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Digit sums of polynomial outputs

Source: ISL 2022 A7

July 9, 2023
algebrapolynomial

Problem Statement

For a positive integer nn we denote by s(n)s(n) the sum of the digits of nn. Let P(x)=xn+an1xn1++a1x+a0P(x)=x^n+a_{n-1}x^{n-1}+\cdots+a_1x+a_0 be a polynomial, where n2n \geqslant 2 and aia_i is a positive integer for all 0in10 \leqslant i \leqslant n-1. Could it be the case that, for all positive integers kk, s(k)s(k) and s(P(k))s(P(k)) have the same parity?