# What is the expected value for this

If there are $8$ random points in the plane whose horizontal coordinate and vertical coordinate are uniformly distributed on the open interval $\left(0,1\right)$, what is the expected largest size of a subset in which the points form the vertices of a convex polygon? Thanks!

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By the way, "size" only means cardinality here. – user0o Jan 17 '13 at 20:14
– Sam Hopkins Jan 17 '13 at 20:22
Also posted to math.stackexchange.com/questions/280648/… originally with $16$ points, then edited to $4$ points. – Gerry Myerson Jan 17 '13 at 22:02
Probability distribution not clear: Are the points independent? For a given point, are the horizontal and vertical coordinates independent? – Gerald Edgar Jan 19 '13 at 13:35
Dear Gerald, yes, the random points are independent, and for a given point, the horizontal and vertical coordinates are also independent. Thank you for your questions that make this clear. – user0o Jan 21 '13 at 7:06

For specific and not very small $n$ this would be quit a messy computation.
There are esults about the asymptotics for large $n$ in a paper of Ambrus and Barany http://arxiv.org/abs/0906.5452 . They consider a slightly different problem, but their methods work and the result is $c n^{1/3}$ for some computable $c$.
The exponent would not change. For example, take random points in a circular disk. Fit a triangle in the disk. A constant fraction of the n points falls inside the triangle (w.h.p.), and thus you get at least $c′n^{1/3}$ points in convex position; similarly, an ellipse (affine image of a disk) fits into any triangle, showing the other direction of the inequality. Any two convex shapes are related like this (after affine transformation). – Günter Rote Apr 28 '13 at 23:15