A Robust Approximation to a Lambert-Type Function

The function $y=g(x)=\mathrm{l}\mathrm{o}\mathrm{g}(W(e^x))$, where $W()$ denotes the Lambert W function, is the solution to the equation $y+e^y=x$. It appears in various problem situations, for instance the calculation of current-voltage curves for solar cells. A direct calculation of $g(x)$ may be inaccurate because of arithmetic underflow or overflow. We present a simple algorithm for calculating $g(x)$ that is robust, in that it will work for almost all $x$ values which are representable in the arithmetic of one’s chosen computer language. The algorithm does not assume that the chosen computer language implements the Lambert W function.