Robert M. Corless Gaston H. Gonnet D. E. G. Hare David J. Jeffrey Donald E. Knuth On the Lambert<i>W</i> function. 329-359 1996 5 Adv. Comput. Math. 1 http://dx.doi.org/10.1007/BF02124750 db/journals/adcm/adcm5.html#CorlessGHJK96