Average Error: 13.6 → 1.2
Time: 21.5s
Precision: 64
Internal Precision: 128
\[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
\[\frac{x}{e^{wj} \cdot wj + e^{wj}} + \left({wj}^{4} - \left(\sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj} \cdot \sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj}\right) \cdot \sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj}\right)\]

Error

Bits error versus wj

Bits error versus x

Target

Original13.6
Target12.9
Herbie1.2
\[wj - \left(\frac{wj}{wj + 1} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)\]

Derivation

  1. Initial program 13.6

    \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
  2. Using strategy rm
  3. Applied div-sub13.6

    \[\leadsto wj - \color{blue}{\left(\frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)}\]
  4. Applied associate--r-7.1

    \[\leadsto \color{blue}{\left(wj - \frac{wj \cdot e^{wj}}{e^{wj} + wj \cdot e^{wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}}\]
  5. Taylor expanded around 0 1.1

    \[\leadsto \color{blue}{\left(\left({wj}^{2} + {wj}^{4}\right) - {wj}^{3}\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
  6. Simplified1.1

    \[\leadsto \color{blue}{\left({wj}^{4} - \left(wj \cdot wj - wj\right) \cdot wj\right)} + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
  7. Using strategy rm
  8. Applied add-cube-cbrt1.2

    \[\leadsto \left({wj}^{4} - \color{blue}{\left(\sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj} \cdot \sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj}\right) \cdot \sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj}}\right) + \frac{x}{e^{wj} + wj \cdot e^{wj}}\]
  9. Final simplification1.2

    \[\leadsto \frac{x}{e^{wj} \cdot wj + e^{wj}} + \left({wj}^{4} - \left(\sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj} \cdot \sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj}\right) \cdot \sqrt[3]{\left(wj \cdot wj - wj\right) \cdot wj}\right)\]

Reproduce

herbie shell --seed 2019053 
(FPCore (wj x)
  :name "Jmat.Real.lambertw, newton loop step"

  :herbie-target
  (- wj (- (/ wj (+ wj 1)) (/ x (+ (exp wj) (* wj (exp wj))))))

  (- wj (/ (- (* wj (exp wj)) x) (+ (exp wj) (* wj (exp wj))))))