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Number theory Difficulty 5.3 AIME, harder Prove it Croatia

Prove that there are no positive integers mm and nn such that
5m3=27n42n2+n. 5m^3 = 27n^4 - 2n^2 + n.

Solution

Let us write the equation in the form
5m3=n(27n32n+1) 5m^3 = n(27n^3 - 2n + 1)
and note that nn and 27n32n+127n^3 - 2n + 1 are relatively prime numbers. We consider two cases:

a) n=5a3n = 5a^3 and 27n32n+1=b327n^3 - 2n + 1 = b^3 for some positive integers aa and bb.
Inequality (3n1)3=27n327n2+9n1<27n32n+1(3n-1)^3 = 27n^3 - 27n^2 + 9n - 1 < 27n^3 - 2n + 1 is equivalent to the inequality 27n211n+2>027n^2 - 11n + 2 > 0, which holds for all positive integers nn. Also, we have 27n32n+1<27n32+1<27n3=(3n)327n^3 - 2n + 1 < 27n^3 - 2 + 1 < 27n^3 = (3n)^3.
Hence, (3n1)3<27n32n+1<(3n)3(3n-1)^3 < 27n^3 - 2n + 1 < (3n)^3, which is impossible.

b) n=a3n = a^3 and 27n32n+1=5b327n^3 - 2n + 1 = 5b^3 for some positive integers aa and bb.
From the second equation we conclude that nn cannot be divisible by 55. If we have n±1(mod5)n \equiv \pm1 \pmod 5, then 27n32n+11(mod5)27n^3 - 2n + 1 \equiv 1 \pmod 5. If we have n2(mod5)n \equiv 2 \pmod 5, then 27n32n+13(mod5)27n^3 - 2n + 1 \equiv 3 \pmod 5, and if n3(mod5)n \equiv 3 \pmod 5, then 27n32n+14(mod5)27n^3 - 2n + 1 \equiv 4 \pmod 5. In all cases the left-hand side of the equation cannot be divisible by 55.
Hence the equation has no solution.

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