How many positive integers n≤2009 have the property that ⌊log2(n)⌋ is odd?
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Solution
We wish to find n such that there is some natural number k for which 2k−1≤log2n<2k. Since n≤2009 we must have k≤5. This is equivalent to finding the number of positive integers n≤2009 satisfying 22k−1≤n<22k for some k≤5, so the number of such integers is 2+23+25+27+29=682
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