ProductLog

[WilliamG]

I have discovered that the solution to a problem I am trying to solve is: B
= A + ProductLog [-Ae^-A]. I know A and, obviously, e. Is there a formula I
can type into Excel that will give me the output B? Many thanks indeed for
any help you can offer.

 

[Barb Reinhardt]

If I can assume that ProductLog = we^w then do this

Let's also assume the your value of w (from the above) is in cell A1

Product Log =A1*EXP(A1)

FWIW, I had to Google this function. I don't believe that there is a built
in function in Excel, but you could create a UDF if you use it enough.

 

[Barb Reinhardt]

Oops, correction to function

=A1+ A1*EXP(A1)

 

[Dana DeLouis]

If I can assume that ProductLog = we^w then do this

Hi. I don't believe that is correct.
A quick copy form Help in a math program...

ProductLog[z] gives the principal solution for w in z=we^w.

For Excel, we need to solve via iteration AFAIK.

Dana DeLouis

 

[Dana DeLouis]

I have discovered that the solution to a problem I am trying to solve is: B
= A + ProductLog [-Ae^-A]. I know A and, obviously, e. Is there a formula I
can type into Excel that will give me the output B? Many thanks indeed for
any help you can offer.

Hi. Here's 1 idea for a vba function.
It does not have error checking. It's very basic.
It goes for 15 loops, and will not exit if it hits it's fixed point earlier.

Function ProductLog(n)
Dim g As Double
Dim j As Long

g = 1

For j = 1 To 15
g = (n * Exp(-g) + g ^ 2) / (1 + g)
Next j

ProductLog = g
End Function

HTH
Dana DeLouis

 

[Dana DeLouis]

= A + ProductLog [-Ae^-A]

As a side note... I would double check my equations if in fact you used
"e" and not "E"

Dana DeLouis