what is the definition of bessel function

Hi
I want use bessel function for fitting so I need to difine this function in new fit function but I couldn't find equation of bessel in Igor. For example it explain BessJ but how can I use it as a equation?
Can somebody please help me?
Best regards
You can just write the command of the Bessel function inside the Fit Expression dialog while passing the independent variable as the argument.
(Assuming you defined x as the independent variable)
f(x)= Besselj(2,x)


will fit the 2nd order Bessel function to your data.
I hope this was the information you were looking for.