OEIS/3x+1 Problem

Introduction

Collatz sequences are sequences of non-negative integer numbers with a simple construction rule:

Even elements are halved, and odd elements are multiplied by 3 and then incremented by 1.

Since decades it is unknown whether the final cyle 4 - 2 - 1 is always reached for any start value. This problem is the Collatz conjecture, for which the english Wikipedia states:

It is also known as the 3n + 1 conjecture, the Ulam conjecture (after Stanisław Ulam), Kakutani's problem (after Shizuo Kakutani), the Thwaites conjecture (after Sir Bryan Thwaites), Hasse's algorithm (after Helmut Hasse), or the Syracuse problem; the sequence of numbers involved is referred to as the hailstone sequence or hailstone numbers (because the values are usually subject to multiple descents and ascents like hailstones in a cloud), or as wondrous numbers.

Straightforward visualizations of the Collatz sequences no obvious structure. The sequences for the first dozen of start values are rather short, but the sequence for 27 suddenly has 112 elements.

References

• Jeffry C. Lagarias, Ed.: The Ultimate Challenge: The 3x+1 Problem, Amer. Math. Soc., 2010, ISBN 978-8218-4940-8. MBK78
• OEIS A07165: File of first 10K Collatz sequences, ascending start values, with lengths
• Gottfried Helms: The Collatz-Problem. A view into some 3x+1-trees and a new fractal graphic representation. Univ. Kassel.
• Klaus Brennecke: Collatzfolgen und Schachbrett, on Wikibooks

Collatz Graph

When all Collatz sequences are read backwards, they form the Collatz graph starting with 1, 2, 4, 8 ... . At each node m > 4 in the graph, the path from the root (4) can be continued

• always to m * 2, and
• to (m - 1) / 3 if m ≡ 1 mod 3.

When m ≡ 0 mod 3, the path will continue with duplications only, since these maintain the divisibility by 3.

The Collatz conjecture claims that the graphs contains all numbers, and that - except for the leading cycle 1 - 2 - 4 - 1 - 2 - 4 ... - it has the form of a tree without cycles. We will not be concerned with this cycle, and we start with node 4, the root.

Graph Operations

Following Trümper, we use abbreviations for the elementary operations which transform a node (element, number) in the Collatz graph:

Name	Mnemonic	Distance to root	Mapping	Condition
d	down	-1	m ↦ m / 2	m ≡ 0 mod 2
u	up	-1	m ↦ 3 * m + 1	(none)
s := ud	spike	-2	m ↦ (m / 2) * 3 + 1	m ≡ 0 mod 2
δ	divide	+1	m ↦ (m - 1) / 3	m ≡ 1 mod 3
µ	multiply	+1	m ↦ m * 2	(none)
σ := δµ	step	+2	m ↦ ((m - 1) / 3) * 2	m ≡ 1 mod 3

We will mainly be interested in the reverse mappings (denoted with greek letters) which move away from the root of the graph.

3-by-2 Replacement

The σ operation, applied to numbers of the form 6 * m - 2, has an interesting property:

(6 * (3 * n) - 2) σ = 4 * 3 * n - 2 =  6 * (2 * n) - 2

In other words, as long as m contains a factor 3, the σ operation maintains the form 6 * x - 2, and it replaces the factor 3 by 2. In the opposite direction, the s operation replaces a factor 2 in m by 3.

Kernels

By the kernel of a number n = 6 * m - 2 we denote the "2-3-free" factor of m, that is the factor which remains when all powers of 2 and 3 have been removed from m.

• The kernel is not affected by σ and s operations.

Segment Construction

We will now construct special subsets of paths in the Collatz graph which we call segments. They lead away from the root, and they always start with a node m ≡ -2 mod 6. Then they split and follow two subpaths in a prescribed sequence of operations. The segment construction process is stopped when the next node in one of the two subpaths is divisible by 3, resp. when a δ operation is no more possible. We assemble the segments as rows of an infinite array C[i,j], the so-called segment directory. One row i of C is constructed according to the following table:

Column	Operation	Formula	Condition	Sequence
C[i,1]		6 * i - 2		4, 10, 16, 22, 28, ...
C[i,2]	C[i,1] µµ	24 * (i - 1) + 16		16, 40, 64, 88, 112, ...
C[i,3]	C[i,1] δµµ	24 * (i - 1) / 3 + 4	i ≡ 1 mod 3	4, 28, 52, 76, 100, ...
C[i,4]	C[i,2] σ	48 * (i - 1) / 3 + 10	i ≡ 1 mod 3	10, 58, 106, 134, ...
C[i,5]	C[i,3] σ	48 * (i - 7) / 9 + 34	i ≡ 7 mod 9	34, 82, 130, 178, ...
C[i,6]	C[i,2] σσ	96 * (i - 7) / 9 + 70	i ≡ 7 mod 9	70, 166, 262, 358, ...
C[i,7]	C[i,3] σσ	96 * (i - 7) / 27 + 22	i ≡ 7 mod 27	22, 118, 214, 310, ...
C[i,8]	C[i,2] σσσ	192 * (i - 7) / 27 + 46	i ≡ 7 mod 27	46, 238, 430, 622, ...
C[i,9]	C[i,3] σσσ	192 * (i - 61) / 81 + 142	i ≡ 61 mod 81	142, 334, ...
...	...	...	...	...

The first column(s) C[i,1] will be denoted as the left part of the segment (or of the whole directory), while the columns C[i,j], j > 1 will be the right part.

• All nodes in the segment directory are of the form 6 * n - 2.

This follows from the formula for columns C[i,1..3], and for any higher column numbers from the 3-by-2 replacement property of the σ operation.

• All segments have a finite length.

At some point the σ operations will have replaced all factors 3 by 2.

• All nodes in the right part of a segment have the same kernel.

This follows from the formula for columns C[i,1..3], and since the σ operation maintains this property.

• All nodes in the right part of a segment are different.

For C[i,1..2] we see that the values modulo 24 are different. For the remaining columns, we see that the exponents of the factors 2 and 3 are different. They are shifted by the σ operations, but they alternate, for example (right part of segment starting with 40, kernel 1):

160 = 6 * 33 * 20 - 2
 52 = 6 * 32 * 20 - 2
106 = 6 * 32 * 21 - 2
 34 = 6 * 31 * 21 - 2
 70 = 6 * 31 * 22 - 2
 22 = 6 * 30 * 22 - 2
 46 = 6 * 30 * 23 - 2

• There is no cycle in a segment.

Segment Lengths

The segment directories are obviously very structured. The lengths of the compressed segments follow the pattern

4 2 2 4 2 2 L1 2 2 4 2 2 4 2 2 L2 2 2 4 2 2 ...

with two fixed lengths 2 and 4 and some variable lengths L₁, L₂ ... > 4. At the starting values 4, 40, 364, 3280, 29524 (OEIS A191681), the segment lengths have high values 4, 8, 12, 16, 20 which did not occur before. Those starting values are (9ⁿ⁺¹ - 1) / 2, or 4 * Sum(9ⁱ, i=0..n).

Coverage of Segments

We now examine the modular conditions which result from the segment construction table in order to find out how the numbers of the form 6 * n -2 are covered by the right part of the segment directory:

Columns j	Covered	Remaining
2-3	4, 16 mod 24	10, 22, 34, 46 mod 48
3-4	10, 34 mod 48	22, 46, 70, 94 mod 96
5-6	70, 22 mod 96	46, 94, 142, 190 mod 192
7-8	46, 142 mod 192	94, 190, 286, 382 mod 384
...	...	...

We can always exclude the first and the third element remaining so far by looking in the next two columns of segments with sufficient length.

• There is no limit on the length of a segment.

We only need to take a segment which, in its right part, has a factor of 3 with a sufficiently high power, and the σ operations will stretch out the segment accordingly.

Therefore we can continue the modulus table above indefinitely, which leads us to the proposition:

• All numbers of the form 6 * n - 2 occur exactly once in the right part C[i,j], j > 1 of the segment directory.

Collatz Tree

Obviously we can replace all compressed segments by the corresponding detailed segments. The claim is that this will lead to the Collatz tree which reaches all numbers.

In the directory of detailed segments we see that we can quickly get reach all odd numbers:

C[i,1] δ = ((6*i-2) - 1) / 3 = 2*i - 1 : 1, 3, 5, 7 

We remain concerned with the numbers

x ≡ 0,2 mod 6 resp. x ≡ 0, 2, 6, 8, 12, 14, 18, 20  mod 24

The following table lists the operations which are necessary to reach most of these by starting at C[i,1]:

Operation	Expression	Result	Condition	Coverage
µ	(6*i-2)*2	12*i - 4		8, 20 mod 24
δµ	((6*i-2-1)/3)*2	4*i - 2		2, 6, 10, 14, 18, 22 mod 24
δµµ	(6*i-2-1)/3*2*2	8*i - 4	i ≡ 2 mod 3	12 mod 24

The multiples of 12 can be reached from the multiples of 3 (contained in the odd numbers) by a µµ operation.

---

We will now construct special portions of paths in the Collatz graph which we call segments. They lead away from the root, and they always start with a node m ≡ 4 mod 6. Then they split into a northern and a southern subsegment by applying the following operations:

• northern: µ µ δ µ δ µ δ ...
• southern: δ µ µ δ µ δ µ ...

The two subsegements are built in parallel, and the process is stopped when one of the two new nodes becomes divisible by 3, resp. when a δ operation is not possible.

We will call these segments detailed, and a segment directory can easily be created by a Perl program.