Average Error: 13.8 → 0.9
Time: 36.8s
Precision: 64
Internal Precision: 128
\[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
\[\begin{array}{l} \mathbf{if}\;wj \le 4.608621108479766 \cdot 10^{-09}:\\ \;\;\;\;x + wj \cdot \left(x \cdot -2 + wj\right)\\ \mathbf{else}:\\ \;\;\;\;wj - \left(\frac{1}{1 + {wj}^{3}} \cdot \left(\left(1 - wj\right) + wj \cdot wj\right)\right) \cdot \left(wj - \frac{x}{e^{wj}}\right)\\ \end{array}\]

Error

Bits error versus wj

Bits error versus x

Target

Original13.8
Target13.1
Herbie0.9
\[wj - \left(\frac{wj}{wj + 1} - \frac{x}{e^{wj} + wj \cdot e^{wj}}\right)\]

Derivation

  1. Split input into 2 regimes
  2. if wj < 4.608621108479766e-09

    1. Initial program 13.4

      \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
    2. Taylor expanded around 0 0.9

      \[\leadsto \color{blue}{\left({wj}^{2} + x\right) - 2 \cdot \left(x \cdot wj\right)}\]
    3. Simplified0.9

      \[\leadsto \color{blue}{x + \left(wj + -2 \cdot x\right) \cdot wj}\]

    if 4.608621108479766e-09 < wj

    1. Initial program 25.3

      \[wj - \frac{wj \cdot e^{wj} - x}{e^{wj} + wj \cdot e^{wj}}\]
    2. Using strategy rm
    3. Applied distribute-rgt1-in25.4

      \[\leadsto wj - \frac{wj \cdot e^{wj} - x}{\color{blue}{\left(wj + 1\right) \cdot e^{wj}}}\]
    4. Applied *-un-lft-identity25.4

      \[\leadsto wj - \frac{\color{blue}{1 \cdot \left(wj \cdot e^{wj} - x\right)}}{\left(wj + 1\right) \cdot e^{wj}}\]
    5. Applied times-frac25.3

      \[\leadsto wj - \color{blue}{\frac{1}{wj + 1} \cdot \frac{wj \cdot e^{wj} - x}{e^{wj}}}\]
    6. Simplified2.6

      \[\leadsto wj - \frac{1}{wj + 1} \cdot \color{blue}{\left(wj - \frac{x}{e^{wj}}\right)}\]
    7. Using strategy rm
    8. Applied flip3-+2.8

      \[\leadsto wj - \frac{1}{\color{blue}{\frac{{wj}^{3} + {1}^{3}}{wj \cdot wj + \left(1 \cdot 1 - wj \cdot 1\right)}}} \cdot \left(wj - \frac{x}{e^{wj}}\right)\]
    9. Applied associate-/r/2.5

      \[\leadsto wj - \color{blue}{\left(\frac{1}{{wj}^{3} + {1}^{3}} \cdot \left(wj \cdot wj + \left(1 \cdot 1 - wj \cdot 1\right)\right)\right)} \cdot \left(wj - \frac{x}{e^{wj}}\right)\]
    10. Simplified2.5

      \[\leadsto wj - \left(\frac{1}{{wj}^{3} + {1}^{3}} \cdot \color{blue}{\left(\left(1 - wj\right) + wj \cdot wj\right)}\right) \cdot \left(wj - \frac{x}{e^{wj}}\right)\]
  3. Recombined 2 regimes into one program.
  4. Final simplification0.9

    \[\leadsto \begin{array}{l} \mathbf{if}\;wj \le 4.608621108479766 \cdot 10^{-09}:\\ \;\;\;\;x + wj \cdot \left(x \cdot -2 + wj\right)\\ \mathbf{else}:\\ \;\;\;\;wj - \left(\frac{1}{1 + {wj}^{3}} \cdot \left(\left(1 - wj\right) + wj \cdot wj\right)\right) \cdot \left(wj - \frac{x}{e^{wj}}\right)\\ \end{array}\]

Reproduce

herbie shell --seed 2019051 
(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))))))