MathDB
Iterated function of a polynomial

Source: 11th CHKMO 2009

December 15, 2008
functionalgebrapolynomialinductionalgebra proposed

Problem Statement

Let f(x) \equal{} c_m x^m \plus{} c_{m\minus{}1} x^{m\minus{}1} \plus{}...\plus{} c_1 x \plus{} c_0, where each ci c_i is a non-zero integer. Define a sequence {an} \{ a_n \} by a_1 \equal{} 0 and a_{n\plus{}1} \equal{} f(a_n) for all positive integers n n. (a) Let i i and j j be positive integers with i<j i<j. Show that a_{j\plus{}1} \minus{} a_j is a multiple of a_{i\plus{}1} \minus{} a_i. (b) Show that a20080 a_{2008} \neq 0