Maths Olympiad Prep

Library / /5 of 24

Algebra Difficulty 5.3 AIME, harder Prove it Argentina

Decide whether there exist 20182018 distinct positive integers such that the sum of their squares is a perfect cube and the sum of their cubes is a perfect square.

Solution

The answer is affirmative. There exist 20182018 distinct positive numbers satisfying the required conditions:
a,2a,,2018a, where a=(2018201940376)4. a, 2a, \dots, 2018a, \text{ where } a = \left( \frac{2018 \cdot 2019 \cdot 4037}{6} \right)^4.
Note that aa is an integer number, since 20192019 is a multiple of 33 and 20182018 is a multiple of 22.

The sum of the squares of these numbers is
a2+(2a)2++(2018a)2=a2(12+22++20182)=a2(2018201940376)==(2018201940376)8(2018201940376)=((2018201940376)3)3, a^2 + (2a)^2 + \dots + (2018a)^2 = a^2(1^2 + 2^2 + \dots + 2018^2) = a^2 \left( \frac{2018 \cdot 2019 \cdot 4037}{6} \right) = \\ = \left( \frac{2018 \cdot 2019 \cdot 4037}{6} \right)^8 \left( \frac{2018 \cdot 2019 \cdot 4037}{6} \right) = \left( \left( \frac{2018 \cdot 2019 \cdot 4037}{6} \right)^3 \right)^3,
which is a perfect cube, and the sum of their cubes is
a3+(2a)3++(2018a)3=a3(13+23++20183)=a3(201820192)2==(2018201940376)12(201820192)2=((2018201940376)6(201820192))2, a^3 + (2a)^3 + \dots + (2018a)^3 = a^3(1^3 + 2^3 + \dots + 2018^3) = a^3 \left(\frac{2018 \cdot 2019}{2}\right)^2 = \\ = \left(\frac{2018 \cdot 2019 \cdot 4037}{6}\right)^{12} \left(\frac{2018 \cdot 2019}{2}\right)^2 = \left(\left(\frac{2018 \cdot 2019 \cdot 4037}{6}\right)^6 \left(\frac{2018 \cdot 2019}{2}\right)\right)^2,
which is a perfect square.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement and solution reproduced as published; topic and difficulty added by this site.