Average Error: 19.5 → 0.4
Time: 30.8s
Precision: 64
Internal Precision: 128
\[\frac{1}{\sqrt{x}} - \frac{1}{\sqrt{x + 1}}\]
\[\frac{\frac{1}{\frac{x}{\sqrt{x}} + \frac{x}{\sqrt{x + 1}}}}{x + 1}\]

Error

Bits error versus x

Target

Original19.5
Target0.7
Herbie0.4
\[\frac{1}{\left(x + 1\right) \cdot \sqrt{x} + x \cdot \sqrt{x + 1}}\]

Derivation

  1. Initial program 19.5

    \[\frac{1}{\sqrt{x}} - \frac{1}{\sqrt{x + 1}}\]
  2. Using strategy rm
  3. Applied flip--19.5

    \[\leadsto \color{blue}{\frac{\frac{1}{\sqrt{x}} \cdot \frac{1}{\sqrt{x}} - \frac{1}{\sqrt{x + 1}} \cdot \frac{1}{\sqrt{x + 1}}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}}\]
  4. Using strategy rm
  5. Applied frac-times24.5

    \[\leadsto \frac{\frac{1}{\sqrt{x}} \cdot \frac{1}{\sqrt{x}} - \color{blue}{\frac{1 \cdot 1}{\sqrt{x + 1} \cdot \sqrt{x + 1}}}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}\]
  6. Applied frac-times19.6

    \[\leadsto \frac{\color{blue}{\frac{1 \cdot 1}{\sqrt{x} \cdot \sqrt{x}}} - \frac{1 \cdot 1}{\sqrt{x + 1} \cdot \sqrt{x + 1}}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}\]
  7. Applied frac-sub19.4

    \[\leadsto \frac{\color{blue}{\frac{\left(1 \cdot 1\right) \cdot \left(\sqrt{x + 1} \cdot \sqrt{x + 1}\right) - \left(\sqrt{x} \cdot \sqrt{x}\right) \cdot \left(1 \cdot 1\right)}{\left(\sqrt{x} \cdot \sqrt{x}\right) \cdot \left(\sqrt{x + 1} \cdot \sqrt{x + 1}\right)}}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}\]
  8. Simplified5.6

    \[\leadsto \frac{\frac{\color{blue}{1}}{\left(\sqrt{x} \cdot \sqrt{x}\right) \cdot \left(\sqrt{x + 1} \cdot \sqrt{x + 1}\right)}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}\]
  9. Simplified5.5

    \[\leadsto \frac{\frac{1}{\color{blue}{x \cdot x + x}}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}\]
  10. Using strategy rm
  11. Applied *-un-lft-identity5.5

    \[\leadsto \frac{\frac{1}{x \cdot x + x}}{\frac{1}{\sqrt{x}} + \color{blue}{1 \cdot \frac{1}{\sqrt{x + 1}}}}\]
  12. Applied div-inv5.5

    \[\leadsto \frac{\frac{1}{x \cdot x + x}}{\color{blue}{1 \cdot \frac{1}{\sqrt{x}}} + 1 \cdot \frac{1}{\sqrt{x + 1}}}\]
  13. Applied distribute-lft-out5.5

    \[\leadsto \frac{\frac{1}{x \cdot x + x}}{\color{blue}{1 \cdot \left(\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}\right)}}\]
  14. Applied div-inv5.5

    \[\leadsto \frac{\color{blue}{1 \cdot \frac{1}{x \cdot x + x}}}{1 \cdot \left(\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}\right)}\]
  15. Applied times-frac5.5

    \[\leadsto \color{blue}{\frac{1}{1} \cdot \frac{\frac{1}{x \cdot x + x}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}}\]
  16. Simplified5.5

    \[\leadsto \color{blue}{1} \cdot \frac{\frac{1}{x \cdot x + x}}{\frac{1}{\sqrt{x}} + \frac{1}{\sqrt{x + 1}}}\]
  17. Simplified0.8

    \[\leadsto 1 \cdot \color{blue}{\frac{1}{\left(\frac{x}{\sqrt{x + 1}} + \frac{x}{\sqrt{x}}\right) \cdot \left(x + 1\right)}}\]
  18. Using strategy rm
  19. Applied associate-/r*0.4

    \[\leadsto 1 \cdot \color{blue}{\frac{\frac{1}{\frac{x}{\sqrt{x + 1}} + \frac{x}{\sqrt{x}}}}{x + 1}}\]
  20. Final simplification0.4

    \[\leadsto \frac{\frac{1}{\frac{x}{\sqrt{x}} + \frac{x}{\sqrt{x + 1}}}}{x + 1}\]

Reproduce

herbie shell --seed 2019093 
(FPCore (x)
  :name "2isqrt (example 3.6)"

  :herbie-target
  (/ 1 (+ (* (+ x 1) (sqrt x)) (* x (sqrt (+ x 1)))))

  (- (/ 1 (sqrt x)) (/ 1 (sqrt (+ x 1)))))