Suppose that \( x, y, z \) are positive integers satisfying
0 \leq x^2 + y^2 – xyz \leq z+1.
Prove that \( x^2 + y^2 – xyz \) is a perfect square.
The best solution was submitted by 임상호 (수리과학과 2016학번). Congratulations!
Here is his solution of problem 2020-07.
Another solution was submitted by 김기수 (수리과학과 2018학번, +3), 홍의천 (수리과학과 2017학번, +3)