Collatz Conjecture – Is It the Only Stable Solution of Its Type?

collatz conjecturedynamical systemselementary-number-theorynumber theorysequences-and-series

The Collatz Conjecture is well known with the sequence

$$f(n) = \begin{cases} n/2 &;\text{if } n \equiv 0 \pmod{2}\\ k\,n+1 &; \text{if } n\equiv 1 \pmod{2} \end{cases}$$

and $k=3$; the sequence converging $1$ (so called oneness).

Is there any conjecture/theorem on whether the sequence would converge for any other value of $k$; or could it be shown that the sequence diverges for values of $k$ other than $3$?

By convergence here I mean that the sequence after finite steps ends with a stable fixed number such as in case of Collatz it is the case with the number $1$.

Append: In the mean time I wrote out a conjecture on this over here >>>, for those who might be interested.

Best Answer

Here are some pictures for your/our intuition. I graphed the trajectories for initial values $x=5,15,25,...$ for the first $256$ steps of $x_{k+1}=(5x_k+1)/2^A$.
To get the curves to a meaningfully visual interval I show logarithmic scales. The pictures show how most trajectories begin to diverge (not really a safe indication of what characteristic the infinite curves really have) but some show cycling already at early iteration indexes $k$ .

I find $2$ cycles besides the "trivial" one.


$x=5,15,25,35,...,95$ detail of the first few iterations . At the bottom we see the "trivial" cyle (brown curve):
picture

$x=5,15,25,35,...,95$ first $2^8 = 256$ iterations. At later iteration-indexes $k$ a first "non-trivial" cycle occurs (red line):
picture


$x=105,115,125,135,...,195$ first $2^8 = 256$ iterations .
picture


$x=205,215,225,235,...,295$ first $2^8 = 256$ iterations . Here a second "non-trivial" cycle becomes visible:
picture


$x=205,215,225,235,...,295$ first $2^{11} = 2048$ iterations
It seems really that all trajectories which are divergent up to iteration $k=256$ are also divergent up to iteration $k=2048$ . In general: I doubt that there are "later" cycles:
picture