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P(Q(x)) = Q(P(x), trinomials - All-Russian MO 2003 Regional (R4) 11.5

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September 17, 2024
algebratrinomial

Problem Statement

Square trinomials P(x)=x2+ax+bP(x) = x^2 + ax + b and Q(x)=x2+cx+dQ(x) = x^2 + cx + d are such that the equation P(Q(x))=Q(P(x))P(Q(x)) = Q(P(x)) has no real roots. Prove that bdb \ne d.