Let n be the number of integers x such that 2 2 0 1 3 ≤ x < 2 2 0 1 4 and when x is written in binary there are at least as many 0s as 1s. What is the sum of the digits of n when it is written in binary?
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From the given conditions, x must be a string of 2014 0s and 1s where the first digit is 1. There must be at least 1007 0s. Let the answer be s , then we can evaluate the answer by looking at how we can place the 0s in the 2013 free spaces. ie. s = i = 1 0 0 7 ∑ i = 2 0 1 3 ( i 2 0 1 3 ) Since ( k 2 0 1 3 ) = ( 2 0 1 3 − k 2 0 1 3 ) we get: 2 s = i = 0 ∑ i = 2 0 1 3 ( i 2 0 1 3 ) . Using a well known result this gives us that 2 s = 2 2 0 1 3 so s = 2 2 0 1 2 which, when written in binary, is just a 1 followed by 2012 0s, so the sum of the digits is 1
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Oh yeah right, I messed up, I thought 2 2 0 1 3 had 2 0 1 3 digits instead of 2 0 1 4 , you're right there.
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Actually I'm pretty sure the answer is 2 2 0 1 1 − 2 1 ( 1 0 0 6 2 0 1 2 ) , but I think just it was a mistake in the problem, so I inputed 1 .