\[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
Test:
Jmat.Real.lambertw, newton loop step
Bits:
128 bits
Bits error versus wj
Bits error versus x
Time: 11.5 s
Input Error: 18.8
Output Error: 0.0
Log:
Profile: 🕒
\((wj * \left(wj - {wj}^2\right) + \left({wj}^{4}\right))_* + \frac{x}{(wj * \left(e^{wj}\right) + \left(e^{wj}\right))_*}\)
  1. Started with
    \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
    18.8
  2. Using strategy rm
    18.8
  3. Applied div-sub to get
    \[wj - \color{red}{\frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}} \leadsto wj - \color{blue}{\left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)}\]
    18.8
  4. Applied associate--r- to get
    \[\color{red}{wj - \left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)} \leadsto \color{blue}{\left(wj - \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}}\]
    13.4
  5. Applied simplify to get
    \[\color{red}{\left(wj - \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}} \leadsto \color{blue}{\left(wj - \frac{wj}{1 + wj}\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    13.4
  6. Applied taylor to get
    \[\left(wj - \frac{wj}{1 + wj}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}} \leadsto \left(\left({wj}^{4} + {wj}^2\right) - {wj}^{3}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    0.0
  7. Taylor expanded around 0 to get
    \[\color{red}{\left(\left({wj}^{4} + {wj}^2\right) - {wj}^{3}\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}} \leadsto \color{blue}{\left(\left({wj}^{4} + {wj}^2\right) - {wj}^{3}\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
    0.0
  8. Applied simplify to get
    \[\color{red}{\left(\left({wj}^{4} + {wj}^2\right) - {wj}^{3}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}} \leadsto \color{blue}{(wj * \left(wj - {wj}^2\right) + \left({wj}^{4}\right))_* + \frac{x}{(wj * \left(e^{wj}\right) + \left(e^{wj}\right))_*}}\]
    0.0

Original test:


(lambda ((wj default) (x default))
  #:name "Jmat.Real.lambertw, newton loop step"
  (- wj (/ (- (* wj (exp wj)) x) (+ (exp wj) (* wj (exp wj)))))
  #:target
  (- wj (- (/ wj (+ wj 1)) (/ x (+ (exp wj) (* wj (exp wj)))))))