olymmath

KURSCHAK 2006

Kürschák Competition 2006.

Other problems

Problem 1

Is there a set S⊂R3S\subset\mathbb{R}^3 of 20062006 points such that not all its points are coplanar, no three of the points are collinear, and for any A,B∈SA,B\in S we can find points C,D∈SC,D\in S for which AB∣∣CDAB||CD ?

Problem 2

Let a,t,na,t,n be positive integers such that a≤na\le n . Consider the subsets of {1,2,…,n}\{1,2,\dots,n\} whose any two elements differ by at least tt . Prove that the number of such subsets not containing aa is at most t2t^2 times the number of those that do contain aa .

Problem 3

We deal n−1n-1 cards in some way to nn people sitting around a table. From then on, in one move a person with at least 22 cards gives one card to each of his/her neighbors. Prove that eventually a state will be reached where everyone has at most one card.