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A208981
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Number of iterations required to reach a power of 2 in the 3x+1 sequence starting at n.
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12
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0, 0, 3, 0, 1, 4, 12, 0, 15, 2, 10, 5, 5, 13, 13, 0, 8, 16, 16, 3, 1, 11, 11, 6, 19, 6, 107, 14, 14, 14, 102, 0, 22, 9, 9, 17, 17, 17, 30, 4, 105, 2, 25, 12, 12, 12, 100, 7, 20, 20, 20, 7, 7, 108, 108, 15, 28, 15, 28, 15, 15, 103, 103, 0, 23, 23, 23, 10, 10, 10
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OFFSET
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1,3
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COMMENTS
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The original name was: Number of iterations of the Collatz recursion required to reach a power of 2.
The statement that all paths must eventually reach a power of 2 is equivalent to the Collatz conjecture.
A006577(n) - a(n) gives the exponent for the first power of 2 reached in the Collatz trajectory of n. - Alonso del Arte, Mar 05 2012
Number of nonpowers of 2 in the 3x+1 sequence starting at n. - Omar E. Pol, Sep 05 2021
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LINKS
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FORMULA
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For x>0 an integer, define f_0(x)=x, and for r=1,2,..., f_r(x)=f_{r-1}(x)/2 if f_{r-1}(x) is even, else f_r(x)=3*f_{r-1}(x)+1. Then a(n) = min(k such that f_k(n) is equal to a power of 2).
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EXAMPLE
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a(7) = 12 because the Collatz trajectory for 7 is 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1, 4, 2, 1, ... which reached 16 = 2^4 in 12 steps.
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MAPLE
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a:= proc(n) option remember; `if`(n=2^ilog2(n), 0,
1+a(`if`(n::odd, 3*n+1, n/2)))
end:
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MATHEMATICA
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Collatz[n_?OddQ] := 3*n + 1; Collatz[n_?EvenQ] := n/2; Table[-1 + Length[NestWhileList[Collatz, n, Not[IntegerQ[Log[2, #]]] &]], {n, 50}] (* Alonso del Arte, Mar 04 2012 *)
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PROG
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(Haskell)
a208981 = length . takeWhile ((== 0) . a209229) . a070165_row
(PARI) ispow2(n)=n>>=valuation(n, 2); n==1
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CROSSREFS
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Cf. A006577 (and references therein).
Cf. A347270 (gives all 3x+1 sequences).
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KEYWORD
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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