

A008908


(1 + number of halving and tripling steps to reach 1 in the Collatz (3x+1) problem), or 1 if 1 is never reached.


31



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
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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

R. Zumkeller, Table of n, a(n) for n = 1..10000
J. C. Lagarias, The 3x+1 problem and its generalizations, Amer. Math. Monthly, 92 (1985), 323.
Nitrxgen, Collatz Calculator
Wikipedia, Collatz conjecture
Index entries for sequences related to 3x+1 (or Collatz) problem


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 nth row in A070165.  Reinhard Zumkeller, May 11 2013


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 xrange(1, 101)] # Indranil Ghosh, Apr 15 2017


CROSSREFS

Cf. A006577, A006370, A006667, A075677.
Sequence in context: A169844 A076123 A021783 * A050077 A261715 A185576
Adjacent sequences: A008905 A008906 A008907 * A008909 A008910 A008911


KEYWORD

nonn,nice,look


AUTHOR

N. J. A. Sloane, Bill Gosper


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



