- #1
VVS
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Homework Statement
I have to prove that [itex]I(a,b)=\int_{-\infty}^{+\infty} exp(-ax^2+bx)dx=\sqrt{\frac{\pi}{a}}exp(b^2/4a)[/itex] where [itex]a,b\in\mathbb{C}[/itex].
I have already shown that [itex]I(a,0)=\sqrt{\frac{\pi}{a}}[/itex].
Now I am supposed to find a relation between [itex]I(a,0)[/itex] and [itex]\int_{-\infty}^{+\infty} exp(-a(x-c)^2)dx[/itex] where [itex]c\in\mathbb{C}[/itex] using the Cauchy integral theorem and prove using this the result above.
Homework Equations
The Cauchy integral theorem states that [itex]\oint_\gamma f(z) dz=2\pi i Res(f(z))[/itex].
The Attempt at a Solution
This is what I got, but I am pretty sure it doesn't lead me anywhere.
Now I am not sure whether [itex]\int_{-\infty}^{+\infty} exp(-a(x-c)^2)dx[/itex] is analytic in c. But I think what I can do is integrate over a closed contour [itex]\gamma[/itex] and use Cauchy's integral theorem, change the order of integration and thus equate it to the Residue.
[itex]\int_{-\infty}^{+\infty} \oint_\gamma exp(-a(x-c)^2)dcdx=2\pi i Res(f(z))[/itex]
Then I can expand to yield:
[itex]\int_{-\infty}^{+\infty} exp(-ax^2) \oint_\gamma exp(-ac^2+2axc)dcdx=2\pi i Res(f(z))[/itex]
I can sort of see the relation between the integrals now but I am kind of stuck.