### IMO Shortlist 2014 problem A3

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7. svibnja 2017.

For a sequence $x_1, x_2, \ldots, x_n$ of real numbers, we define its price as

Given $n$ real numbers, Dave and George want to arrange them into a sequence with a low price. Diligent Dave checks all possible ways and finds the minimum possible price $D$. Greedy George, on the other hand, chooses $x_1$ such that $|x_1|$ is as small as possible; among the remaining numbers, he chooses $x_2$ such that $|x_1 + x_2|$ is as small as possible, and so on. Thus, in the $i^{\text{th}}$ step he chooses $x_i$ among the remaining numbers so as to minimise the value of $|x_1 + x_2 + \dots + x_i|$. In each step, if several numbers provide the same value, George chooses one at random. Finally he gets a sequence with price $G$.

Find the least possible constant $c$ such that for every positive integer $n$, for every collection of $n$ real numbers, and for every possible sequence that George might obtain, the resulting values satisfy the inequality $G \leq c D$.

(Georgia)

Izvor: https://www.imo-official.org/problems/IMO2014SL.pdf