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Algebra Difficulty 5.3 AIME, harder Find the answer

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.

A number or a short expression. Spacing and $ signs are ignored.

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<1x_{2 k-1}+\frac{1}{x_{2 k}}<0 \Longrightarrow\left|x_{2 k-1} x_{2 k}\right|<1 and x2k+1x2k+1<0x2kx2k+1>1x_{2 k}+\frac{1}{x_{2 k+1}}<0 \Longrightarrow\left|x_{2 k} x_{2 k+1}\right|>1. 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,,2019,12020,10000,100001,-\frac{1}{2}, 3,-\frac{1}{4}, \ldots, 2019,-\frac{1}{2020},-10000,-10000.

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.