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Bulgarian National Olympiad - Final Round

Bulgaria algebra

Problem

Do there exist non-zero reals , such that
Solution
Suppose that such exist. The condition rewrites as , which means that for all . It's easy to obtain consecutively that for any , , , so . Since , all are equal to some real . However, doesn't have a real solution, contradiction.
Final answer
No

Techniques

Recurrence relationsLinear and quadratic inequalitiesQuadratic functions