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SHARYGIN 2019 Problem 17

    Three circles ω1\omega_1 , ω2\omega_2 , ω3\omega_3 are given. Let A0A_0 and A1A_1 be the common points of ω1\omega_1 and ω2\omega_2 , B0B_0 and B1B_1 be the common points of ω2\omega_2 and ω3\omega_3 , C0C_0 and C1C_1 be the common points of ω3\omega_3 and ω1\omega_1 . Let Oi,j,kO_{i,j,k} be the circumcenter of triangle AiBjCkA_iB_jC_k . Prove that the four lines of the form OijkO1−i,1−j,1−kO_{ijk}O_{1 - i,1 - j,1 - k} are concurrent or parallel.

    Dataset text AI-MO AoPS. Quoted with source.

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