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is 100 000 a plum number

Source: Flanders Math Olympiad 2021 p1

December 24, 2022
number theorycombinatorics

Problem Statement

Johnny once saw plums hanging, like eggs so big and numbered according to the first natural numbers. He is the first to pick the plum with number 22. After that, Jantje picks the plum each time with the smallest number nn that satisfies the following two conditions: \bullet nn is greater than all numbers on the already picked plums, \bullet nn is not the product of two equal or different numbers on already picked plums. We call the numbers on the picked plums plum numbers. Is 100000100 000 a plum number? Justify your answer.