- #1
wotanub
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Homework Statement
This is from Cahill's Physical Mathematics. Exercise 5.23.
For [itex]a \gt 0[/itex] and [itex]b^{2} – 4ac \lt 0[/itex], use a ghost contour to do the integral
[itex]\int^{\infty}_{-\infty} \frac{1}{ax^{2}+bx+c} \mathrm{d}x[/itex]
Homework Equations
Use contour integration and the residue theorem.
The Attempt at a Solution
Mathematica is giving a different result than I got.
It gives [itex]\int^{\infty}_{-\infty} \frac{1}{ax^{2}+bx+c} \mathrm{d}x = \frac{2\pi}{\sqrt{4ac-b^{2}}}[/itex]
My solution:
The roots of the polynomial are
[itex]x = \frac{-b \pm \sqrt{b^{2}-4ac}}{2a}[/itex]
For convenience, write
[itex]x = \alpha \pm \mathrm{i}\beta[/itex]
where [itex] \alpha \equiv \frac{-b}{2a}, \beta \equiv \frac{-\mathrm{i}}{2a}\sqrt{b^{2}-4ac} \gt 0[/itex]
My contour [itex]\mathcal{C}[/itex] will be the real axis, and a large CCW semicircle that encloses the entire UHP. If [itex]z = R \mathrm{e}^{\mathrm{i}\theta}[/itex] then the integral along the arc trivially vanishes as [itex]R→\infty[/itex]
[itex]\int^{\infty}_{-\infty} \frac{1}{ax^{2}+bx+c} \mathrm{d}x[/itex]
[itex]= \oint_{\mathcal{C}} \frac{1}{az^{2}+bz+c} \mathrm{d}z[/itex]
[itex]= \oint_{\mathcal{C}} \frac{1}{(z-(\alpha - \mathrm{i}\beta))(z-(\alpha + \mathrm{i}\beta))} \mathrm{d}z[/itex]
[itex]= \mathrm{Res}(\frac{1}{(z-(\alpha - \mathrm{i}\beta))(z-(\alpha + \mathrm{i}\beta))}, z = \alpha + \mathrm{i}\beta)[/itex], since this is the only pole in the UHP
[itex] = 2\pi\mathrm{i}\frac{1}{\alpha + \mathrm{i}\beta - (\alpha - \mathrm{i}\beta)}[/itex]
[itex] = 2 \pi\mathrm{i}\frac{1}{2\mathrm{i}\beta}[/itex]
[itex] = \frac{\pi}{\beta}[/itex]
[itex] = \frac{2a\mathrm{i}\pi}{\sqrt{b^{2}-4ac}}[/itex]
[itex] = \frac{2\pi a}{\sqrt{4ac-b^{2}}}[/itex]
So I've got an extra factor of [itex]a[/itex] here? Mistakes?
My Mathematica code is
Code:
Integrate[1/(a*x^2 + b*x + c), {x, -Infinity, Infinity}, Assumptions -> {a > 0, 4*a*c > b^2}]