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Number theory Difficulty 4.9 AIME Prove it China

Do there exist integers aa, bb and cc such that a2bc+2a^2bc + 2, ab2c+2ab^2c + 2, abc2+2abc^2 + 2 are perfect squares?

Solution

No. Suppose the contrary that there are such integers aa, bb and cc.
If one of them is even, say aa, then a2bc+22(mod4)a^2bc + 2 \equiv 2 \pmod 4, which contradicts the assumption that a2bc+2a^2bc + 2 is a perfect square. We may then assume that aa, bb and cc are odd, so they are either 11 or 3(mod4)3 \pmod 4. It follows from the Pigeon-hole Principle that two of them are congruent modulo 44. Relabel if necessary, we may assume that ab(mod4)a \equiv b \pmod 4, so abc2+2c2+21+23(mod4)abc^2 + 2 \equiv c^2 + 2 \equiv 1 + 2 \equiv 3 \pmod 4, which violates the perfect square assumption. \square

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