public static double atan (double a)

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, 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.

`a` | the value whose arc tangent is to be returned. |

Diagram: Math