Average Error: 44.0 → 2.9
Time: 6.0s
Precision: binary64
\[\left(\left(1.1102230246251565 \cdot 10^{-16} < a \land a < 9007199254740992\right) \land \left(1.1102230246251565 \cdot 10^{-16} < b \land b < 9007199254740992\right)\right) \land \left(1.1102230246251565 \cdot 10^{-16} < c \land c < 9007199254740992\right)\]
\[\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a} \]
↓
\[\mathsf{fma}\left(\frac{\left(a \cdot a\right) \cdot {c}^{3}}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{c \cdot \left(a \cdot c\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right) \]
\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a}
↓
\mathsf{fma}\left(\frac{\left(a \cdot a\right) \cdot {c}^{3}}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{c \cdot \left(a \cdot c\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right)

(FPCore (a b c)
 :precision binary64
 (/ (+ (- b) (sqrt (- (* b b) (* (* 3.0 a) c)))) (* 3.0 a)))
↓
(FPCore (a b c)
 :precision binary64
 (fma
  (/ (* (* a a) (pow c 3.0)) (pow b 5.0))
  -0.5625
  (fma
   (/ (* (pow a 3.0) (pow c 4.0)) (pow b 7.0))
   -1.0546875
   (fma (/ (* c (* a c)) (pow b 3.0)) -0.375 (* -0.5 (/ c b))))))
double code(double a, double b, double c) {
	return (-b + sqrt(((b * b) - ((3.0 * a) * c)))) / (3.0 * a);
}
↓
double code(double a, double b, double c) {
	return fma((((a * a) * pow(c, 3.0)) / pow(b, 5.0)), -0.5625, fma(((pow(a, 3.0) * pow(c, 4.0)) / pow(b, 7.0)), -1.0546875, fma(((c * (a * c)) / pow(b, 3.0)), -0.375, (-0.5 * (c / b)))));
}

Error

Bits error versus a

Bits error versus b

Bits error versus c

Derivation

  1. Initial program 44.0

    \[\frac{\left(-b\right) + \sqrt{b \cdot b - \left(3 \cdot a\right) \cdot c}}{3 \cdot a} \]

  2. Taylor expanded in b around inf 2.9

    \[\leadsto \color{blue}{-\left(0.5625 \cdot \frac{{c}^{3} \cdot {a}^{2}}{{b}^{5}} + \left(1.0546875 \cdot \frac{{c}^{4} \cdot {a}^{3}}{{b}^{7}} + \left(0.375 \cdot \frac{{c}^{2} \cdot a}{{b}^{3}} + 0.5 \cdot \frac{c}{b}\right)\right)\right)} \]

  3. Simplified2.9

    \[\leadsto \color{blue}{\mathsf{fma}\left(\frac{\left(a \cdot a\right) \cdot {c}^{3}}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{c \cdot \left(c \cdot a\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right)} \]

  4. Final simplification2.9

    \[\leadsto \mathsf{fma}\left(\frac{\left(a \cdot a\right) \cdot {c}^{3}}{{b}^{5}}, -0.5625, \mathsf{fma}\left(\frac{{a}^{3} \cdot {c}^{4}}{{b}^{7}}, -1.0546875, \mathsf{fma}\left(\frac{c \cdot \left(a \cdot c\right)}{{b}^{3}}, -0.375, -0.5 \cdot \frac{c}{b}\right)\right)\right) \]

Reproduce

herbie shell --seed 2022125 
(FPCore (a b c)
  :name "Cubic critical, medium range"
  :precision binary64
  :pre (and (and (and (< 1.1102230246251565e-16 a) (< a 9007199254740992.0)) (and (< 1.1102230246251565e-16 b) (< b 9007199254740992.0))) (and (< 1.1102230246251565e-16 c) (< c 9007199254740992.0)))
  (/ (+ (- b) (sqrt (- (* b b) (* (* 3.0 a) c)))) (* 3.0 a)))