BESSELI
BESSELI(j, x) returns the value of jth-order Modified Bessel function of the first kind for argument value x where x>0. If j=0, this function automatically calls the BESSEL_I0 function.
Note that the BESSEL_I0 and BESSEL_I1 functions provide values for the zeroth and first order integer Bessel functions and allow values of negative x as low as -3.75.