acos (arc cosine function)
The acos function computes the arc cosine of its argument; that is the value whose cosine is the given argument. The result is in radians and lies between 0 and PI (the range is approximate due to rounding errors).
Null is returned if the argument is not between -1 and 1.
q)acos -0.4 1.982313
For a complete list of functions, see the kdb+ Function Reference.