AT&T 6300 Programmer's Manual page 174

ATN
Function
Syntax
Remarks
Example
7-18
Returns the arctangent of the argument.
ATN (numexp)
The evaluation of ATN is performed in single
precision, unless you specify /D, (double
precision), when you invoke GWBASIC.
The result is expressed in radians and falls in
the range -PI/2 to PI/2 (where PI =3.141593).
10lNPUTX
20 PRINT ATN[X]
RUN
?3
1.249046
Ok