update: adds a ATAN page to documentation

docs/src/functions/math_and_trigonometry/atan.md

@@ -4,8 +4,34 @@ outline: deep
 lang: en-US
 ---
 
-# ATAN
+# ATAN function
+## Overview
+ATAN is a function of the Math and Trigonometry category that calculates the inverse tangent (arctangent) of a number, returning an angle in the range [-$\pi$/2 to +$\pi$/2], expressed in radians.
+## Usage
+### Syntax
+**ATAN (<span title="Number" style="color:#1E88E5">number</span>) => <span title="Number" style="color:#1E88E5">atan</span>**
+### Argument descriptions
+* *number* ([number](/features/value-types#numbers), required). The number whose arctangent is to be calculated, in the range [-$\infty$, +$\infty$]. 
+### Additional guidance
+None.
+### Returned value
+ATAN returns a number in radians in the range [-$\pi$/2 to +$\pi$/2] that is the angle whose tangent is the specified number.
+### Error conditions
+* In common with many other IronCalc functions, ATAN propagates errors that are found in its argument.
+* If no argument, or more than one argument, is supplied, then ATAN returns the [`#ERROR!`](/features/error-types.md#error) error.
+* If the value of the *number* argument is not (or cannot be converted to) a [number](/features/value-types#numbers), then ATAN returns the [`#VALUE!`](/features/error-types.md#value) error.
+* For some argument values, ATAN may return a [`#DIV/0!`](/features/error-types.md#div-0) error.
+<!--@include: ../markdown-snippets/error-type-details.txt-->
+## Details
+* The ATAN function utilizes the *atan()* method provided by the [Rust Standard Library](https://doc.rust-lang.org/std/).
+* The figure below illustrates the output of the ATAN function for angles $x$ in the range [-$\infty$, +$\infty$].
+<center><img src="/functions/images/arctangent-curve.png" width="350" alt="Graph showing atan(x) for x between [-$\infty$, +$\infty$]."></center>
 
-::: warning
-🚧 This function is implemented but currently lacks detailed documentation. For guidance, you may refer to the equivalent functionality in [Microsoft Excel documentation](https://support.microsoft.com/en-us/office/excel-functions-by-category-5f91f4e9-7b42-46d2-9bd1-63f26a86c0eb).
-:::
+## Examples
+[See some examples in IronCalc](https://app.ironcalc.com/?example=acos).
+
+## Links
+* For more information about inverse trigonometric functions, visit Wikipedia's [Inverse trigonometric functions](https://en.wikipedia.org/wiki/Inverse_trigonometric_functions) page.
+* See also IronCalc's [SIN](/functions/math_and_trigonometry/sin), [COS](/functions/math_and_trigonometry/cos) and [TAN](/functions/math_and_trigonometry/tan) functions.
+* Visit Microsoft Excel's [ATAN function](https://support.microsoft.com/en-us/office/atan-function-50746fa8-630a-406b-81d0-4a2aed395543) page.
+* Both [Google Sheets](https://support.google.com/docs/answer/3093395) and [LibreOffice Calc](https://wiki.documentfoundation.org/Documentation/Calc_Functions/ATAN) provide versions of the ATAN function.