1973 IMO Problems/Problem 2

Problem

Determine whether or not there exists a finite set $M$ of points in space not lying in the same plane such that, for any two points $A$ and $B$ of $M$ ; one can select two other points $C$ and $D$ of $M$ so that lines $AB$ and $CD$ are parallel and not coincident.

Solution

In order to solve this problem we can start by finding at least one finite set $M$ that satisfies the condition.

We start by defining our first set $M$ with the vertices of a cube of side $k$ as follows: $M = \{ (0,0,0), (k,0,0), (k,k,0), (0,k,0), (0,0,k), (k,0,k), (k,k,k), (0,k,k) \}$

Since all the faces of this cube have a parallel face, then any two points on one face will have corresponding 2 points on the opposite face that is parallel. However we have four diagonals on this cube that do not have two points that are parallel to any of these diagonals.

By doing a reflection of the points on the $x=k$ plane along the $yz-plane$ these four diagonals will have their respective parallel diagonals on the $x \le 0& space

If set$ (Error compiling LaTeX. Unknown error_msg)M$of points in space consist of 3 points or less, then we can't satisfy the condition because we would need at least 4 points.

If set$ (Error compiling LaTeX. Unknown error_msg)M $of points in space consist of 4 points, then we can't satisfy the condition because for the condition of lines$ AB $and$ CD $to be parallel the 4 points would need to be co-planar. But the points in set$ M $shall not be lying in the same plane. So, a finite set$ M$with 4 points would not satisfy the condition.

If set$ (Error compiling LaTeX. Unknown error_msg)M $of points in space consist of 5 points, then we can't satisfy the condition either because even though we can construct a parallelogram in space with 4 co-planar vertices in the set$ M $with a 5th point outside of the parallelogram plane, the condition is to select any two points. So if one of the$ A $or$ B $points is that 5th point, there would be no other two other points$ C $and$ D $for which will make lines$ AB $and$ CD$parallel because any of those combinations of lines will be skewed. be parallel because those other two points will provide skew lines.

If set$ (Error compiling LaTeX. Unknown error_msg)M $of points in space consist of 6 points, then let's consider a the 6 vertices of a pentahedron$ ABCDEF $with triangular faces$ ABC $, and$ DEF $of the same size parallel to each other with the other faces being parallelograms. Here one might be tempted to think that this set complies with the condition because all lines of the solid are parallel to at least another line. But when we take a diagonal on that solid like$ AE$, such diagonal can't be parallel with anything else.

If set$ (Error compiling LaTeX. Unknown error_msg)M $of points in space consist of 8 points, then let's consider the 8 vertices of parallelepiped$ ABCDEFG $where all faces are parallelograms. Like in the example of the set$ M $despite almost all combinations of two vertices having another two points in parallel even the diagonal to the faces, when we take a diagonal of this solid like diagonal$ AG $that doesn't lie on any of the faces, such line does not have a parallel two points. So, then you add more points$ H $and$ I $parallel to the diagonals, but then the two new points have the lines$ AH$not parallel to anything else. ...and you keep adding points until infinity at which time the condition will be satisfied. But that would make the set infinite and not finite.

Therefore the finite set$ (Error compiling LaTeX. Unknown error_msg)M$ of points in space for this problem does not exist.

NOTE: I made a mistake. I found a case that it does exist. I'm working on a solution to update this.

Alternate solutions are always welcome. If you have a different, elegant solution to this problem, please add it to this page.

1973 IMO (Problems) • Resources
Preceded by Problem 1	1 • 2 • 3 • 4 • 5 • 6	Followed by Problem 3
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1973 IMO Problems/Problem 2

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