A Floor Function Dilemma!

Calculus Level 5

lim n 1 n 2 k = 1 n 1 k x + n k 1 n = ? \large{ \lim_{n \to \infty} \dfrac{1}{n^2} \sum_{k=1}^{n-1} \ k \cdot \left \lfloor x + \dfrac{n-k-1}{n} \right \rfloor = \ ? }

Find the value of the above limit upto 3 decimal places when x = 2015 x = \sqrt{2015} .

Bonus - Generalize the above limit in terms of x x .


The answer is 22.395.

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2 solutions

Iliya Hristov
Feb 26, 2020

? = lim n 1 n 2 k = 1 n 1 k x + n k 1 n 0 n k 1 n < 1 { x } = x x ?=\lim_{ n \to \infty }\frac{1}{n^2}\sum_{k=1}^{n-1}{k\left\lfloor x+\frac{n-k-1}{n}\right\rfloor}\;\;\;\;\;\;\;\;\;\;\;\;0≤\frac{n-k-1}{n}<1\;\;\;\;\;\;\;\;\;\;\;\;\;\{x\}=x-\lfloor x\rfloor L e t 1 = { x } + n p 1 n p = n { x } 1 a n d x + n k 1 n = { x + 1 i f k p p x i f k p + 1 > p Let\;\;\;1= \{x\}+\frac{n-p-1}{n}\;\;\;⇒\;\;\;p=n\{x\}-1\;\;\;and\;\;\;\left\lfloor x+\frac{n-k-1}{n} \right\rfloor=\begin{cases} \lfloor x\rfloor+1\;\;\;if\;\;\;k≤\lfloor p\rfloor≤p\\ \lfloor x\rfloor\;\;\;if\;\;\;k≥\lfloor p\rfloor+1>p\end{cases} ? = lim n 1 n 2 ( ( x + 1 ) k = 1 p k + x k = p + 1 n 1 k ) U s i n g t h e a r i t h m e t i c s e r i e r s f o r m u l a ?=\lim_{n\to\infty}\frac{1}{n^2}\left((\lfloor x\rfloor+1)\sum_{k=1}^{\lfloor p\rfloor}{k }+\lfloor x\rfloor\sum_{k=\lfloor p\rfloor+1 }^{n-1}{ k}\right) \;\;\;Using\;the\;arithmetic\;seriers\;formula\;\;\;⇒ ? = lim n 1 n 2 ( ( x + 1 ) ( p + 1 ) ( p 1 + 1 ) 2 + x ( n 1 + p + 1 ) ( n 1 p 1 + 1 ) 2 ) ?=\lim_{n\to\infty}\frac{1}{n^2}\left((\lfloor x\rfloor+1)\frac{(\lfloor p\rfloor +1)(\lfloor p\rfloor-1+1 )}{2}+\lfloor x\rfloor\frac{(n-1+\lfloor p\rfloor +1)(n-1-\lfloor p\rfloor -1+1)}{2}\right) I f n p p n { x } If\;\;\;n\to\infty\;\;\;⇒\;\;\;\lfloor p\rfloor\to p\to n\{x\} \to\infty ? = lim n ( x + 1 ) p 2 + x ( n 2 p 2 ) 2 n 2 = lim n x n 2 + p 2 2 n 2 = x + { x } 2 2 = 44 + ( 2015 44 ) 2 2 22.395 ?=\lim_{n\to\infty}\frac{(\lfloor x\rfloor+1)p^2+\lfloor x\rfloor(n^2-p^2)}{2n^2}=\lim_{n\to\infty}\frac{\lfloor x\rfloor n^2+p^2 }{2n^2}=\frac{\lfloor x\rfloor +\{x\}^2}{2}=\frac{44+(\sqrt{2015}-44)^2}{2}\approx\boxed{22.395}

Ankit Kumar Jain
Jun 15, 2018

@Satyajit Mohanty Sir , can you please post a solution, because I got it correct by mistake , I got exactly 22 but the answer was 22.395 . I would like to know the solution. Please sir.

I got 22.448 :P I also think i made a mistake

Md Zuhair - 2 years, 7 months ago

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