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Homework Help: Need help getting exp equation to lambert form!

  1. Oct 24, 2009 #1
    I have an equation that I have numerically solved in matlab - but can't figure out how MatLab has done it! It has used a Lambert W equation to get an answer for my formula which is in the style:


    which MuPad simplifies down to:

    z = 1/(Wk(-1/e1) + 1)

    But I've been trying to figure out how to even get z into the correct form for Lambert W [XeX = Y -> X = W(Y)] and can't figure it out.

    Hopefully someone can open my eyes as I can't help but think it should be a simple enough solution.

    Thanks for any help!

  2. jcsd
  3. Oct 24, 2009 #2


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    Rewrite your equation as:

    [tex]e^{-\frac 1 z} = 1 - \frac 1 z[/tex]

    Let u = 1 - 1/z giving eu-1 = u. This gives:

    1 = ue1-u) = ue1e-u. Now let v = -u, giving vev= -1/e. So v = W(-1/e). Now you can just back substitute for z.
  4. Oct 24, 2009 #3


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    Back substitute for z if you dare. (-1)e^(-1)=(-1/e) so W(-1/e)=(-1). Your original equation doesn't have any real roots.
  5. Oct 24, 2009 #4
    Thank you very much LCKurtz! My maths brain is mucho rusty.

    Ah I noticed that just after I posted. It's just a style example of a more long winded question I've got.

    Just worked through my actual question there and got the same result as matlab (give or take a very little) so thank you all for your help!
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