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Algebraic Magic Trick

Source: ISL 2020 A5

July 20, 2021
algebraIMO ShortlistIMO Shortlist 2020polynomialalgorithmlagrange s interpolationGerhard Woeginger

Problem Statement

A magician intends to perform the following trick. She announces a positive integer nn, along with 2n2n real numbers x1<<x2nx_1 < \dots < x_{2n}, to the audience. A member of the audience then secretly chooses a polynomial P(x)P(x) of degree nn with real coefficients, computes the 2n2n values P(x1),,P(x2n)P(x_1), \dots , P(x_{2n}), and writes down these 2n2n values on the blackboard in non-decreasing order. After that the magician announces the secret polynomial to the audience. Can the magician find a strategy to perform such a trick?