Lambert W function for Excel, work on real and complex number

[x I'm tc]

This is a very old thread, but there's some great code in it! However, all these formulae apparently work on the principal branch. I need a formula that will work on the -1 branch (as well as the principal branch). Is there such a thing?

[blguerke]

Updated with Principal and Lower Branch...
Validated:
LambertW(-0.271,-1)=-2.00
LambertW(-0.366,-1)=-1.10
LambertW(-0.368,0)=-1.00
LambertW(-0.366,)=-0.90
LambertW(-0.090,0)=-0.10
LambertW(0,0)=0
LambertW(0.111,0)=0.10
LambertW(2.178,0)=1.00
...
LambertW(89.472,0)=3.30

Doesn't Work above y=100
LambertW(101.78,0)=5.719 --> Actually should be 3.40LambertW.bas