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Winning strategy involving coefficents of a polynomial

Source: Bulgarian National Olympiad 2012 Problem 3

May 21, 2012
algebrapolynomialalgebra proposed

Problem Statement

We are given a real number aa, not equal to 00 or 11. Sacho and Deni play the following game. First is Sasho and then Deni and so on (they take turns). On each turn, a player changes one of the “*” symbols in the equation: x4+x3+x2+x1+=0*x^4+*x^3+*x^2+*x^1+*=0 with a number of the type ana^n, where nn is a whole number. Sasho wins if at the end the equation has no real roots, Deni wins otherwise. Determine (in term of aa) who has a winning strategy