The convolution theorem
[Convolution](https://texonom.com/convolution-54def9d34de64e199ea46393675af2ce) 


The Fourier transform of the convolution of two functions is the product of their Fouriertransforms and inverse too
**Using the convolution theorem, we can interpret and implement all types of linearshift-invariant filtering as multiplication in frequency domain.**




> [Convolution theorem](https://en.wikipedia.org/wiki/Convolution_theorem#Functions_of_discrete_variable_sequences)





