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Number theory Difficulty 5.7 AIME, harder Prove it Romania

Find the smallest positive integer nn for which there exists three pairwise distinct positive integers a,b,ca, b, c, such that n=a+b+cn = a + b + c and (a+b)(b+c)(c+a)(a+b)(b+c)(c+a) is a perfect cube.

Solution

Let nn be the required minimum value, and a,b,cN1a, b, c \in \mathbb{N}_{\ge 1} pairwise distinct, such that n=a+b+cn = a + b + c and (a+b)(b+c)(c+a)=k3(a+b)(b+c)(c+a) = k^3, where kNk \in \mathbb{N}.

If a,b,ca, b, c are all odd, then n1+3+5=9n \ge 1+3+5=9. If n=9n=9, then {a,b,c}={1,3,5}\{a, b, c\} = \{1, 3, 5\}, which leads to (a+b)(b+c)(c+a)=468=192(a+b)(b+c)(c+a) = 4 \cdot 6 \cdot 8 = 192, which isn't a perfect square. Consequently, n11n \ge 11.

Suppose now that not all the numbers a,b,ca, b, c have the same parity. WLOG, suppose that aa and bb have the same parity. It follows that a+ba+b is even and c+ac+a and b+cb+c are both odd. Then 2(a+b)(b+c)(c+a)=k32 \mid (a+b)(b+c)(c+a) = k^3, so 2k2 \mid k, which implies 8(a+b)(b+c)(c+a)8 \mid (a+b)(b+c)(c+a). Since b+cb+c and c+ac+a are odd, it follows that 8a+b8 \mid a+b, so a+b8a+b \ge 8.
If n=9n=9, then a+b=8a+b=8 and c=1c=1, and also (b+c)(c+a)=ab+9(b+c)(c+a) = ab+9 must be a perfect cube, so ab18ab \ge 18. Since 4=a+b2ab4 = \frac{a+b}{2} \ge \sqrt{ab}, it follows that ab16ab \le 16, a contradiction.

Consequently, n10n \ge 10. Notice that for n=10n=10 we may consider a=1,b=2,c=7a=1, b=2, c=7, for which (a+b)(b+c)(c+a)=216=63(a+b)(b+c)(c+a) = 216 = 6^3, so the required minimum value is n=10n=10.

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