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| {{for|the Fred Frith album|The Happy End Problem}}
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| [[Image:Happy-End-problem.svg|thumb|300px|The Happy Ending problem: every set of five points in general position contains the vertices of a convex quadrilateral.]]
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| The '''Happy Ending problem''' (so named by [[Paul Erdős]] because it led to the marriage of [[George Szekeres]] and [[Esther Szekeres|Esther Klein]]) is the following statement:
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| :'''Theorem.''' Any set of five points in the plane in [[general position]]<ref>In this context, general position means that no two points coincide and no three points are collinear.</ref> has a subset of four points that form the vertices of a [[convex polygon|convex]] [[quadrilateral]].
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| This was one of the original results that led to the development of [[Ramsey theory]].
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| 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 point set 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 {{harvtxt|Peterson|2000}} for an illustrated explanation of this proof, and {{harvtxt|Morris|Soltan|2000}} for a more detailed survey of the problem than we provide here.
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| The '''Erdős–Szekeres conjecture''' states precisely a more general relationship between the number of points in a general-position point set and its largest convex polygon. It remains unproven, but less precise bounds are known.
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| ==Larger polygons==
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| [[Image:8-points-no-pentagon.svg|thumb|A set of eight points in general position with no convex pentagon.]]
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| {{harvtxt|Erdős|Szekeres|1935}} proved the following generalisation:
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| :'''Theorem.''' For any positive [[integer]] ''N'', any sufficiently large finite set of points in the plane in general position has a subset of ''N'' points that form the vertices of a convex polygon.
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| The proof appeared in the same paper that proves the [[Erdős–Szekeres theorem]] on monotonic subsequences in sequences of numbers.
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| Let ''f''(''N'') denote the minimum ''M'' for which any set of ''M'' points in general position must contain a convex ''N''-gon. It is known that
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| * ''f''(3) = 3, trivially.
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| * ''f''(4) = 5.<ref>This was the original problem, proved by Esther Klein.</ref>
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| * ''f''(5) = 9.<ref>According to {{harvtxt|Erdős|Szekeres|1935}}, this was first proved by E. Makai; the first published proof appeared in {{harvtxt|Kalbfleisch|Kalbfleisch|Stanton|1970}}.</ref> A set of eight points with no convex pentagon is shown in the illustration, demonstrating that ''f''(5) > 8; 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.
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| * ''f''(6) = 17.<ref>This has been proved by {{harvtxt|Szekeres|Peters|2006}}. They carried out a computer search which eliminated all possible configurations of 17 points without convex hexagons while examining only a tiny fraction of all configurations.</ref>
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| * The value of ''f''(''N'') is unknown for all ''N'' > 6; by the result of {{harvtxt|Erdős|Szekeres|1935}} it is known to be finite.
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| On the basis of the known values of ''f''(''N'') for ''N'' = 3, 4 and 5, Erdős and Szekeres conjectured in their original paper that
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| :<math>f(N) = 1 + 2^{N-2} \quad \mbox{for all } N \geq 3.</math>
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| They proved later, by constructing explicit examples, that
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| :<math>f(N) \geq 1 + 2^{N-2},</math><ref>{{harvtxt|Erdős|Szekeres|1961}}</ref>
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| but the best known upper bound when ''N'' ≥ 7 is
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| :<math>f(N) \leq {2N-5 \choose N-2} + 1 = O\left(\frac{4^N}{\sqrt N}\right).</math><ref>{{harvtxt|Tóth|Valtr|2005}}. See [[binomial coefficient]] and [[Big O notation]] for notation used here and [[Catalan number]]s or [[Stirling's approximation]] for the asymptotic expansion.</ref>
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| ==Empty polygons==
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| One may also consider the question of whether any sufficiently large set of points in general position has an ''empty'' quadrilateral, [[pentagon]], etc.,
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| 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.<ref>{{harvtxt|Harborth|1978}}.</ref> However, there exist arbitrarily large sets of points in general position that contain no empty [[heptagon]].<ref>{{harvtxt|Horton|1983}}</ref>
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| For a long time the question of the existence of empty [[hexagon]]s remained open, but {{harvtxt|Nicolás|2007}} and {{harvtxt|Gerken|2008}} proved that every sufficiently large point set in general position contains a convex empty hexagon. More specifically, Gerken showed that the number of points needed is no more than ''f''(9) for the same function ''f'' defined above, while Nicolás showed that the number of points needed is no more than ''f''(25). Valtr (2006) supplies a simplification of Gerken's proof that however requires more points, ''f''(15) instead of ''f''(9). At least 30 points are needed: there exists a set of 29 points in general position with no empty convex hexagon.<ref>{{harvtxt|Overmars|2003}}.</ref>
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| ==Related problems==
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| The problem of finding sets of ''n'' points minimizing the number of convex quadrilaterals is equivalent to minimizing the [[Crossing number (graph theory)|crossing number]] in a straight-line [[graph drawing|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.<ref>{{harvtxt|Scheinerman|Wilf|1994}}</ref>
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| It is straightforward to show that, in higher dimensional [[Euclidean space]]s, 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 ''k''-gons 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]].<ref>{{harvtxt|Grünbaum|2003}}, Ex. 6.5.6, p.120. Grünbaum attributes this result to a private communication of Micha A. Perles.</ref> 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]].<ref>{{harvtxt|Grünbaum|2003}}, Ex. 7.3.6, p. 126. This result follows by applying a Ramsey-theoretic argument similar to Szekeres' original proof together with Perles' result on the case ''k'' = ''d'' + 2.</ref>
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| ==Notes==
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| {{refend}}
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| ==External links==
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| * [http://planetmath.org/encyclopedia/HappyEndingProblem.html Happy ending problem] and [http://planetmath.org/encyclopedia/RamseyTheoreticProofOfTheErdHosSzekeresTheorem.html Ramsey-theoretic proof of the Erdős-Szekeres theorem] on [[PlanetMath]]
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| *{{mathworld | urlname = HappyEndProblem | title = Happy End Problem}}
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| [[Category:Discrete geometry]]
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| [[Category:Euclidean plane geometry]]
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| [[Category:Mathematical problems]]
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| [[Category:Ramsey theory]]
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