"I am Back" says Integration Part 6

Calculus Level 5

x e 2 x e e 2 x d x \large \int_{-\infty}^{\infty} {xe^{2x}e^{-{e}^{2x}} \, dx}

The above Integral can be expressed as γ a , -\dfrac{\gamma}{a},

where γ \gamma denotes the Euler-Mascheroni constant γ = lim n ( ln n + k = 1 n 1 k ) 0.5772. \displaystyle \gamma = \lim_{n\to\infty} \left( - \ln n + \sum_{k=1}^n \dfrac1k \right) \approx 0.5772 .

Find a a .


The answer is 4.

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1 solution

Aareyan Manzoor
Feb 15, 2016

Substitute 10 e 2 x = t x = ln ( t ) 2 d t = 2 e 2 x d t 0 \begin{array}{c}10e^{2x}=t\to x=\dfrac{\ln(t)}{2}\\ dt= 2e^{2x} dt\\ \mid_{-\infty}^\infty \to \mid_0^\infty\end{array} The integrand becomes: 1 4 0 ln ( t ) e t dt \dfrac{1}{4}\int_0^\infty \ln(t) e^{-t} \text{dt} Consider the gamma functions: Γ ( a ) = 0 t a 1 e t dt \Gamma(a)=\int_0^\infty t^{a-1}e^{-t}\text{dt} d.w.r.a Γ ( a ) = 0 ln ( t ) t a 1 e t dt \Gamma'(a)=\int_0^\infty \ln(t) t^{a-1} e^{-t}\text{dt} Put a=1 to get our original integrand. Now Γ ( 1 ) = ψ ( 1 ) Γ ( 1 ) = γ \Gamma'(1)=\psi(1)\Gamma(1)=-\gamma so the answer is γ 4 \dfrac{-\gamma}{\boxed{4}}

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