These two equations determine the diode current and the diode voltage. To solve these two equations, we could substitute the current I from the second equation into the first equation, and then try to rearrange the resulting equation to get V D in terms of V S . A difficulty with this method is that the diode law is nonlinear. Nonetheless, a formula expressing I directly in terms of V S without involving V D can be obtained using the Lambert W -function , which is the inverse function of f ( w ) = w e w , that is, w = W ( f ) . This solution is discussed next.