Suppose positive integers , , satisfy the following four conditions:
.
.
.
.
Here we denote by the greatest common divisor of the numbers in .
Determine the minimum possible value the sum can take.
, 2015
Solution
Let , , . If there exists a prime which divides both and , then must divide both and . Furthermore, since divides as well, must be divisible by , but this contradicts the assumption that . Therefore, we conclude that and are relatively prime. Similarly, and are relatively prime and so are and . Combining these facts with the assumption that , , are all greater than , we get .
Furthermore, since divides both and , it divides as well. Similarly, both and divide . Therefore, we conclude that the product divides also, and this implies that must hold.
On the other hand, we see that the triple satisfies all the conditions of the problem and that is satisfied. Therefore, we conclude that is the minimum value we seek.
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