usage: lambertw(z) or lambertw(n,z)

Compute the Lambert W function of z.  This function satisfies
W(z).*exp(W(z)) = z, and can thus be used to express solutions
of transcendental equations involving exponentials or logarithms.

n must be integer, and specifies the branch of W to be computed;
W(z) is a shorthand for W(0,z), the principal branch.  Branches
0 and -1 are the only ones that can take on non-complex values.

If either n or z are non-scalar, the function is mapped to each
element; both may be non-scalar provided their dimensions agree.

This implementation should return values within 2.5*eps of its
counterpart in Maple V, release 3 or later.  Please report any
discrepancies to the author, Nici Schraudolph .

For further details, see:

Corless, Gonnet, Hare, Jeffrey, and Knuth (1996), "On the Lambert
W Function", Advances in Computational Mathematics 5(4):329-359.