3
$\begingroup$

Design 25 cubes so that each cube's six faces display integer numbers in the range 0,1,...,31 such that:
On each cube, all the numbers are different.
Any given two cubes share exactly one common number.
Please give your answer as 25 lines of six numbers. Bonus question: Use as small a range of numbers as possible.

After seeing the solution, I read that this can be solved using "Projective plane". So my questions are:

  1. How to solve such a question? (What is the strategy..)
  2. How can I decide that a given problem (like this) has a solution? For example, given 25 cubes and a set from 1 to 10.. Logically it's impossible to be solved.
  3. What field of maths does "Projective plane" belong to? I mean, what is (more general) topic in maths that I have to study to understand "Projective plane"?


    Note: I don't know what tags do I have to choose for this question.
$\endgroup$

2 Answers 2

Reset to default
3
$\begingroup$

Here is a simplified version of your problem with the geometrical diagram.

It might be phrased like, how many committees of 3 can be formed by a group of 7 people such that no pair of people serve on two committees.

The figure is called the "Fano Plane" and a finite protective plane.

enter image description here

Your figure will be several orders of magnitude more complicated, but a similar idea.

You might want to look up "the Sunday golfer problem" as it is a well known and very similar problem.

$\endgroup$
2
  • $\begingroup$ "the Fano plane is the finite projective plane of order 2, having the smallest possible number of points and lines, 7 each, with 3 points on every line and 3 lines through every point." Another clear description for me.. Then to solve such a question I have to draw such things? $\endgroup$
    – Ahmed Amir
    Commented Nov 2, 2016 at 17:27
  • 1
    $\begingroup$ Each "line" (the circle in the middle counts as a line) represents one of your dice. In the Fano plane these are 3 sided dice, you will need a more complicated figure with 6 points on each line. The letters are the equivalents of the numbers on the faces. So, you need a figure with 25 intersecting curves, six points on each curve, and no more than 31 points of intersection. $\endgroup$
    – Doug M
    Commented Nov 2, 2016 at 17:56
1
+50
$\begingroup$

  1. As you should already be suggested, :-) (as far as I know) there is no direct plan or program of solution, it is a creative task. Math contests winners are good in that. They proceed by trials and errors, based on their experience, knowledge, intuition, conjectures, guesses, using symmetries, brute force search, trying to adapt known constructions for the problem solution. Sometimes you may be just lucky.

  2. It depends on the problem, what is (much) harder to find its solution or to prove that it does not exist

I’ll illustrate the above ideas by a story about the magic hexagon told by M.A.B. Deakin.

  1. Projective planes which you are referenced are studied by linear algebra over finite fields. As instances of relevant topics your may read about Steiner systems and Kirkman’s schoolgirl problem. Topological projective planes are very different topic.

Note: I’ll retag your question.

$\endgroup$
1
  • 1
    $\begingroup$ While I was trying to solve the question, I figured many things like I can't use the same number on more than six cubes. Also, I figured that I can design 31 cubes using the same set. Not only 25. I mean, I got many ideas just by thinking or as you said by (trials and errors...etc). But I was wondering if there is a "direct" mathematical way to solve such problems. That's why I asked. Thanks for your answer. All the links you provided are great. $\endgroup$
    – Ahmed Amir
    Commented Nov 12, 2016 at 11:59

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .