The ATAN function returns a numeric value in radians that approximates the arctangent of argument-1. The type of this function
is numeric.

- Argument-1 must be class numeric.

- The returned value is the approximation of the arctangent of argument-1 and is greater than -pi/2 and less than +pi/2.
Note: Conversion factors from radians to degrees and Gon (grad) are specified in the section Trigonometric Functions.
- Floating-point format is used for numeric non-integer results.