Maths Olympiad Prep

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Number theory Difficulty 6.0 National olympiad Prove it

Example 5 Let nn be a positive integer greater than 2. Prove: there exists a prime pp, such that n<p<n!n<p<n!.

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Solution

Proof: Let p1<p2<<pkp_{1}<p_{2}<\cdots<p_{k} be all the prime numbers not exceeding nn, and let qq be the smallest prime number greater than nn. If n=2n=2, then q=3q=3, and the proposition holds. If n=3n=3, then q=5q=5, and the proposition holds. When n>3n>3, 2,32,3 both appear in p1,,pkp_{1}, \cdots, p_{k}, so 5qn!1n5 \leqslant q \leqslant n!-1n, thus n<pq<n!n<p \leqslant q<n!. Therefore, the proposition holds.

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