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Number theory Difficulty 6.3 National olympiad Prove it China

Let aa, bb and cc be coprime positive integers so that a2(b3+c3)a^2 \mid (b^3 + c^3), b2(a3+c3)b^2 \mid (a^3 + c^3) and c2(a3+b3)c^2 \mid (a^3 + b^3). Find the values of aa, bb and cc. (posed by Yang Xiaoming)

Solution

By the condition of the problem, we have a2(a3+b3+c3)a^2 \mid (a^3 + b^3 + c^3), b2(a3+b3+c3)b^2 \mid (a^3 + b^3 + c^3) and c2(a3+b3+c3)c^2 \mid (a^3 + b^3 + c^3). Since aa, bb and cc are coprime, we see that a2b2c2(a3+b3+c3)a^2 b^2 c^2 \mid (a^3 + b^3 + c^3).

Without loss of generality, suppose that abca \ge b \ge c, so
3a3a3+b3+c3a2b2c2ab2c23, 3a^3 \ge a^3 + b^3 + c^3 \ge a^2 b^2 c^2 \Rightarrow a \ge \frac{b^2 c^2}{3},
and
2b3b3+c3a22b3b4c49b18c4. 2b^3 \ge b^3 + c^3 \ge a^2 \Rightarrow 2b^3 \ge \frac{b^4 c^4}{9} \Rightarrow b \le \frac{18}{c^4}.
We see that if c2b1c \ge 2 \Rightarrow b \le 1, the result contradicts bcb \ge c. Thus, c=1c = 1.

If c=1c = 1 and b=1b = 1, then a=1a = 1, so (a,b,c)=(1,1,1)(a, b, c) = (1, 1, 1) is a solution.

If c=1c = 1, b2b \ge 2 and a=ba = b, then b2b3+1b^2 \mid b^3 + 1, which is a contradiction!

If b2b \ge 2 and a>b>c=1a > b > c = 1, then
a2b2(a3+b3+1)2a3a3+b3+1a2b2ab22, a^2 b^2 \mid (a^3 + b^3 + 1) \Rightarrow 2a^3 \ge a^3 + b^3 + 1 \ge a^2 b^2 \Rightarrow a \ge \frac{b^2}{2},
and by c=1c = 1,
a2(b3+1)b3+1a2b444b3+4b4. a^2 \mid (b^3 + 1) \Rightarrow b^3 + 1 \ge a^2 \ge \frac{b^4}{4} \Rightarrow 4b^3 + 4 \ge b^4.

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