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The Excel BESSELK Function
Basic Description
The Excel Besselk function calculates the modified Bessel functions, K_{n}(x), which are also known as the hyperbolic Bessel Functions. These are the equivalent of the Bessel functions, evaluated for purely imaginary arguments.
The syntax of the function is :
BESSELK( x, n )
where the function arguments are:
x    The value at which the function is to be evaluated 
n    A positive integer, denoting the order of the modified Bessel function (If the supplied value of n is a decimal, it is truncated to an integer by Excel) 
Besselk Function Example
Order 1 Modified Bessel Function, K
_{n}(x)
The above chart on the right shows the modified Bessel function, K_{n}(x), of order 1.
If you want to calculate the value of this function at x = 0.05, this can be done using the following formula:
=BESSELK( 0.05, 1 )
which gives the result 19.90967433.
Further examples of the Excel Besselk function can be found on the Microsoft Office website.
Besselk Function Errors
If you get an error from the Excel Besselk function this is likely to be one of the following :
Common Errors
#NUM!    Occurs if either:  the supplied value of x is ≤ 0
 the supplied value of n is < 0

#VALUE!    Occurs if any of the supplied arguments are nonnumeric 
#NAME?    Occurs when Analysis ToolPak addin is not enabled in your Excel. You will need to enable the addin if you want to use the Excel engineering functions.
+ Show How to Do This in Excel 2003:
 From the Tools dropdown menu, select the option AddIns ...
 An 'AddIns' window will pop up. From this, select the option Analysis ToolPak and click OK
+ Show How to Do This in Excel 2007:
 Click the Microsoft button on the top left of your spreadsheet and select
the Excel Options button
 From the menu on the left hand side, select AddIns
 In the 'Manage:' box, select Excel Addins and click Go...
 An 'AddIns' window will pop up. From this, select the option Analysis ToolPak and click OK
+ Show How to Do This in Excel 2010 or Excel 2013:
 Click the File tab (top left of your spreadsheet) and select Options
 From the menu on the left hand side, select AddIns
 If the window that pops up doesn't show the 'Addins' list, use the 'Manage:'
dropdown menu (at the bottom of the window) to select Excel Addins. Click Go...
 From the 'AddIns' window, select the option Analysis ToolPak and click OK
