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A210516 The length-1 of the Collatz (3k+1) sequence for all odd fractions and integers. 4
0, 1, 2, 7, 3, 3, 2, 0, 3, 6, 5, 4, 15, 7, 5, 8, 9, 3, 11, 6, 7, 16, 1, 0, 8, 2, 7, 4, 3, 4, 16, 5, 7, 25, 4, 17, 19, 5, 13, 12, 6, 7, 17, 18, 8, 6, 7, 3, 0, 3, 22, 4, 3, 8, 31, 14, 10, 6, 9, 11, 26, 12, 19, 21, 32, 10, 9, 10, 1, 31, 8, 7, 18, 2, 8, 16, 11, 76 (list; table; graph; refs; listen; history; text; internal format)
OFFSET
1,3
COMMENTS
This sequence is the unification, in the limit, of the length of Collatz sequences for all fractions whose denominator is odd, and also all integers.
The sequence A210483 gives the triangle read by rows giving trajectory of k/(2n+1) in Collatz problem, k = 1..2n, but particular attention should be paid to numbers in the triangle T(n,k) = (n-k)/(2k+1) for n = 1,2,... and k = 0..n-1.
The example shown below gives a general idea of this regular triangle. This contains all fractions whose denominator is odd and all integers. Now, from T(n,k) we could introduce a 3D triangle in order to produce a complete Collatz sequence starting from each rational T(n,k).
Remark: a(A000124(n)) = A006577(n) because the first column of this triangle generates A006577.
The triangle T(n,k) begins
1;
2, 1/3;
3, 2/3, 1/5;
4, 3/3, 2/5, 1/7;
5, 4/3, 3/5, 2/7, 1/9;
6, 5/3, 4/5, 3/7, 2/9, 1/11;
...
LINKS
J. C. Lagarias, The set of rational cycles for the 3x+1 problem, Acta Arith. 56 (1990), 33-53.
EXAMPLE
The triangle of lengths begins
0;
1, 2;
7, 3, 3;
2, 0, 3, 6;
5, 4, 15, 7, 5;
...
Individual numbers have the following Collatz sequences:
[1] => [0] (0 iteration);
[2 1/3] => [1, 2] because: 2 -> 1 => 1 iteration; 1/3 -> 2 -> 1 => 2 iterations;
[3 2/3 1/5] => [7, 3, 3] because: 3->10->5->16->8->4->2->1 => 7 iterations; 2/3 -> 1/3 -> 2 -> 1 => 3 iterations; 1/5 -> 8/5 -> 4/5 -> 2/5 => 3 iterations.
MATHEMATICA
Collatz2[n_] := Module[{lst = NestWhileList[If[EvenQ[Numerator[#]], #/2, 3 # + 1] &, n, Unequal, All]}, If[lst[[-1]] == 1, lst = Drop[lst, -3], If[lst[[-1]] == 2, lst = Drop[lst, -2], If[lst[[-1]] == 4, lst = Drop[lst, -1], If[MemberQ[Rest[lst], lst[[-1]]], lst = Drop[lst, -1]]]]]]; t = Table[s = Collatz2[(n - k)/(2*k + 1)]; Length[s] - 1, {n, 12}, {k, 0, n - 1}]; Flatten[t] (* T. D. Noe, Jan 28 2013 *)
CROSSREFS
Sequence in context: A021370 A248140 A088538 * A257102 A226626 A249778
KEYWORD
nonn,tabl
AUTHOR
Michel Lagneau, Jan 26 2013
STATUS
approved

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Last modified March 29 02:23 EDT 2024. Contains 371264 sequences. (Running on oeis4.)