Find all integers , such that the equation has only integer solutions.
Solution
Let and be the integer solutions of this quadratic equation. We may assume that . By Viete's formulas we have and . Adding both identities together we get , so
Since , we have four possible cases to consider. In the first case we have , , in the second case , , in the third case , and in the final case , . Using the identity we see that is equal to or .
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