Number theoryDifficulty 4.8AIMEProve itUnited States
Problem:
Find all prime numbers p such that p2+2007p−1 is prime as well.
Solution
Solution:
If p=3, then p2+2007p−1=6029 which is a prime.
For p=3, we know that p=3k±1 for some integer k. Hence p2+2007p−1=(3k±1)2+2007(3k±1)−1=9k2±6k+1+2007⋅3k±2007−1=9k2±6k+2007⋅3k±2007. This expression is divisible by 3 and can't be prime (unless it equals 3, but for p>3 it is much larger than 3). Thus the only such prime number is p=3.
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Source: MathNet,
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