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Algebra, Iran MO 2019

Source: Iran MO 2019 , secound round , day 2 , p5

May 3, 2019
combinatoricsGame Theoryalgebra

Problem Statement

Ali and Naqi are playing a game. At first, they have Polynomial P(x)=1+x1398P(x) = 1+x^{1398}. Naqi starts. In each turn one can choice natural number k[0,1398]k \in [0,1398] in his trun, and add xkx^k to the polynomial. For example after 2 moves PP can be : P(x)=x1398+x300+x100+1P(x) = x^{1398} + x^{300} + x^{100} +1. If after Ali's turn, there exist tRt \in R such that P(t)<0P(t)<0 then Ali loses the game. Prove that Ali can play forever somehow he never loses the game!