Great deal of simplification can be achieved if we choose 
.
Putting , we get 
which is vector poisson equation.
In Cartesian coordinates, the above equation can be written in terms of the components as
......................................(4.27a)
......................................(4.27b)
......................................(4.27c)
The form of all the above equation is same as that of
..........................................(4.28)
for which the solution is
..................(4.29)