Maths Olympiad Prep

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Number theory Difficulty 5.2 AIME, harder Prove it JBMO

Problem:
Are there any positive integers mm and nn satisfying the equation
m3=9n4+170n2+289? m^{3}=9 n^{4}+170 n^{2}+289 ?

Solution

Solution:
We will prove that the answer is no. Note that
m3=9n4+170n2+289=(9n2+17)(n2+17) m^{3}=9 n^{4}+170 n^{2}+289=(9 n^{2}+17)(n^{2}+17)
If nn is odd then mm is even, therefore 8m38 \mid m^{3}. However,
9n4+170n2+2899+170+2894(mod8) 9 n^{4}+170 n^{2}+289 \equiv 9+170+289 \equiv 4 \pmod{8}
which leads to a contradiction. If nn is a multiple of 1717 then so is mm and hence 289289 is a multiple of 17317^{3}, which is absurd. For nn even and not multiple of 1717, since
gcd(9n2+17,n2+17)9(n2+17)(9n2+17)=2317 \gcd\left(9 n^{2}+17, n^{2}+17\right) \mid 9\left(n^{2}+17\right)-\left(9 n^{2}+17\right)=2^{3} \cdot 17
this gcd must be 11. Therefore n2+17=a3n^{2}+17=a^{3} for an odd aa, so
n2+25=(a+2)(a22a+4) n^{2}+25=(a+2)\left(a^{2}-2 a+4\right)
For a1(mod4)a \equiv 1 \pmod{4} we have a+23(mod4)a+2 \equiv 3 \pmod{4}, while for a3(mod4)a \equiv 3 \pmod{4} we have a22a+43(mod4)a^{2}-2 a+4 \equiv 3 \pmod{4}. Thus (a+2)(a22a+4)(a+2)\left(a^{2}-2 a+4\right) has a prime divisor of type 4+34 \ell+3. As it divides n2+25n^{2}+25, it has to divide nn and 55, which is absurd.

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Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.