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OIMU 2005 Problem 5

    Arnaldo and Bernaldo play a game where they alternate saying natural numbers, and the winner is the one who says 00 . In each turn except the first the possible moves are determined from the previous number nn in the following way: write n=∑m∈On2m;n =\sum_{m\in O_n}2^m; the valid numbers are the elements mm of OnO_n . That way, for example, after Arnaldo says 42=25+23+2142= 2^5 + 2^3 + 2^1 , Bernaldo must respond with 55 , 33 or 11 .

    We define the sets A,B⊂NA,B\subset \mathbb{N} in the following way. We have n∈An\in A iff Arnaldo, saying nn in his first turn, has a winning strategy; analogously, we have n∈Bn\in B iff Bernaldo has a winning strategy if Arnaldo says nn during his first turn. This way, A={0,2,8,10,⋯ },B={1,3,4,5,6,7,9,⋯ }A =\{0, 2, 8, 10,\cdots\}, B = \{1, 3, 4, 5, 6, 7, 9,\cdots\} Define f:N→Nf:\mathbb{N}\to \mathbb{N} by f(n)=∣A∩{0,1,⋯ ,n−1}∣f(n)=|A\cap \{0,1,\cdots,n-1\}| . For example, f(8)=2f(8) = 2 and f(11)=4f(11)=4 . Find lim⁡n→∞f(n)log⁡(n)2005n\lim_{n\to\infty}\frac{f(n)\log(n)^{2005}}{n}

    Dataset text AI-MO AoPS. Quoted with source.

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