<!--[[File:03_freespace_tn.gif]]-->
where [[File:i_tn.gif]] <math>\mathbf{\overline{\overline{I}}}</math> is the unit dyad, [[File:delta_tn.gif]] <math>\nabla</math> is the gradient operator, '''r''' and '''r'''' are the position vectors of the observation and source points, respectively, and k<sub>0</sub> is the free-space propagation constant. This implies that electromagnetic waves propagate in free space in a spherical form away from the source. Note that the Green’s function has a singularity at the source, i.e. when '''r''' = '''r''''. This singularity must be removed when solving the integral equations.
=== 3D Integral Equations ===