Since this is also a convolution, the Fourier transformation of the same equation is a product V ~ ~ ( k ) = d e f ρ ρ ~ ~ u c ( k ) Φ Φ ~ ~ ( k ) where the Fourier transform is defined V ~ ~ ( k ) = ∫ ∫ d r v ( r ) e − − i k ⋅ ⋅ r