Transformer Integrals

Calculus Level 5

The shaded area is equal to 2 e 2 K 1 ( 2 ) 2e^2\text{K}_1(2) . It is also equal to 0 1 ln 2 x α ln x d x \displaystyle \int_0^1 \sqrt{\ln^2{x}-\alpha\ln{x}}\, dx , for some positive number α \alpha . Find α \alpha .


Notation: K n ( x ) \text{K}_n (x) is the modified Bessel function of the second kind . (You don't have to worry about it in this question) .


The answer is 4.

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1 solution

Digvijay Singh
Mar 2, 2018

ln ( x y ) = ( ln x ) ( ln y ) \ln(xy)=(\ln{x})(\ln{y})

Converting to polar form

r = exp ( 1 1 2 ln 2 tan θ + 4 ) sin θ cos θ \implies r=\dfrac{\exp\left(1-\dfrac{1}{2}\sqrt{\ln^2\tan{\theta}+4}\right)}{\sqrt{\sin{\theta}\cos{\theta}}}

Required area = A = 1 2 0 π 2 r 2 d θ =A=\dfrac{1}{2}\displaystyle\int_0^{\frac{\pi}{2}}r^2\,d\theta

= 0 π 4 csc θ sec θ exp ( 2 ln 2 tan θ + 4 ) d θ =\displaystyle\int_0^{\frac{\pi}{4}}\csc{\theta}\sec{\theta}\exp\left(2-\sqrt{\ln^2\tan{\theta}+4}\right)\,d\theta

Let ln tan θ = t d t = csc θ sec θ d θ \ln\tan{\theta}=t\,\implies\,dt=\csc{\theta}\sec{\theta}\,d\theta

A = 0 e 2 t 2 + 4 d t = 0 e 2 t 2 + 4 d t \implies A=\displaystyle\int_{-\infty}^0 e^{2-\sqrt{t^2+4}}\,dt=\displaystyle\int_0^\infty e^{2-\sqrt{t^2+4}}\,dt

By Inverting the function, the integral becomes:

0 1 ln 2 x 4 ln x d x \displaystyle\int_0^1\sqrt{\ln^2x-4\ln{x}}\,dx

α = 4 \implies \boxed{\alpha=4}

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