Returns the double factorial of a number.
If this function is not available, and returns the #NAME? error, install and load the Analysis ToolPak addin.
How?
 On the Tools menu, click AddIns.
 In the AddIns available list, select the Analysis ToolPak box, and then click OK.
 If necessary, follow the instructions in the setup program.
Syntax
FACTDOUBLE(number)
Number is the value for which to return the double factorial. If number is not an integer, it is truncated.
Remarks
 If number is nonnumeric, FACTDOUBLE returns the #VALUE! error value.
 If number is negative, FACTDOUBLE returns the #NUM! error value.
 If number is even:
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.

A 
B 
Formula 
Description (Result) 
=FACTDOUBLE(6) 
Double factorial of 6 (48) 
=FACTDOUBLE(7) 
Double factorial of 7 (105) 
