"I am Back" says Integration Part 7

Calculus Level 5

x e 3 x e e 2 x d x \large \int_{-\infty}^{\infty} {xe^{3x}e^{-{e}^{2x}} \, dx}

If the above Integral can be expressed as π A / 2 B ( C γ D ln ( E ) ) , \dfrac{{\pi}^{A/2}}{B} \left( C - \gamma - D\ln(E)\right),

where A , B , C , D , E A,B,C,D, E are positive integers, with E E prime, find A + B + C + D + E A+B+C+D+E .

Notation : 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 .


The answer is 15.

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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} {10}e^{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 ) t 1 / 2 e t dt \dfrac{1}{4}\int_0^\infty \ln(t)t^{1/2} 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=3/2 to get that Γ ( 3 / 2 ) = ψ ( 3 / 2 ) Γ ( 3 / 2 ) = ( 1 1 / 2 + ψ ( 1 / 2 ) ) 1 2 Γ ( 1 / 2 ) = π 1 / 2 2 ( 2 γ 2 ln ( 2 ) ) \Gamma'(3/2)=\psi(3/2)\Gamma(3/2)=\left(\dfrac{1}{1/2}+\psi(1/2)\right)\dfrac{1}{2}\Gamma(1/2)=\dfrac{\pi^{1/2}}{2} (2-\gamma-2\ln(2)) we know ψ ( 1 / 2 ) = γ + 0 1 1 x 1 / 2 1 x dx = γ 2 ln ( 2 ) \psi(1/2)=-\gamma+\int_0^1 \dfrac{1-x^{-1/2}}{1-x}\text{dx}=-\gamma-2\ln(2) final answer is x e 3 x e e 2 x d x = π 1 / 2 8 ( 2 γ 2 ln ( 2 ) ) \int_{-\infty}^{\infty} {xe^{3x}e^{-{e}^{2x}} \, dx}=\dfrac{\pi^{1/2}}{8} (2-\gamma-2\ln(2))

Do you know how to derive the value of ψ ( 1 2 ) \psi(\frac{1}{2}) ?

Aditya Kumar - 5 years, 4 months ago

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Google it.

Aareyan Manzoor - 5 years, 4 months ago

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I know how. i just wanted you to add that in your solution for users.

Aditya Kumar - 5 years, 4 months ago

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@Aditya Kumar I dont, i googled it.

Aareyan Manzoor - 5 years, 4 months ago

@Aditya Kumar : @Aareyan Manzoor

Ψ ( x ) = d d x ln Γ ( x ) = Γ ( x ) Γ ( x ) \Psi (x)=\frac{d}{dx}\ln\Gamma (x)=\frac{\Gamma '(x)}{\Gamma (x)}

A Former Brilliant Member - 5 years, 4 months ago

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Wow! Thanks for the new knowledge! I didnt know that, i just knew how to apply it. You know sometimes you know how to apply stuff before defination. i also know Γ = ψ Γ \Gamma'=\psi\Gamma but not that defination.

All sarcasm aside, i actually learned later how to calculate it. Do the integral ψ ( 1 / 2 ) = γ + 0 1 1 x 1 / 2 1 x dx \psi(1/2)=-\gamma+\int_0^1 \dfrac{1-x^{-1/2}}{1-x}\text{dx} The integral is easy.

Aareyan Manzoor - 5 years, 4 months ago

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@Aareyan Manzoor I am glad that you found it useful...:)

A Former Brilliant Member - 5 years, 4 months ago

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