A. 601. Let q1 be an integer. Prove that there is an integer Cq such that for every finite set A of integers |A+q.A|(q+1)|A|-Cq holds. (A+q.A is the set of those integers that can be expressed as a+qa' with some a,a'A.)
Schweitzer competition, 2013
Deadline expired on 10 December 2013.