[…] It is useful to think of the infinite series in brackets as the Taylor series for the function Q(x) = (exp(x)-1)/x, which has a removeable singularity at x=0. A stable method for approximating W(x) is to compute it as x*Q(x) for various approximations of Q(x). For example, the following DATA step uses a 6-term Taylor series approximation of Q(x). The Taylor polynomial is evaluated by using Horner's method: …