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Maximum number elements in A U B

Source: Dutch IMO TST II Problem 2

July 17, 2014
number theory unsolvednumber theory

Problem Statement

The sets AA and BB are subsets of the positive integers. The sum of any two distinct elements of AA is an element of BB. The quotient of any two distinct elements of BB (where we divide the largest by the smallest of the two) is an element of AA. Determine the maximum number of elements in ABA\cup B.