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A309154
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Function of natural numbers satisfying the properties a(2*n) = 2*a(n) and a(2*n+1) = -3 + 2*a(3*n+2).
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1
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0, 1, 2, 23, 4, 13, 46, 1595, 8, 6377, 26, 799, 92, 101, 3190, 3283, 16, 401, 12754, 12775, 52, 61, 1598, 1643, 184, 51097, 202, 946891009738223808271, 6380, 6389, 6566, 118361376217277976035, 32, 204385, 802, 823, 25508, 25517, 25550, 6540635, 104, 473445504869111904137
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OFFSET
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0,3
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COMMENTS
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This integer sequence exists if and only if the Collatz conjecture is true. The proof is relatively trivial.
This is -3 times the Q function from Rozier restricted to the natural numbers.
The only multiple of 3 in the sequence is 0.
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LINKS
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FORMULA
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a(2*n) = 2*a(n); a(2*n+1) = -3 + 2*a(3*n+2).
a(n) = -3*(n mod 2) + 2*a(A014682(n)) where A014682 is the Collatz function.
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EXAMPLE
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For n = 0, the equation a(0) = 2*a(0) implies a(0) = 0.
For n = 1, the equation becomes a(1) = -3 + 2*a(2) = -3 + 4*a(1), so a(1) = 1.
For n = 3, a bit more calculating gives a(3) = -3 + 2*a(5) = -9 + 4*a(8) = -9 + 32*a(1) = 23.
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MAPLE
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a:= proc(n) option remember; `if`(n<2, n,
`if`(irem(n, 2, 'r')=0, 2*a(r), 2*a(n+r+1)-3))
end:
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MATHEMATICA
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a[n_] := a[n] = If[n < 2, n, If[EvenQ[n], 2 a[n/2], 2 a[(3n + 1)/2] - 3]];
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PROG
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(Python)
def a(x):
if x <= 1: return x
elif x%2: return -3 + 2 * a((3*x + 1)//2)
else: return 2*a(x//2)
(PARI) a(n)=if(n<=1, n, if(n%2, -3 + 2*a((3*n+1)/2), 2*a(n/2))) \\ Richard N. Smith, Jul 16 2019
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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