Definition:Lambert W Function/Lower Branch

Definition
The lower branch of the Lambert W function is the real function $W_{-1}: \left[{-\dfrac 1 e \,.\,.\, 0}\right) \to \left({\gets \,.\,.\, 0}\right)$ such that:


 * $x = W_{-1} \left({x}\right) e^{W_{-1} \left({x}\right)}$