BESSELI function
Returns the modified Bessel function, which is equivalent to the Bessel function evaluated for purely imaginary arguments.
Syntax
BESSELI(x,n)
X is the value at which to evaluate the function.
N is the order of the Bessel function. If n is not an integer, it is truncated.
Remarks

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

If n is nonnumeric, BESSELI returns the #VALUE! error value.

If n < 0, BESSELI returns the #NUM! error value.

The nth order modified Bessel function of the variable x is:
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.

