Maths Olympiad Prep

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Algebra Difficulty 4.0 AIME Find the answer United States

Problem:
How many positive integers n2009n \leq 2009 have the property that log2(n)\left\lfloor\log_{2}(n)\right\rfloor is odd?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Solution:
We wish to find nn such that there is some natural number kk for which 2k1log2n<2k2k-1 \leq \log_{2} n < 2k. Since n2009n \leq 2009 we must have k5k \leq 5. This is equivalent to finding the number of positive integers n2009n \leq 2009 satisfying 22k1n<22k2^{2k-1} \leq n < 2^{2k} for some k5k \leq 5, so the number of such integers is 2+23+25+27+29=6822 + 2^{3} + 2^{5} + 2^{7} + 2^{9} = 682.

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