Find all positive integer solutions to the equation 4z2(2z2+1)=4xy−2x−y.
Solution
Answer: There is no solution. Our equation is equivalent to (4z2+1)2=(4x−1)(2y−1). Since 4x−1≡3(mod4), there exists a prime divisor p≥3 of 4x−1 such that p≡3(mod4). Then p∣4z2+1, thus 4z2≡−1(modp). By Fermat's theorem, we have 1≡(2z)p−1≡(−1)2p−1≡−1(modp). This is a contradiction. So there is no solution.
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