Average Error: 0.2 → 0.3
Time: 1.7m
Precision: 64
Internal Precision: 128
\[\left|\frac{1}{\sqrt{\pi}} \cdot \left(\left(\left(2 \cdot \left|x\right| + \frac{2}{3} \cdot \left(\left(\left|x\right| \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right) + \frac{1}{5} \cdot \left(\left(\left(\left(\left|x\right| \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right) + \frac{1}{21} \cdot \left(\left(\left(\left(\left(\left(\left|x\right| \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right)\right|\]
\[\left|\frac{\left({\left(\left|x\right|\right)}^{5} \cdot \frac{1}{5} + \left(2 + \frac{2}{3} \cdot \left(\left|x\right| \cdot \left|x\right|\right)\right) \cdot \left|x\right|\right) + {\left(\left|x\right|\right)}^{7} \cdot \frac{1}{21}}{\sqrt{\sqrt{\pi}}} \cdot \frac{1}{\sqrt{\sqrt{\pi}}}\right|\]

Error

Bits error versus x

Derivation

  1. Initial program 0.2

    \[\left|\frac{1}{\sqrt{\pi}} \cdot \left(\left(\left(2 \cdot \left|x\right| + \frac{2}{3} \cdot \left(\left(\left|x\right| \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right) + \frac{1}{5} \cdot \left(\left(\left(\left(\left|x\right| \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right) + \frac{1}{21} \cdot \left(\left(\left(\left(\left(\left(\left|x\right| \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right)\right|\]
  2. Taylor expanded around -inf 0.2

    \[\leadsto \left|\color{blue}{\sqrt{\frac{1}{\pi}} \cdot \left(\frac{1}{5} \cdot {\left(\left|x\right|\right)}^{5} + \left(2 \cdot \left|x\right| + \left(\frac{2}{3} \cdot {\left(\left|x\right|\right)}^{3} + \frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7}\right)\right)\right)}\right|\]
  3. Simplified0.2

    \[\leadsto \left|\color{blue}{\sqrt{\frac{1}{\pi}} \cdot \left(\left(\frac{1}{5} \cdot {\left(\left|x\right|\right)}^{5} + \left|x\right| \cdot \left(2 + \left(\frac{2}{3} \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right) + \frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7}\right)}\right|\]
  4. Using strategy rm
  5. Applied sqrt-div0.2

    \[\leadsto \left|\color{blue}{\frac{\sqrt{1}}{\sqrt{\pi}}} \cdot \left(\left(\frac{1}{5} \cdot {\left(\left|x\right|\right)}^{5} + \left|x\right| \cdot \left(2 + \left(\frac{2}{3} \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right) + \frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7}\right)\right|\]
  6. Applied associate-*l/0.6

    \[\leadsto \left|\color{blue}{\frac{\sqrt{1} \cdot \left(\left(\frac{1}{5} \cdot {\left(\left|x\right|\right)}^{5} + \left|x\right| \cdot \left(2 + \left(\frac{2}{3} \cdot \left|x\right|\right) \cdot \left|x\right|\right)\right) + \frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7}\right)}{\sqrt{\pi}}}\right|\]
  7. Simplified0.6

    \[\leadsto \left|\frac{\color{blue}{\frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7} + \left(\left|x\right| \cdot \left(2 + \left(\left|x\right| \cdot \left|x\right|\right) \cdot \frac{2}{3}\right) + {\left(\left|x\right|\right)}^{5} \cdot \frac{1}{5}\right)}}{\sqrt{\pi}}\right|\]
  8. Using strategy rm
  9. Applied add-sqr-sqrt0.6

    \[\leadsto \left|\frac{\frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7} + \left(\left|x\right| \cdot \left(2 + \left(\left|x\right| \cdot \left|x\right|\right) \cdot \frac{2}{3}\right) + {\left(\left|x\right|\right)}^{5} \cdot \frac{1}{5}\right)}{\sqrt{\color{blue}{\sqrt{\pi} \cdot \sqrt{\pi}}}}\right|\]
  10. Applied sqrt-prod0.4

    \[\leadsto \left|\frac{\frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7} + \left(\left|x\right| \cdot \left(2 + \left(\left|x\right| \cdot \left|x\right|\right) \cdot \frac{2}{3}\right) + {\left(\left|x\right|\right)}^{5} \cdot \frac{1}{5}\right)}{\color{blue}{\sqrt{\sqrt{\pi}} \cdot \sqrt{\sqrt{\pi}}}}\right|\]
  11. Applied *-un-lft-identity0.4

    \[\leadsto \left|\frac{\color{blue}{1 \cdot \left(\frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7} + \left(\left|x\right| \cdot \left(2 + \left(\left|x\right| \cdot \left|x\right|\right) \cdot \frac{2}{3}\right) + {\left(\left|x\right|\right)}^{5} \cdot \frac{1}{5}\right)\right)}}{\sqrt{\sqrt{\pi}} \cdot \sqrt{\sqrt{\pi}}}\right|\]
  12. Applied times-frac0.3

    \[\leadsto \left|\color{blue}{\frac{1}{\sqrt{\sqrt{\pi}}} \cdot \frac{\frac{1}{21} \cdot {\left(\left|x\right|\right)}^{7} + \left(\left|x\right| \cdot \left(2 + \left(\left|x\right| \cdot \left|x\right|\right) \cdot \frac{2}{3}\right) + {\left(\left|x\right|\right)}^{5} \cdot \frac{1}{5}\right)}{\sqrt{\sqrt{\pi}}}}\right|\]
  13. Final simplification0.3

    \[\leadsto \left|\frac{\left({\left(\left|x\right|\right)}^{5} \cdot \frac{1}{5} + \left(2 + \frac{2}{3} \cdot \left(\left|x\right| \cdot \left|x\right|\right)\right) \cdot \left|x\right|\right) + {\left(\left|x\right|\right)}^{7} \cdot \frac{1}{21}}{\sqrt{\sqrt{\pi}}} \cdot \frac{1}{\sqrt{\sqrt{\pi}}}\right|\]

Reproduce

herbie shell --seed 2019090 
(FPCore (x)
  :name "Jmat.Real.erfi, branch x less than or equal to 0.5"
  (fabs (* (/ 1 (sqrt PI)) (+ (+ (+ (* 2 (fabs x)) (* (/ 2 3) (* (* (fabs x) (fabs x)) (fabs x)))) (* (/ 1 5) (* (* (* (* (fabs x) (fabs x)) (fabs x)) (fabs x)) (fabs x)))) (* (/ 1 21) (* (* (* (* (* (* (fabs x) (fabs x)) (fabs x)) (fabs x)) (fabs x)) (fabs x)) (fabs x)))))))