atan

Returns the arc tangent of a value; the returned angle is in the range -pi/2 through pi/2.

Special cases:

If the argument is NaN or its absolute value is greater than 1, then the result is NaN.
If the argument is zero, then the result is a zero with the same sign as the argument.

The computed result must be within 1 ulp of the exact result. Results must be semi-monotonic.

Works with:

atan(var)