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Calculus Level 5

Find the value of

1 0 ( ln ( 1 x ) ) d x \displaystyle \int_1^0\left(\ln\left(\frac{1}{x}\right)\right)^{dx}

If the answer is α \alpha input the value of ln α \ln\alpha

Note

The d x dx is raised to the power. If you think that the given notation is meaningless, input 2.718 2.718


The answer is 0.57721.

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1 solution

Rohan Shinde
Apr 25, 2019

These type of integrals are called Multiplicative Integrals or Product Integral. The above integral is represented as 1 0 f ( x ) d x \prod_1^0 f(x)^{dx}

Where f ( x ) = ln ( 1 x ) f(x)=\ln\left(\frac 1x\right)

Using a property of Multiplicative integrals which is stated as a b f ( x ) d x = exp ( a b ln f ( x ) d x ) \displaystyle \prod_a^b f(x)^{dx}=\exp {\left(\int_a^b \ln f(x) dx\right)}

Thus in this case I = 1 0 ( ln ( 1 x ) ) d x = 1 0 ( ln ( 1 x ) ) d x \displaystyle I=\int_1^0 \left(\ln\left(\frac 1x\right)\right)^{dx}=\prod_1^0 \left(\ln\left(\frac 1x\right)\right)^{dx} = exp ( 1 0 ln ( ln ( 1 x ) ) d x ) =\displaystyle \exp {\left(\int_1^0 \ln\left(\ln\left(\frac 1x\right)\right)dx\right)}

Now using that 0 1 ln ( ln ( 1 x ) ) d x = γ \displaystyle \int_0^1 \ln\left(\ln\left(\frac 1x\right)\right)dx=-\gamma

Where γ \gamma is the Euler Mascheroni Constant, we get I = exp ( 1 0 ln ( ln ( 1 x ) ) d x ) = e γ = α \displaystyle I=\exp {\left(\int_1^0 \ln\left(\ln\left(\frac 1x\right)\right)dx\right)}=e^{\gamma}=\alpha

Hence ln α = γ 0.5772 \ln \alpha=\gamma\approx \boxed {0.5772}

Edit: \pmb{\text{Edit:}}

For the Evaluation of 0 1 ln ( ln ( 1 x ) ) d x \displaystyle \int_0^1 \ln\left(\ln\left(\frac 1x\right)\right)dx

Just substitute ln x = t -\ln x=t , You would get 0 ( ln t ) e t d t \displaystyle\int_0^{\infty} (\ln t )e^{-t} dt . Now if I ( a ) = 0 t a 1 e t d t = Γ ( a ) \displaystyle I(a)=\int_0^{\infty} t^{a-1}e^{-t}dt=\Gamma(a) then what we need is I ( 1 ) = Γ ( 1 ) ψ ( 1 ) = γ I'(1)=\Gamma(1)\psi(1)=-\gamma

Yes absolutely.........although, shouldn't you prove that the value of that integral is the euler mascheroni constant????

Aaghaz Mahajan - 2 years, 1 month ago

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@Aaghaz Mahajan That is quite a basic result, just substitute ln x = t -\ln x=t , You would get 0 ( ln t ) e t d t \displaystyle\int_0^{\infty} (\ln t )e^{-t} dt . Now if I ( a ) = 0 t a 1 e t d t = Γ ( a ) \displaystyle I(a)=\int_0^{\infty} t^{a-1}e^{-t}dt=\Gamma(a) then what we need is I ( 1 ) = Γ ( 1 ) ψ ( 1 ) = γ I'(1)=\Gamma(1)\psi(1)=-\gamma

Rohan Shinde - 2 years, 1 month ago

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No no.......I know that already.....what I meant was, it would be better if you add this part in your solution :)

Aaghaz Mahajan - 2 years, 1 month ago

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@Aaghaz Mahajan Yeah done.... edited the answer accordingly....

Rohan Shinde - 2 years, 1 month ago

We can even advance this problem.

Naren Bhandari - 1 year, 2 months ago

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Yeah, I mean Product Integrals were something new for me, and it just turns out to be a normal integration problem of a different function.....it is good if you can advance this!!

Aaghaz Mahajan - 1 year, 1 month ago

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It was too new for me. I advance it here . I even tried up doing generalization but I got stuck and didn't go further , haha.

Naren Bhandari - 1 year, 1 month ago

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