Limit problem by Sutirtha Datta

Calculus Level pending

lim n 1 n i = 1 n 4 i n \large \lim_{n\to\infty} \dfrac1n \sum_{i=1}^n \left \lfloor \sqrt{ \dfrac{4i}n} \right \rfloor

Find the value of the closed form of the above limit.


Notation: \lfloor \cdot \rfloor denotes the floor function .


The answer is 0.75.

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1 solution

Sutirtha Datta
Feb 24, 2017

Evaluation of limits using definite integrals:

lim n 1 n i = 1 n 4 i n = 0 1 4 x d x = 0 1 4 0. d x + 1 4 1 . 1. d x = [ x ] 1 4 1 = 1 1 4 = 3 4 \lim_{n\to\infty} \dfrac1n \sum_{i=1}^n \left \lfloor \sqrt{ \dfrac{4i}n} \right \rfloor = \int_0^1\left\lfloor\sqrt{4x}\right\rfloor{dx} = \int_0^{\frac{1}{4}}0.dx + \int_{\frac{1}{4}}^1.1.dx =[x]_\frac{1}{4}^1 =1-\frac{1}{4}= \frac{3}{4}

[The graph of 4 x \left\lfloor\sqrt{4x}\right\rfloor has jump discontinuities at x = 1 4 , 1 , 9 4 . . . x=\frac{1}{4},1,\frac{9}{4}... ]

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