## Section18.6Examples of Fourier Transforms

This section asks you to find the Fourier transform of a cosine function and a Gaussian. Hints and answers are provided, but the details are left for the reader.

### Activity18.7.The Fourier Transform of the Cosine.

Find the Fourier transform of the cosine function $$f(x)=\cos kx\text{.}$$

Hint.

Hint: You must integrate a combination of an exponential and a cosine. It is ALWAYS easier to integrate exponentials, so use the exponential form of the cosine function, (2.5.3).

\begin{align} {\cal{F}}(\cos kx)\amp =\tilde{f}(k^{\prime})\tag{18.6.1}\\ \amp = \pi\, \left(\delta(k-k^{\prime})+\delta(k+k^{\prime})\right)\tag{18.6.2} \end{align}

### Activity18.8.The Fourier Transform of a Gaussian.

Find the Fourier transform of the normalized Gaussian (18.2.10)

$$f(x)=\frac{1}{\sqrt{2\pi}\,\sigma}\, e^{-\frac{(x-x_0)^2}{2\sigma^2}}\tag{18.6.3}$$
Hint.

Complete the square in the exponential, see SectionÂ A.1, and use the formula for the integral of a Gaussian, see (18.2.1).

Notice that the Fourier transform of a Gaussian is also a Gaussian, but now the factor of $$\sigma^2$$ is in the numerator of the exponential instead of the denominator. How are the shapes of the two Gaussian's related to each other?