16.11

Algebra Level 4

n = 0 2016 n 2016 n 2015 + 1 = ? \large \sum_{n=0}^{2016} \left \lfloor \dfrac{ n^{2016}}{n^{2015}+1} \right \rfloor = \, ?


The answer is 2031120.

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1 solution

To begin this problem, let us attempt to find the value of n 2016 n 2015 + 1 \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor for integers n n such that 0 n 2016 0 \leq n \leq 2016 .

Looking at the value n 2016 n 2015 + 1 \frac{n^{2016}}{n^{2015}+1} , we see that this will be extremely close to n 2016 n 2015 = n \frac{n^{2016}}{n^{2015}} = n because of the exponents which will lead to extremely large numbers for n 2 n \geq 2 . Seeing the additional 1 1 that is added in the denominator of the fraction, we can deduce that the fraction n 2016 n 2015 + 1 \frac{n^{2016}}{n^{2015}+1} will have a value slightly lower than n n for n 2 n \geq 2 . Therefore the greatest integer less than n 2016 n 2015 + 1 \frac{n^{2016}}{n^{2015}+1} , or n 2016 n 2015 + 1 \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor , will be equivalent to n 1 n-1 for n 2 n\geq 2 .

So, we now know the value of n 2016 n 2015 + 1 \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor for the integers n = 2 , 3 , 4 2014 , 2015 , 2016 n=2,3,4 \ldots 2014,2015,2016 . Therefore, to find the value of n = 0 2016 n 2016 n 2015 + 1 \sum_{n=0}^{2016} \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor , we must simply add the values of n 2016 n 2015 + 1 \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor for n = 0 , 1 n=0,1 to the value of n = 2 2016 n 2016 n 2015 + 1 \sum_{n=2}^{2016} \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor .

Therefore, n = 0 2016 n 2016 n 2015 + 1 = 0 2016 0 2015 + 1 + 1 2016 1 2015 + 1 + n = 2 2016 n 2016 n 2015 + 1 = 0 1 + 1 2 + n = 2 2016 n 1 = 0 + 0 + 1 + 2 + 3 + + 2013 + 2014 + 2015 = \sum_{n=0}^{2016} \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor = \left\lfloor \frac{0^{2016}}{0^{2015}+1} \right\rfloor + \left\lfloor \frac{1^{2016}}{1^{2015}+1} \right\rfloor + \sum_{n=2}^{2016} \left\lfloor \frac{n^{2016}}{n^{2015}+1} \right\rfloor = \left\lfloor \frac{0}{1} \right\rfloor+ \left\lfloor \frac{1}{2} \right\rfloor + \sum_{n=2}^{2016} n-1 = 0 + 0 + 1 + 2 + 3 +\ldots + 2013 +2014 + 2015 =

1 + 2 + 3 + + 2013 + 2014 + 2015 = ( 2015 + 1 ) 2015 2 = 2016 2015 2 = 1008 2015 = 2031120 1 + 2 + 3 +\ldots + 2013 +2014 + 2015 = \frac{(2015+1)\cdot2015}{2} = \frac{2016\cdot2015}{2} = 1008\cdot2015=\boxed{2031120}

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