Maths Olympiad Prep

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Number theory Difficulty 5.3 AIME, harder Prove it Estonia

Prove that for any positive integer kk there exist kk pairwise distinct integers for which the sum of their squares equals the sum of their cubes.

Solution

For any integer m>1m > 1 the numbers 2m2+12m^2 + 1, m(2m2+1)m(2m^2 + 1), m(2m2+1)-m(2m^2 + 1) satisfy the conditions of the problem, because they are pairwise different and
(2m2+1)2+(m(2m2+1))2+(m(2m2+1))2=(1+m2+m2)(2m2+1)2=(2m2+1)3=(1+m3m3)(2m2+1)3=(2m2+1)3+(m(2m2+1))3+(m(2m2+1))3. \begin{aligned} & (2m^2 + 1)^2 + (m(2m^2 + 1))^2 + (-m(2m^2 + 1))^2 \\ &= (1 + m^2 + m^2) \cdot (2m^2 + 1)^2 = (2m^2 + 1)^3 = (1 + m^3 - m^3) \cdot (2m^2 + 1)^3 \\ &= (2m^2 + 1)^3 + (m(2m^2 + 1))^3 + (-m(2m^2 + 1))^3. \end{aligned}
With mm growing, the numbers in these triples get arbitrarily large, hence for any set of these triples one can find a new triple, where all numbers are larger than the ones already used.
Any positive integer kk can be written as k=3q+rk = 3q + r with 0r<30 \le r < 3. Choose qq triples as above so that the numbers in them do not coincide. If r=1r = 1, then add 00, and if r=2r = 2, then add 00 and 11. Since for each group the sum of the squares of the numbers equals the sum of the cubes of the numbers, the same property holds for the whole set.

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