atan

Inverse circular tangent (arctangent).

SYNOPSIS:

double x, y, atan();
y = atan(x);

DESCRIPTION:

Returns the radian angle between -pi/2 and +pi/2 whose tangent is x.
Range reduction is from three intervals into the interval from zero to 0.66. The approximant uses a rational function of degree 4/5 of the form x + x**3 P(x)/Q(x).

ACCURACY:
                      Relative error:
arithmetic   domain     # trials      peak         rms
    DEC       -10, 10     50000       2.4e-17     8.3e-18
    IEEE      -10, 10      10^6       1.8e-16     5.0e-17