### Author Topic: TT2--P5  (Read 1219 times)

#### Victor Ivrii

• Elder Member
• Posts: 2511
• Karma: 0
##### TT2--P5
« on: March 23, 2018, 06:18:55 AM »
Find Fourier transforms of the  function

f(x)=\cos^2(x)e^{-|x|}

and write this function as a Fourier integral.
« Last Edit: March 23, 2018, 06:21:11 AM by Victor Ivrii »

#### Tristan Fraser

• Full Member
• Posts: 30
• Karma: 11
##### Re: TT2--P5
« Reply #1 on: March 23, 2018, 07:40:11 PM »
We first note that $\cos^2 x = (\frac{(e^{ix} + e^{-ix})}{2})^2 = \frac{2 + e^{2ix} + e^{-2ix}}{4}$

From there, we take the fourier transform:  $$\hat{f(}k) = \frac{1}{2\pi} \int_{-\infty }^{\infty } e^{-ikx} e^{-|x|} ( \frac{2 + e^{2ix} + e^{-2ix}}{4}) = \frac{1}{2\pi }\left ( \int_{0}^{\infty } \frac{2e^{-x(1+ki)} + e^{-x(1+ki - 2i)} + e^{-x(1+ki + 2i))}}{4} dx +\int_{-\infty }^{0} \frac{2e^{x(1- ki)} + e^{x(1- ki - 2i) }+ e^{x(1- ki + 2i))}}{4} dx\right )$$

We integrate that thing, and note that by evaluating at the bounds of $0,\pm \infty$, only the terms evaluated at 0 can be kept, netting us:

$$\hat{f(}k) = \frac{1}{2\pi}( \frac{1}{2(1+ki)} + \frac{1}{4(1+ki - 2i)} + \frac{1}{4(1+ki+2i)} + \frac{1}{2(1-ki)} + \frac{1}{4(1-ki-2i)} + \frac{1}{4(1-ki+2i)})$$

While this expression may be ugly, what is useful about it is the fact that these are fractions of complex numbers and their conjugates (noteably first and fourth terms, 2nd and 6th, 3rd and 5th) so we'd get the following fraction

$$\hat{f(}k) = \frac{1}{2\pi}( \frac{2}{4(1+ k^2)} + \frac{2}{16(1+ (k-2)^2)} + \frac{2}{16(1+ (k+2)^2)} )$$

Thus, we can plug this function into the IFT:

$$f(x) = \int_{-\infty}^{\infty} \hat{f}(k) e^{ikx} dk = \int_{-\infty}^{\infty} \frac{1}{4\pi}( \frac{1}{(1+ k^2)} + \frac{1}{4(1+ (k-2)^2)} + \frac{1}{4(1+ (k+2)^2)}) e^{ikx} dk$$

Leaving the function as a Fourier integral.

#### Jingxuan Zhang

• Elder Member
• Posts: 106
• Karma: 20
##### Re: TT2--P5
« Reply #2 on: March 23, 2018, 08:00:54 PM »
I suggest somewhat different numerical factor. Not much different.

#### Victor Ivrii

• Elder Member
• Posts: 2511
• Karma: 0
##### Re: TT2--P5
« Reply #3 on: March 25, 2018, 04:30:42 AM »
I suggest somewhat different numerical factor. Not much different.