Happy ending problem

The happy ending problem: every set of five points in general position contains the vertices of a convex quadrilateral

In mathematics, the "happy ending problem" (so named by Paul Erdős because it led to the marriage of mathematicians George Szekeres and Esther Klein) is the following statement:

This was one of the original results that led to the development of Ramsey theory.

The happy ending theorem can be proven by a simple case analysis: if four or more points are vertices of the convex hull, any four such points can be chosen. If on the other hand, the convex hull has the form of a triangle with two points inside it, the two inner points and one of the triangle sides can be chosen. See Template:Harvtxt for an illustrated explanation of this proof, and Template:Harvtxt for a more detailed survey of the problem.

The Erdős–Szekeres conjecture states precisely a more general relationship between the number of points in a general-position point set and its largest subset forming a convex polygon, namely that the smallest number of points for which any general position arrangement contains a convex subset of $n$ points is $2^{n-2}+1$. It remains unproven, but less precise bounds are known.

Larger polygons

A set of eight points in general position with no convex pentagon

Template:Harvtxt proved the following generalisation:

The proof appeared in the same paper that proves the Erdős–Szekeres theorem on monotonic subsequences in sequences of numbers.

Let Template:Math denote the minimum Template:Mvar for which any set of Template:Mvar points in general position must contain a convex N-gon. It is known that

• Template:Math, trivially.
• Template:Math.
• Template:Math. A set of eight points with no convex pentagon is shown in the illustration, demonstrating that Template:Math; the more difficult part of the proof is to show that every set of nine points in general position contains the vertices of a convex pentagon.
• Template:Math.
• The value of Template:Math is unknown for all Template:Math. By the result of Template:Harvtxt, Template:Math is known to be finite for all finite Template:Math.

On the basis of the known values of Template:Math for N = 3, 4 and 5, Erdős and Szekeres conjectured in their original paper that

A set of sixteen points in general position with no convex hexagon

$$f(N)=1+2^{N-2}\text{for all }N\ge 3.$$ They proved later, by constructing explicit examples, that $$f(N)\ge 1+2^{N-2}.$$ In 2016 Andrew Suk showed that for Template:Math $$f(N)\le 2^{N+o(N)}.$$

Suk actually proves, for N sufficiently large, $$f(N)\le 2^{N+6N^{2/3}\log N}.$$

This was subsequently improved to:Template:Sfnp

$$f(N)\le 2^{N+O(\sqrt{N\log N})}.$$

Empty convex polygons

There is also the question of whether any sufficiently large set of points in general position has an "empty" convex quadrilateral, pentagon, etc., that is, one that contains no other input point. The original solution to the happy ending problem can be adapted to show that any five points in general position have an empty convex quadrilateral, as shown in the illustration, and any ten points in general position have an empty convex pentagon. However, there exist arbitrarily large sets of points in general position that contain no empty convex heptagon.

Let $N$ be the minimum number of points, such that any $N$ points in general position contains an empty hexagon. For a long time it is open whether $N$ exists. The question is now solved:

• Template:Harvtxt showed that if it exists, then $N\ge 30$, by constructing an example with 29 points.
• Template:Harvtxt showed that $N\le f(25)$.
• Template:Harvtxt showed that $N\le f(9)$.
• Template:Harvtxt made a simpler but looser version of Template:Harvtxt, to show that $N\le f(15)$.
• Template:Harvtxt showed, by using a SAT solving approach, that $N=30$.

Related problems

The problem of finding sets of n points minimizing the number of convex quadrilaterals is equivalent to minimizing the crossing number in a straight-line drawing of a complete graph. The number of quadrilaterals must be proportional to the fourth power of n, but the precise constant is not known.

It is straightforward to show that, in higher-dimensional Euclidean spaces, sufficiently large sets of points will have a subset of k points that forms the vertices of a convex polytope, for any k greater than the dimension: this follows immediately from existence of convex Template:Nowrap in sufficiently large planar point sets, by projecting the higher-dimensional point set into an arbitrary two-dimensional subspace. However, the number of points necessary to find k points in convex position may be smaller in higher dimensions than it is in the plane, and it is possible to find subsets that are more highly constrained. In particular, in d dimensions, every d + 3 points in general position have a subset of d + 2 points that form the vertices of a cyclic polytope. More generally, for every d and k > d there exists a number m(d, k) such that every set of m(d, k) points in general position has a subset of k points that form the vertices of a neighborly polytope.

"Happy ending"

According to George Szekeres, who discussed the problem with his future wife Esther Klein early in their relationship, "it was Paul Erdös who called it the 'Happy Ending'". They became friends in 1933 and married in 1937, and Szekeres says the reason they got married was not entirely because of their work on the problem.

References

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External links

• Happy ending problem and Ramsey-theoretic proof of the Erdős-Szekeres theorem on PlanetMath
• Template:Mathworld