IMO 2009 Shortlist N4
Find all positive integers n such that there exists a sequence of positive integers a1,a2,...,an satisfying ak+1 = a2 k ...
Category: Number Theory
Problem
Find all positive integers n such that there exists a sequence of positive integers a1,a2,...,an satisfying ak+1 = a2 k + 1 ak−1 + 1 − 1 for every k with 2 ≤ k ≤ n − 1.