<atan> computes the arc tangent function.

<atan> takes two arguments: x and y. The y argument defaults to 1. If the x argument is missing, an error occurs.

<atan> returns the principal value of the arc tangent of its argument(s) in radians. If either argument is indeterminate, the result is also indeterminate.

If two arguments are given, <atan> returns the arc tangent of y/x, using the signs of the two arguments to determine the quadrant of the result which will be in the range [-π, π]. If both arguments are 0, the result will also be 0.

If only one argument is given, <atan> returns the arc tangent of its argument. The result will be in the range [-π/2, π/2].