The inverse Fourier transform of the vector potential is A ( r ) = ∫ ∫ d 3 k ( 2 π π ) 3 A ( k ) e i k ⋅ ⋅ r 1 = q 2 2 c 1 r [ 1 + r ^ ^ r ^ ^ ] ⋅ ⋅ v 2 where r = r 1 − − r 2 (see Common integrals in quantum field theory § Transverse potential with mass).