Wikipedia:Reference desk/Archives/Mathematics/2018 August 23

= August 23 =

2 Collatz Conjecture (3x+1) question.
Hello! I was recommended by Jasper Deng to ask my questions here. :) This question is in regards to the Collatz Conjecture or 3x + 1 problem (https://en.wikipedia.org/wiki/Collatz_conjecture). I have done my best to watch all of the videos and read a lot of websites, but my question isn't answered anywhere there. There is something that goes by a few names; Steps or Total Stopping Distance (are they the same thing?) and I was wondering if there existed a simple linear function f(S) that returns any number N that is S Steps from 1? So for example f(111) could return 27, since 27 is 111 steps from 1. Is there a function that can give f(100000) to f(200000) or something?

24.78.157.241 (talk) 03:40, 23 August 2018 (UTC)


 * f(s) will have to output a set of numbers as there can be more than one number that is s steps away from 1. For example, both 5 and 32 are 6 steps away from 1. f(s) can be defined recursively as follows:


 * $$f(4) = \{8\}$$
 * $$f(s) = \{2x | x \in f(s-1)\} \cup \{\frac{x-1}{3} | x \in f(s-1) \land x \equiv 4 \mod 6\} \quad s > 4$$


 * In effect, this is just running the Collatz rule in reverse. Applying this recursively gives:


 * $$f(5) = \{16\}$$
 * $$f(6) = \{5,32\}$$
 * $$f(7) = \{10,64\}$$
 * $$f(8) = \{3,20,21,128\}$$
 * etc.


 * and you could eventually find the set f(111). Gandalf61 (talk) 08:14, 23 August 2018 (UTC)

Hmm your algorithm does 2 things than what I am after: It returns a set; more than one result; and it, as you said "runs the algorithm backwards". I was wondering if you had a simple calculation that provided a small (but not smallest) non-trivial number N that is S steps away from 1? IE: No summations, no loops, no recursion, no look-ups, etc.. Does that type of thing exist? Or is the 3x + 1 system too complicated for such a thing? If such an algorithm could be found, do you think it might help out the people trying to prove the conjecture over-all?

24.78.157.241 (talk) 16:46, 23 August 2018 (UTC)
 * The only easy to implement function that returns a number n steps away from 1 is $$f(n) = 2^n$$, but that returns the largest number for each n. Iffy★Chat -- 18:37, 23 August 2018 (UTC)
 * FYI, this is . –Deacon Vorbis (carbon &bull; videos) 19:17, 23 August 2018 (UTC)

You might like the graph in this xkcd: https://www.xkcd.com/710/ 2607:FCD0:100:8303:5D:0:0:B7D4 (talk) 23:51, 24 August 2018 (UTC)

The short answer is "no", it is too irregular. Bubba73 You talkin' to me? 00:22, 25 August 2018 (UTC)

Thank you everyone for your feedback and responses! 24.78.157.241 (talk) 17:17, 26 August 2018 (UTC)