Definite integral

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  • #1
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Can anyone help me integrating the folowwing integrand from zero to infinity:
x*ln^2(ax)*exp(-bx^2+cx)
where a,b and c are real and pisitive constants.
Thanks
 
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Answers and Replies

  • #2
First term diverges with [tex]x \rightarrow \infty[/tex]. Is the region really from 0 to infinity? If possible, please post in TeX.
But first term is not difficult. Using partial integral,

[tex]\int x (\ln ax)^2 dx = 1/2 x^2 (\ln ax)^2 - \int \frac {ax^2} {ax} \ln ax dx = \frac 1 2 x^2 (\ln ax)^2 - \int x \ln ax dx[/tex].

Using partial integral on the second term, it is

[tex]\frac 1 2 x^2 \ln ax - \frac 1 2 \int ax^2 \frac 1 {ax} dx = \frac 1 2 x^2 \ln ax - \frac 1 4 x^2[/tex]

So in total,

[tex]\int x (\ln ax)^2 dx = \frac 1 2 x^2((\ln ax))^2 - \ln ax) + \frac 1 4 x^2[/tex].

The second term can be integrated from [tex]-\infty \rightarrow \infty[/tex]
 
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  • #3
As for [tex]\exp(-bx^2+cx)[/tex], it can be integrated for some kinds of limited regions...(AFAIK) using Gaussian integral. i.e. Put

[tex]I = \int_{-\infty}^\infty \exp(-x^2) dx[/tex].

Then

[tex]I^2 = \int_{-\infty}^\infty \int_{-\infty}^\infty \exp(-(x^2 + y^2)) dx dy = 2 \pi \int_0^\infty r \exp(-r^2) dr = \pi[/tex]

So [tex]I = \sqrt \pi.[/tex]

But exp(-bx^2+cx) is another thing if the range is different.... If the region is from [tex]-\infty \rightarrow \infty[/tex]

[tex]\int_{-\infty}^\infty \exp(-bx^2 + cx) dx = \int_{\infty}^\infty \exp(-b(x-\frac c {2b}) ^ 2 + \frac {c^2} {4b}) dx = \sqrt \pi \exp (c^2/4b) / \sqrt b[/tex]

I cannot go further..:frown:
 
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  • #4
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I'm very sorry. I was mistaken. It should be a multiplication of the two terms, not sum. The integrand is now corrected in the original message.
Thanks
 

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