Binary Problem

Level pending

Let n be the number of integers x such that 2 2013 x < 2 2014 2^{2013} \le x < 2^{2014} 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?


The answer is 1.

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1 solution

Yong See Foo
Jan 27, 2014

Actually I'm pretty sure the answer is 2 2011 1 2 ( 2012 1006 ) 2^{2011}-\frac{1}{2}{2012 \choose 1006} , but I think just it was a mistake in the problem, so I inputed 1 1 .

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 = 1007 i = 2013 ( 2013 i ) s = \displaystyle\sum_{i=1007}^{i=2013} \dbinom{2013}{i} Since ( 2013 k ) = ( 2013 2013 k ) \dbinom{2013}{k} = \dbinom{2013}{2013-k} we get: 2 s = i = 0 i = 2013 ( 2013 i ) 2s = \displaystyle\sum_{i=0}^{i=2013} \dbinom{2013}{i} . Using a well known result this gives us that 2 s = 2 2013 2s = 2^{2013} so s = 2 2012 s = 2^{2012} which, when written in binary, is just a 1 followed by 2012 0s, so the sum of the digits is 1 \fbox{1}

Josh Rowley - 7 years, 4 months ago

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Oh yeah right, I messed up, I thought 2 2013 2^{2013} had 2013 2013 digits instead of 2014 2014 , you're right there.

Yong See Foo - 7 years, 4 months ago

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