Average Error: 59.8 → 0.4
Time: 29.0s
Precision: 64
\[-0.0259999999999999988065102485279567190446 \lt x \land x \lt 0.0259999999999999988065102485279567190446\]
\[\frac{1}{x} - \frac{1}{\tan x}\]
↓
\[\mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, x \cdot \mathsf{fma}\left(x \cdot x, 0.02222222222222222307030925492199457949027, 0.3333333333333333148296162562473909929395\right)\right)\]
\frac{1}{x} - \frac{1}{\tan x}
↓
\mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, x \cdot \mathsf{fma}\left(x \cdot x, 0.02222222222222222307030925492199457949027, 0.3333333333333333148296162562473909929395\right)\right)
double f(double x) {
        double r101474 = 1.0;
        double r101475 = x;
        double r101476 = r101474 / r101475;
        double r101477 = tan(r101475);
        double r101478 = r101474 / r101477;
        double r101479 = r101476 - r101478;
        return r101479;
}
↓
double f(double x) {
        double r101480 = 0.0021164021164021165;
        double r101481 = x;
        double r101482 = 5.0;
        double r101483 = pow(r101481, r101482);
        double r101484 = r101481 * r101481;
        double r101485 = 0.022222222222222223;
        double r101486 = 0.3333333333333333;
        double r101487 = fma(r101484, r101485, r101486);
        double r101488 = r101481 * r101487;
        double r101489 = fma(r101480, r101483, r101488);
        return r101489;
}

Error

Bits error versus x

Target

Original59.8
Target0.1
Herbie0.4
\[\begin{array}{l} \mathbf{if}\;\left|x\right| \lt 0.0259999999999999988065102485279567190446:\\ \;\;\;\;\frac{x}{3} \cdot \left(1 + \frac{x \cdot x}{15}\right)\\ \mathbf{else}:\\ \;\;\;\;\frac{1}{x} - \frac{1}{\tan x}\\ \end{array}\]

Derivation

  1. Initial program 59.8

    \[\frac{1}{x} - \frac{1}{\tan x}\]

  2. Taylor expanded around 0 0.4

    \[\leadsto \color{blue}{0.3333333333333333148296162562473909929395 \cdot x + \left(0.02222222222222222307030925492199457949027 \cdot {x}^{3} + 0.002116402116402116544841005563171165704262 \cdot {x}^{5}\right)}\]

  3. Simplified0.4

    \[\leadsto \color{blue}{\mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, \mathsf{fma}\left(0.02222222222222222307030925492199457949027, {x}^{3}, 0.3333333333333333148296162562473909929395 \cdot x\right)\right)}\]

  4. Taylor expanded around 0 0.4

    \[\leadsto \mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, \color{blue}{0.3333333333333333148296162562473909929395 \cdot x + 0.02222222222222222307030925492199457949027 \cdot {x}^{3}}\right)\]

  5. Simplified0.4

    \[\leadsto \mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, \color{blue}{x \cdot \mathsf{fma}\left(x \cdot x, 0.02222222222222222307030925492199457949027, 0.3333333333333333148296162562473909929395\right)}\right)\]

  6. Final simplification0.4

    \[\leadsto \mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, x \cdot \mathsf{fma}\left(x \cdot x, 0.02222222222222222307030925492199457949027, 0.3333333333333333148296162562473909929395\right)\right)\]

Reproduce

herbie shell --seed 2019174 +o rules:numerics
(FPCore (x)
  :name "invcot (example 3.9)"
  :pre (and (< -0.026 x) (< x 0.026))

  :herbie-target
  (if (< (fabs x) 0.026) (* (/ x 3.0) (+ 1.0 (/ (* x x) 15.0))) (- (/ 1.0 x) (/ 1.0 (tan x))))

  (- (/ 1.0 x) (/ 1.0 (tan x))))