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Number theory Difficulty 5.8 AIME, harder Prove it Croatia

Find all pairs (m,n)(m, n) of integers such that m2=n5+n4+1m^2 = n^5 + n^4 + 1 and m7nm - 7n divides m4nm - 4n. (Nikola Adžaga, Petar Bakić)

Solution

Note that n5+n4+1=(n3n+1)(n2+n+1)n^5 + n^4 + 1 = (n^3 - n + 1)(n^2 + n + 1), and that
d=gcd(n3n+1,n2+n+1)=gcd(n2+n+1,n22n+1)=gcd(n2+n+1,n2)=gcd(n2,7), \begin{align*} d &= \gcd(n^3 - n + 1, n^2 + n + 1) \\ &= \gcd(n^2 + n + 1, -n^2 - 2n + 1) \\ &= \gcd(n^2 + n + 1, n - 2) \\ &= \gcd(n - 2, 7), \end{align*}
hence we have two cases:

1) d=7d = 7

This implies 7m7 \mid m and 7n7 \nmid n, from which we get 7m7n7 \mid m - 7n and 7m4n7 \nmid m - 4n.
Therefore, there is no solution in this case.

2) d=1d = 1

This implies that n3n+1n^3 - n + 1 and n2+n+1n^2 + n + 1 are both squares of integers. That is true only for n=0n = 0 and n=1n = -1, since n2<n2+n+1<(n+1)2n^2 < n^2 + n + 1 < (n + 1)^2 holds for n1n \ge 1, and (n+1)2<n2+n+1<n2(n+1)^2 < n^2 + n + 1 < n^2 holds for n<1n < -1.
Both n=0n = 0 and n=1n = -1 yield m2=1m^2 = 1, i.e. m=±1m = \pm 1, and among four possibilities only two satisfy the given conditions: (m,n)=(1,0)(m, n) = (-1, 0) and (m,n)=(1,0)(m, n) = (1, 0).

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