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Oct 4, 2020 at 22:38 answer added Christopher Ryba timeline score: 4
Oct 3, 2020 at 9:12 comment added YCor Challenge: from $[1,2,3,4,5,6,7,8,9]$ achieve stopping time 22 without computer aid.
Oct 3, 2020 at 9:10 comment added YCor For $n=9$ by random walk on the graph I obtain the following stopping times (I don't claim these are the only possible ones). I checked once by choosing random $(a,b)$ among possibles, and once by choosing random $(a,b)$ among possible minimizing $b-a$. Did 30000 tests in each case. (Choosing random $(a,b)$ among possible maximizing $b-a$, in all 20000 tests, yields stopping time 4.) The obtained times: $[4,6,\dots,16,18,19,21,22,24,27,30]$. (With the minimizing $b-a$ option, I obtained stopping time 6 with probability about $1/12$, and the most likely is stopping time $24$: almost $1/2$.)
Oct 3, 2020 at 7:37 comment added YCor For $n\le 8$ here's the set of possible stopping times starting from $\{1,\dots,n\}$. For $n=8$ it starts taking some time. $n\le 2$: $[0]$; $n=3$: $[1]$; $n=4$: $[2]$; $n=5$: $[2,5]$; $n=6$: $[2,5,8]$; $n=7$: $[3,5,6,8,11,14]$; $n=8$: $[4,6,8,9,11,12,14,17,20]$.
Oct 3, 2020 at 1:25 history edited Richard Stanley CC BY-SA 4.0
added condition $a\neq b$
Oct 3, 2020 at 1:25 comment added Richard Stanley @YCor: yes, I want $a\neq b$. This has been corrected.
Oct 3, 2020 at 1:07 comment added Sam Hopkins If $n=2m+1$ is odd you can clearly terminate starting from $\{1,2,\ldots,n\}$ in $m$ steps by choosing the $a,b$ with $a+b=n+1$.
Oct 3, 2020 at 0:56 comment added WhatsUp Have you gone through the experiment and pattern spotting procedures?
Oct 3, 2020 at 0:14 comment added YCor You probably want to force $a\neq b$? or else one in most case, at some point, choose a pair of equal numbers to increase artificially the number of moves. So termination is when all numbers in $M$ of the same parity are equal.
Oct 3, 2020 at 0:10 history edited YCor
edited tags
Oct 2, 2020 at 23:55 history asked Richard Stanley CC BY-SA 4.0