When all the functions of time are sinusoids of the same frequency, the time domain equation can be replaced with a frequency domain equation. Retarded time is replaced with a phase term. F ( r ) = ε ε 0 4 π π ∫ ∫ Ω Ω M i ( r ) e − − j k | r − − r ′ | | r − − r ′ | d 3 r ′ , where F and M i are phasor quantities and k is the wave number.