ATAN (Primitive Spread Function) returns the angle whose tangent is the specified value. The angle is in radians between –π/2 and π/2.

ATAN(value)

REMARKS:

The value argument is the tangent of the angle you want to return and must be between –1 and 1. If you want to convert the result to degrees, multiply it by 180/π. 

EXAMPLES:

ATAN(B3)
ATAN(1)=0.7853981634