I addeed besselas in C and Julia.  This function (in julia) returns an array where the first entry is I1 / I0 and the second is I2/ I1.  I'll be using this in the second derivative for ricians.  This is now necessay because, previously, I avoided computing any I2 by turning it into a term proportional to (I1 / v).  With multi component models, I'm unable to cancel out this v, which could make things numerically unstable.