Russian math olympiads are renowned for their creative, unconventional problems that challenge even expert mathematicians
To give you an idea of what awaits you in those PDFs, here is a classic "Russian style" problem.
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: Includes problems involving properties of triangles, polynomials, and sequences.
(t \in [0,1]) ⇒ (x = t^2 + 1 \in [1, 2]).
: This is one of the most famous collections, containing 320 unconventional problems in algebra, number theory, and trigonometry. Full PDFs are available through repositories like Internet Archive and Les-Mathematiques.net .