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Number theory Difficulty 6.1 National olympiad Prove it

Are there any positive integers mm and nn satisfying the equation

m3=9n4+170n2+289? m^{3}=9 n^{4}+170 n^{2}+289 ?

Solutions — 2

Solution 1

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=\left(9 n^{2}+17\right)\left(n^{2}+17\right)

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(\bmod 8)

which leads to a contradiction. If nn is a multiple of 17 then so is mm and hence 289 is a multiple of 17317^{3}, which is absurd. For nn even and not multiple of 17, since

gcd(9n2+17,n2+17)9(n2+17)(9n2+17)=2317 \operatorname{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 1. 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(\bmod 4) we have a+23(mod4)a+2 \equiv 3(\bmod 4), while for a3(mod4)a \equiv 3(\bmod 4) we have a22a+43a^{2}-2 a+4 \equiv 3 (mod4)(\bmod 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 5, which is absurd.

Solution 2

1. We start with the given equation:
m3=9n4+170n2+289 m^3 = 9n^4 + 170n^2 + 289
We can rewrite the right-hand side as a product of two polynomials:
m3=(9n2+17)(n2+17) m^3 = (9n^2 + 17)(n^2 + 17)

2. Next, we consider the case where 1717 divides nn. If 17n17 \mid n, then n=17kn = 17k for some integer kk. Substituting n=17kn = 17k into the equation, we get:
m3=9(17k)4+170(17k)2+289 m^3 = 9(17k)^4 + 170(17k)^2 + 289
Simplifying, we have:
m3=9174k4+170172k2+289 m^3 = 9 \cdot 17^4 k^4 + 170 \cdot 17^2 k^2 + 289
Since 1739174k417^3 \mid 9 \cdot 17^4 k^4 and 173170172k217^3 \mid 170 \cdot 17^2 k^2, it follows that 173m317^3 \mid m^3. Therefore, 17m17 \mid m. Let m=17pm = 17p for some integer pp. Substituting m=17pm = 17p into the equation, we get:
(17p)3=9174k4+170172k2+289 (17p)^3 = 9 \cdot 17^4 k^4 + 170 \cdot 17^2 k^2 + 289
Simplifying, we have:
173p3=9174k4+170172k2+289 17^3 p^3 = 9 \cdot 17^4 k^4 + 170 \cdot 17^2 k^2 + 289
Dividing both sides by 17317^3, we get:
p3=917k4+170k2+289173 p^3 = 9 \cdot 17 k^4 + 170 k^2 + \frac{289}{17^3}
Since 289173\frac{289}{17^3} is not an integer, this leads to a contradiction. Hence, 1717 does not divide nn.

3. Now, we consider the case where 1717 and nn are coprime. Let gcd(9n2+17,n2+17)=d \text{gcd}(9n^2 + 17, n^2 + 17) = d . It follows that d(9n2+17)9(n2+17)d \mid (9n^2 + 17) - 9(n^2 + 17):
d(9n2+17)9(n2+17)=9n2+179n2153=136 d \mid (9n^2 + 17) - 9(n^2 + 17) = 9n^2 + 17 - 9n^2 - 153 = -136
Therefore, d136d \mid 136. The divisors of 136 are 1,2,4,8,17,34,68,1361, 2, 4, 8, 17, 34, 68, 136.

4. If d=1d = 1, assume we have 9n2+17=a39n^2 + 17 = a^3 and n2+17=b3n^2 + 17 = b^3 for positive integers aa and bb. Considering modulo 7:
9n2+172n2+3(mod7) 9n^2 + 17 \equiv 2n^2 + 3 \pmod{7}
n2+17n2+3(mod7) n^2 + 17 \equiv n^2 + 3 \pmod{7}
Since a3b3(mod7)a^3 \equiv b^3 \pmod{7}, we need 2n2+3n2+3(mod7)2n^2 + 3 \equiv n^2 + 3 \pmod{7}, which implies n20(mod7)n^2 \equiv 0 \pmod{7}. Thus, 7n7 \mid n, contradicting that 1717 and nn are coprime.

5. If d=2d = 2, we have 8(9n2+17)(n2+17)8 \mid (9n^2 + 17)(n^2 + 17). Considering modulo 4:
9n2+17n2+1(mod4) 9n^2 + 17 \equiv n^2 + 1 \pmod{4}
n2+17n2+1(mod4) n^2 + 17 \equiv n^2 + 1 \pmod{4}
Since 8(n2+1)(n2+1)8 \mid (n^2 + 1)(n^2 + 1), we need n23(mod4)n^2 \equiv 3 \pmod{4}, which is impossible since n20,1(mod4)n^2 \equiv 0, 1 \pmod{4}.

6. If d=4d = 4 or d=8d = 8, we will get a contradiction by considering modulo 4:
9n2+17n2+1(mod4) 9n^2 + 17 \equiv n^2 + 1 \pmod{4}
n2+17n2+1(mod4) n^2 + 17 \equiv n^2 + 1 \pmod{4}
Since 4(n2+1)(n2+1)4 \mid (n^2 + 1)(n^2 + 1), we need n23(mod4)n^2 \equiv 3 \pmod{4}, which is impossible.

Therefore, there are no positive integers mm and nn satisfying the equation m3=9n4+170n2+289m^3 = 9n^4 + 170n^2 + 289.

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Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.