More fun in 2016

Calculus Level 5

lim n 0 1 x 2016 ( { n x } ) 2016 d x = 1 a 2 \large \displaystyle \lim _{n \to \infty} \int _0^1 x^{2016} \left(\left\{ nx \right\} \right)^{2016} \, dx =\frac {1}{a^2}

Find a |a| .

Notation : { } \{ \cdot \} denotes the fractional part function .


The answer is 2017.

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1 solution

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Jul 6, 2017

lim n 0 1 x 2016 ( { n x } ) 2016 d x = lim n 1 n 2017 0 x 2016 { x } 2016 Now rewrite the fractional part: \displaystyle\lim_{n\to\infty}\int_0^1x^{2016}(\big\{nx\big\})^{2016}dx=\lim_{n\to\infty}\frac1{n^{2017}}\int_0^\infty x^{2016}\big\{x\big\}^{2016}\text{ Now rewrite the fractional part:}

1 n 2017 k = 0 ( t + k ) 2016 t 2016 d t = 1 n k = 0 ( t n + k n ) 2016 t 2016 d t (as n , t n 0 ) \displaystyle\frac1{n^{2017}}\sum_{k=0}^\infty(t+k)^{2016}t^{2016}dt=\frac1{n}\sum_{k=0}^\infty(\frac{t}{n}+\frac{k}{n})^{2016}t^{2016}dt\quad\text{(as }n\to\infty, \frac{t}{n}\to0\text{ )}

lim n 1 n k = 0 ( k n ) 2016 0 1 t 2016 d t = 0 1 x 2016 d x 0 1 t 2016 d t = 1 201 7 2 \displaystyle \lim_{n\to\infty}\frac1{n}\sum_{k=0}^\infty(\frac{k}{n})^{2016}\int_0^1t^{2016}dt=\int_0^1x^{2016}dx\int_0^1t^{2016}dt=\Huge\boxed{\frac1{2017^2}}

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