olymmath

OIMU 2007 Problem 4

    Consider an infinite sequence a1,a2,⋯a_1,a_2,\cdots whose terms all belong to {1,2}\left\{1,2\right\} . A positive integer with nn digits is said to be good if its decimal representation has the form arar+1⋯ar+(n−1)a_ra_{r+1}\cdots a_{r+(n-1)} , for some positive integer rr . Suppose that there are at least 20082008 good numbers with a million digits. Prove that there are at least 20082008 good numbers with 20072007 digits.

    Dataset text AI-MO AoPS. Quoted with source.

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