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Number theory Difficulty 4.8 AIME Prove it United States

Problem:

Find all prime numbers pp such that p2+2007p1p^{2} + 2007p - 1 is prime as well.

Solution

Solution:

If p=3p = 3, then p2+2007p1=6029p^{2} + 2007p - 1 = 6029 which is a prime.

For p3p \neq 3, we know that p=3k±1p = 3k \pm 1 for some integer kk. Hence
p2+2007p1=(3k±1)2+2007(3k±1)1=9k2±6k+1+20073k±20071=9k2±6k+20073k±2007. p^{2} + 2007p - 1 = (3k \pm 1)^{2} + 2007(3k \pm 1) - 1 = 9k^{2} \pm 6k + 1 + 2007 \cdot 3k \pm 2007 - 1 = 9k^{2} \pm 6k + 2007 \cdot 3k \pm 2007.
This expression is divisible by 33 and can't be prime (unless it equals 33, but for p>3p > 3 it is much larger than 33). Thus the only such prime number is p=3p = 3.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.