1
$\begingroup$

I have, cleverly I hope, shuffled my Rubik's Cube for a while. I could not find a position where, for each faces, all colors appear 3 times or less. I always had a or several faces with at least 4 of the same colors.

This puzzle is to determine what is the minimum number of colors $X$ such that there exists a Rubik's Cube position which for all faces, all colors appear less than $X$ times.

Improve this question
$\endgroup$
0

1 Answer 1

Reset to default
2
$\begingroup$

Each color appears $9$ times total, so an average of $9/6$ times per face, which implies a lower bound

$$X \ge \lceil 9/6 \rceil = 2.$$

Without loss of generality, color $c$ appears at the center of face $c$. Here is such an assignment of colors to faces, where each row or column sums to $9$ and the diagonal is positive:

\begin{matrix} \text{color}\backslash\text{face} &1 &2 &3 &4 &5 &6 \\ \hline 1 &1 &2 &2 &2 &0 &2 \\ 2 &2 &2 &0 &2 &2 &1 \\ 3 &2 &0 &2 &1 &2 &2 \\ 4 &1 &1 &1 &2 &2 &2 \\ 5 &1 &2 &2 &2 &2 &0 \\ 6 &2 &2 &2 &0 &1 &2 \\ \end{matrix}

Not sure whether these counts are actually physically realizable on the cube.

I played with an actual cube and got

$X \le 3$:

enter image description here

(The top right corner is orange.)

enter image description here

Improve this answer
$\endgroup$
4
  • $\begingroup$ Do you choose to upvote only the best of the best questions? $\endgroup$
    – JLee
    Jul 30, 2022 at 1:29
  • $\begingroup$ @JLee I haven't kept track, but I probably upvote answers more often than questions. Why do you ask? $\endgroup$
    – RobPratt
    Jul 30, 2022 at 1:36
  • 1
    $\begingroup$ Just something I noticed. You provide very high-quality answers. I always enjoy reading them & learning. So thanks for that. Often I see that there are zero upvotes on a question/puzzle that you answer, & I assume, "He thought this puzzle/question was worth answering (many times in great detail with lots of time spent) but not worth an upvote?" To me, the vast majority of the time, if I am going to spend more than a tiny bit of time on something, my upvote of the puzzle is a "thank you" for providing me with something interesting to think about. But that's just me, and everyone's different. $\endgroup$
    – JLee
    Jul 30, 2022 at 11:07
  • 1
    $\begingroup$ I see. I took some time just now to upvote many questions I have answered, including at least one of yours. 😊 I hit my vote limit for the day but will keep this in mind. $\endgroup$
    – RobPratt
    Jul 30, 2022 at 14:38

Not the answer you're looking for? Browse other questions tagged or ask your own question.