This article describes the formula syntax and usage of the BESSELK function (function: A prewritten formula that takes a value or values, performs an operation, and returns a value or values. Use functions to simplify and shorten formulas on a worksheet, especially those that perform lengthy or complex calculations.) in Microsoft Excel.
Description
Returns the modified Bessel function, which is equivalent to the Bessel functions evaluated for purely imaginary arguments.
Syntax
BESSELK(X, N)
The BESSELK function syntax has the following arguments (argument: A value that provides information to an action, an event, a method, a property, a function, or a procedure.):
 X Required. The value at which to evaluate the function.
 N Required. The order of the function. If n is not an integer, it is truncated.
Remarks
 If x is nonnumeric, BESSELK returns the #VALUE! error value.
 If n is nonnumeric, BESSELK returns the #VALUE! error value.
 If n < 0, BESSELK returns the #NUM! error value.
 The nth order modified Bessel function of the variable x is:
where Jn and Yn are the J and Y Bessel functions, respectively.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?
 Select the example in this article. If you are copying the example in Excel Online, copy and paste one cell at a time.
Important: Do not select the row or column headers.
Selecting an example from Help
 Press CTRL+C.
 Create a blank workbook or worksheet.
 In the worksheet, select cell A1, and press CTRL+V. If you are working in Excel Online, repeat copying and pasting for each cell in the example.
Important: For the example to work properly, you must paste it into cell A1 of the worksheet.
 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.
After you copy the example to a blank worksheet, you can adapt it to suit your needs.

A 
B 
Formula 
Description (Result) 
=BESSELK(1.5, 1) 
Modified Bessel function at 1.5 with an order of 1 (0.277388) 
