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:
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). 
The above chart on the right shows the order 1 modified Bessel function, K_{1}(x).
If you want to calculate the value of this function at x = 0.05, this can be done using the Excel Besselk function as follows:
which gives the result 19.90967433.
See the Microsoft Office website for further information on the Excel Besselk function.
If you get an error from the Excel Besselk function this is likely to be one of the following:
#NUM!   
Occurs if either:

#VALUE!    Occurs if any of the supplied arguments are nonnumeric. 