## XPath/XQuery atan function

### Summary

Returns the arc tangent of the argument.

### Signature

`math:atan`(
`\$arg` as `xs:double?`
) as `xs:double?`

### Properties

This function is deterministic, context-independent, and focus-independent.

### Rules

If `\$arg` is the empty sequence, the function returns the empty sequence.

Otherwise the result is the arc tangent of `\$arg`, as defined in the specification of the `atan` function applied to 64-bit binary floating point values. The result is in the range -π/2 to +π/2 radians.

### Examples

The expression `math:atan(())` returns `()`.

The expression `math:atan(0)` returns `0.0e0`.

The expression `math:atan(-0.0e0)` returns `-0.0e0`.

The expression `math:atan(1.0e0)` returns `0.7853981633974483e0`.

The expression `math:atan(-1.0e0)` returns `-0.7853981633974483e0`.

The expression `math:atan(xs:double('NaN'))` returns `xs:double('NaN')`.

The expression `math:atan(xs:double('INF'))` returns `1.5707963267948966e0`.

The expression `math:atan(xs:double('-INF'))` returns `-1.5707963267948966e0`.

### Notes

The treatment of the `underflow` exception is defined in .

If `\$arg` is positive or negative zero, the result is `\$arg`.

If `\$arg` is `NaN` then the result is `NaN`.

In other cases the result is an `xs:double` value representing an angle θ in radians in the range -π ```/2 <= ``` θ ` <= +` π `/2`.