Average Error: 59.9 → 0.3
Time: 18.0s
Precision: 64
\[-0.0259999999999999988065102485279567190446 \lt x \land x \lt 0.0259999999999999988065102485279567190446\]
\[\frac{1}{x} - \frac{1}{\tan x}\]
↓
\[\mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, x \cdot \mathsf{fma}\left(0.02222222222222222307030925492199457949027, x \cdot x, 0.3333333333333333148296162562473909929395\right)\right)\]
\frac{1}{x} - \frac{1}{\tan x}
↓
\mathsf{fma}\left(0.002116402116402116544841005563171165704262, {x}^{5}, x \cdot \mathsf{fma}\left(0.02222222222222222307030925492199457949027, x \cdot x, 0.3333333333333333148296162562473909929395\right)\right)
double f(double x) {
        double r2974530 = 1.0;
        double r2974531 = x;
        double r2974532 = r2974530 / r2974531;
        double r2974533 = tan(r2974531);
        double r2974534 = r2974530 / r2974533;
        double r2974535 = r2974532 - r2974534;
        return r2974535;
}
↓
double f(double x) {
        double r2974536 = 0.0021164021164021165;
        double r2974537 = x;
        double r2974538 = 5.0;
        double r2974539 = pow(r2974537, r2974538);
        double r2974540 = 0.022222222222222223;
        double r2974541 = r2974537 * r2974537;
        double r2974542 = 0.3333333333333333;
        double r2974543 = fma(r2974540, r2974541, r2974542);
        double r2974544 = r2974537 * r2974543;
        double r2974545 = fma(r2974536, r2974539, r2974544);
        return r2974545;
}

Error

Bits error versus x

Target

Original59.9
Target0.1
Herbie0.3
\[\begin{array}{l} \mathbf{if}\;\left|x\right| \lt 0.0259999999999999988065102485279567190446:\\ \;\;\;\;\frac{x}{3} \cdot \left(1 + \frac{x \cdot x}{15}\right)\\ \mathbf{else}:\\ \;\;\;\;\frac{1}{x} - \frac{1}{\tan x}\\ \end{array}\]

Derivation

  1. Initial program 59.9

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

  2. Taylor expanded around 0 0.3

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

  3. Simplified0.3

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

  4. Final simplification0.3

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

Reproduce

herbie shell --seed 2019172 +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))))