A006577, this sequence counts 1 more. - M. F. Hasler, Nov 05 2017
When Collatz 3N+1 function is seen as an isometry over the dyadics, the halving step necessarily following each tripling is not counted, hence N -> N/2, if even, but N -> (3N+1)/2, if odd. Counting iterations of this map until reaching 1 leads to sequence A064433. [Michael Vielhaber (vielhaber(AT)gmail.com), Nov 18 2009]
REFERENCES
R. K. Guy, Unsolved Problems in Number Theory, E16.
LINKS
J. C. Lagarias, The 3x+1 problem and its generalizations, Amer. Math. Monthly, 92 (1985), 3-23.
FORMULA
a(n) = A006577(n) + 1.
a(n) = f(n,1) with f(n,x) = if n=1 then x else f(A006370(n),x+1).
a(A033496(n)) = A159999(A033496(n)). - Reinhard Zumkeller, May 04 2009
a(n) = A006666(n) + A078719(n).
a(n) = length of n-th row in A070165. - Reinhard Zumkeller, May 11 2013
MAPLE
a:= proc(n) option remember; 1+`if`(n=1, 0,
a(`if`(n::even, n/2, 3*n+1)))
end:
seq(a(n), n=1..100); # Alois P. Heinz, Jan 29 2021
MATHEMATICA
Table[Length[NestWhileList[If[EvenQ[ # ], #/2, 3 # + 1] &, i, # != 1 &]], {i, 75}]
PROG
(Haskell)
a008908 = length . a070165_row
-- Reinhard Zumkeller, May 11 2013, Aug 30, Jul 19 2011
(PARI) a(n)=my(c=1); while(n>1, n=if(n%2, 3*n+1, n/2); c++); c \\ Charles R Greathouse IV, May 18 2015
(Python)
def a(n):
if n==1: return 1
x=1
while True:
if n%2==0: n//=2
else: n = 3*n + 1
x+=1
if n<2: break
return x
print([a(n) for n in range(1, 101)]) # Indranil Ghosh, Apr 15 2017
CROSSREFS
Sequence in context: A169844 A076123 A021783 * A050077 A261715 A309640
KEYWORD
nonn,nice,look
AUTHOR
EXTENSIONS
More terms from Larry Reeves (larryr(AT)acm.org), Apr 27 2001
"Escape clause" added to definition by N. J. A. Sloane, Jun 06 2017
Edited by M. F. Hasler, Nov 05 2017
STATUS
approved

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Last modified March 19 03:33 EDT 2024. Contains 370952 sequences. (Running on oeis4.)