Maths Olympiad Prep

Library / /1042 of 1394

, 2022

Combinatorics Difficulty 5.5 AIME, harder Prove it United States

Problem:
Let x1,x2,,x2022x_{1}, x_{2}, \ldots, x_{2022} be nonzero real numbers. Suppose that xk+1xk+1<0x_{k}+\frac{1}{x_{k+1}}<0 for each 1k20221 \leq k \leq 2022, where x2023=x1x_{2023}=x_{1}. Compute the maximum possible number of integers 1n20221 \leq n \leq 2022 such that xn>0x_{n}>0.

Proposed by: Akash Das

Solution

Solution:
Let the answer be MM. If M>1011M>1011, there would exist two consecutive positive terms xk,xk+1x_{k}, x_{k+1} which contradicts the assumption that xk+1xk+1<0x_{k}+\frac{1}{x_{k+1}}<0. Thus, M1011M \leq 1011. If M=1011M=1011, then the 2022xi2022 x_{i} s must alternate between positive and negative. WLOG, assume x2k1>0x_{2 k-1}>0 and x2k<0x_{2 k}<0 for each kk. Then, we have
x2k1+1x2k<0x2k1x2k<1,x2k+1x2k+1<0x2kx2k+1>1. \begin{aligned} & x_{2 k-1}+\frac{1}{x_{2 k}}<0 \Longrightarrow\left|x_{2 k-1} x_{2 k}\right|<1, \\ & x_{2 k}+\frac{1}{x_{2 k+1}}<0 \Longrightarrow\left|x_{2 k} x_{2 k+1}\right|>1 . \end{aligned}
Multiplying the first equation over all kk gives us i=12022xi<1\prod_{i=1}^{2022}\left|x_{i}\right|<1, while multiplying the second equation over all kk gives us i=12022xi>1\prod_{i=1}^{2022}\left|x_{i}\right|>1. Thus, we must have M<1011M<1011.
M=1010M=1010 is possible by the following construction:
1,12,3,14,,2019,12020,10000,10000 1,-\frac{1}{2}, 3,-\frac{1}{4}, \ldots, 2019,-\frac{1}{2020},-10000,-10000

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