A surprising integral result

Calculus Level 4

0 d x x 2 x \large \displaystyle\int_{0}^{\infty} \dfrac{dx}{\lceil x \rceil 2^x}

Find the value of the above integral, to 2 decimal places if necessary.


Notation: \lceil \cdot \rceil denotes the ceiling function .


The answer is 1.00.

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1 solution

Sabhrant Sachan
Mar 21, 2017

I = 0 d x x 2 x = 0 1 d x 1 2 x + 1 2 d x 2 2 x + 2 3 d x 3 2 x + = r = 1 r 1 r d x r 2 x = 1 ln 2 r = 1 1 r 2 r \begin{aligned} I & = \displaystyle\int_{0}^{\infty} \dfrac{dx}{ \lceil x \rceil 2^x} = \displaystyle\int_{0}^{1} \dfrac{dx}{1\cdot 2^x}+\displaystyle\int_{1}^{2} \dfrac{dx}{2\cdot 2^x}+\displaystyle\int_{2}^{3} \dfrac{dx}{3\cdot 2^x}+\cdots \\ & = \displaystyle\sum_{r=1}^{\infty}\int_{r-1}^{r} \dfrac{dx}{r\cdot 2^x} \\ & = \dfrac{1}{\ln{2}}\displaystyle\sum_{r=1}^{\infty} \dfrac{1}{r\cdot 2^r} \end{aligned}


ln ( 1 + x ) = x x 2 2 + x 3 3 ln ( 1 x ) = x + x 2 2 + x 3 3 + \begin{aligned} \ln{(1+x)} & = x-\dfrac{x^2}{2}+\dfrac{x^3}{3}- \cdots \\ -\ln{(1-x)} & = x+\dfrac{x^2}{2}+\dfrac{x^3}{3}+ \cdots \end{aligned}

ln 2 = 1 2 + 1 2 2 2 + 1 3 2 3 + = r = 1 1 r 2 r \ \ln{2} = \dfrac{1}{2}+\dfrac{1}{2\cdot 2^2}+\dfrac{1}{3\cdot 2^3}+ \cdots = \displaystyle\sum_{r=1}^{\infty} \dfrac{1}{r\cdot 2^r}


I = 1 ln 2 × ln 2 = 1 \boxed{I = \dfrac{1}{\ln{2}} \times \ln{2} =1}

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