ProductLog

ProductLog[z] gives the principal solution for w in z = w*e^w (Lambert W function).

ProductLog[k, z] gives the kth solution.

Examples

Principal branch:

ProductLog[1]
(* 0.567143 *)

Verify solution:

w = ProductLog[1];
w Exp[w]
(* 1. *)

Please visit the official Wolfram Language Reference for more details.