Maths Olympiad Prep

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Number theory Difficulty 5.5 AIME, harder Prove it

11.4. Given 15 different pairwise coprime natural numbers from the interval [2,2022][2,2022]. Prove that one of these numbers is prime.

Solution

Solution. Suppose this is not the case. Let these numbers be denoted by a1,a2,,a15a_{1}, a_{2}, \ldots, a_{15}, and their smallest prime divisors by p1,p2,,p15p_{1}, p_{2}, \ldots, p_{15}, respectively. Since all the numbers are pairwise coprime, these prime divisors are distinct. The first 15 prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37,41,43,4737,41,43,47. This means that the largest of the prime divisors pi47p_{i} \geq 47. Then the corresponding number ai472=2209>2022a_{i} \geq 47^{2}=2209>2022, which contradicts the condition. Therefore, one of the numbers a1,a2,,a15a_{1}, a_{2}, \ldots, a_{15} is prime.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.