atan (arc tangent function)
The atan function computes the arc tangent of its argument; that is the value whose tangent is the given argument. The result is in radians and lies between -PI/2 and PI/2 (the range is approximate due to rounding errors).
q)atan 0.5 0.4636476 q)atan 42 1.546991
For a complete list of functions, see the kdb+ Function Reference.