/sys$common/syshlp/helplib.hlb CC, Run-time functions, atan  *Conan The Librarian (sorry for the slow response - running on an old VAX)

  Returns a value in the range -pi/2 to pi/2, which is the arc
  tangent of its radian argument.

  Syntax:

       #include <math.h>

       double atan(double x);