Using Bessele in Maple returns zero

Question

I am so new in maple . I want to execute this this function on my formula as you can see in the below image :

As you can see every operations return zero .why ?

I am so new in maple

Answer 1

Which version? Here are the inputs/outputs for Maple V or Maple 7, they should work with yours too:

diff(BesselI(a, z*r),r);
> (BesselI(a+1,z*r)+a/z/r*BesselI(a,z*r))*z