Number theoryDifficulty 4.7AIMEProve itSaudi Arabia
Let a and b be integers such that a−b=a2c−b2d for some consecutive integers c and d. Prove that ∣a−b∣ is a perfect square.
Solution
Let d=c+1. The equality a−b=a2c−b2(c+a) implies (a−b)[c(a+b)−1]=b2. But c(a+b)−1 and a−b are relatively prime. Indeed, if p is a prime dividing a−b, then the above equality shows that p also divides b, so p will divide a+b=(a−b)+2b. Hence p cannot divide c(a+b)−1. It follows that ∣a−b∣ is a perfect square as well. A first nontrivial example is 18−22=182(−3)−222(−2).
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