What Is the Fourier Transform?

Shameless plug: If you are interested in Fourier Transform and signal processing you might enjoy my somewhat artistic 3D visualisation of the fourier transform as well as the fractional fourier transform [1]

(Fractional fourier transform on the top face of the cube)

And for short time fourier transform showing how a filter kernel is shiftes across the signal. [2]

[1]: https://static.laszlokorte.de/frft-cube/

[2]: https://static.laszlokorte.de/time-frequency/

If I might also plug ‘the Atlas of Fourier Transforms’. If your interested in understanding building intuition of symmetry and phase in fourier space, the book illustrates many structures.