Number theoryDifficulty 4.8AIMEProve itUnited States
Problem: Find the greatest common divisor of the numbers 2002+2,20022+2,20023+2,….
Solution
Solution: Notice that 2002+2 divides 20022−22, so any common divisor of 2002+2 and 20022+2 must divide (20022+2)−(20022−22)=6. On the other hand, every number in the sequence is even, and the nth number is always congruent to 1n+2≡0 modulo 3. Thus, 6 divides every number in the sequence.
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Source: MathNet,
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