Determine all pairs of positive integers satisfying the equation
Solution
We first compute the entries of the following matrix
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | |
|---|---|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 6 | 24 | 120 | 720 | 5040 | 40320 | 362880 | 3628800 | |
| 1 | 3 | 9 | 33 | 153 | 873 | 5913 | 46233 | 409113 | 4037913 |
Obviously, the pairs and are solutions. We will show that the unique solution with is .
We observe that since is divided by for , the sum leaves a remainder when divided by for . If equality holds
for some , then there is some natural number such that
or
The discriminant is and it should be a perfect square. On the other hand we cannot have
as otherwise would divide . Therefore the given relation cannot hold for , as well as, for . Since
we observe that , a product of two consecutive integers . Therefore the only solution is .
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