The following table gives an overview of Green's functions of frequently appearing differential operators, where r = x 2 + y 2 + z 2 , ρ ρ = x 2 + y 2 , Θ Θ ( t ) is the Heaviside step function, J ν ν ( z ) is a Bessel function, I ν ν ( z ) is a modified Bessel function of the first kind, and K ν ν ( z ) is a modified Bessel function of the second kind. Where time (t) appears in the first column, the retarded (causal) Green's function is listed.