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Convolution, or called folding, of two functions, f(r) and h(r), is defined as,
 [2738a]
In addition, convolution theorem is a useful theorem connecting the convolution in direct space to multiplication in Fourier transform space. The convolution theorem is given by,
 [2738b]
Multiplication theorem connects the multiplication in direct space to the convolution in Fourier transform space,
 [2738c]
