I am currently reading Digital Image Processing by Gonzalez and Woods and in the the chapter of Filtering in Frequency Domain there is a huge mention about Fourier Transform.
The author writes,
" Fourier's Contribution in this field states that any periodic function can be expressed as the sum of sines and /or cosines of different frequencies each multiplied by a different coefficient (which we now call Fourier Series)"
Can anyone explain me on a basic mathematical level on how this modelling of every periodic function just with sines and/or cosines is possible?