GAMMALN function
Returns the natural logarithm of the gamma function, Γ(x).
Syntax
GAMMALN(x)
X is the value for which you want to calculate GAMMALN.
Remarks

If x is nonnumeric, GAMMALN returns the #VALUE! error value.

If x ≤ 0, GAMMALN returns the #NUM! error value.

The number e raised to the GAMMALN(i) power, where i is an integer, returns the same result as (i  1)!.

GAMMALN is calculated as follows:
where:
Example
The example may be easier to understand if you copy it to a blank worksheet.
How to copy an example

Create a blank workbook or worksheet.

Select the example in the Help topic.
Note Do not select the row or column headers.
Selecting an example from Help

Press CTRL+C.

In the worksheet, select cell A1, and press CTRL+V.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.

