atan - arc tangent function
#include <math.h> double atan(double x);
The atan() function calculates the arc tangent of x; that is the value whose tangent is x.
The atan() function returns the arc tangent in radians and the value is mathematically defined to be between -PI/2 and PI/2 (inclusive).
SVID 3, POSIX, BSD 4.3, ISO 9899
acos(3), asin(3), atan2(3),