In two dimensions, the equation
is rewritten as
For simplicity, we have considered that along an axis u, we have
. The term
is split into its components along x and y: 
and 
(only the equation for has been written). The form of the one
dimensional system is retrieved by considering for each axis a source term
as a sum of the physical source term J and the coupling with the
other axis. The extension to higher dimension follows readily from these lines.