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Some x_i so 1/2 < P(x_i)/P(x_j) < 2 is d-th power

Source: 2016 IMO Shortlist N8

July 19, 2017
number theoryIMO Shortlistpolynomial

Problem Statement

Find all polynomials P(x)P(x) of odd degree dd and with integer coefficients satisfying the following property: for each positive integer nn, there exists nn positive integers x1,x2,,xnx_1, x_2, \ldots, x_n such that 12<P(xi)P(xj)<2\frac12 < \frac{P(x_i)}{P(x_j)} < 2 and P(xi)P(xj)\frac{P(x_i)}{P(x_j)} is the dd-th power of a rational number for every pair of indices ii and jj with 1i,jn1 \leq i, j \leq n.