Modified Bessel functions of the first kind

Use only in the MuPAD Notebook Interface.

This functionality does not run in MATLAB.


besselI(v, z)


besselI(v, z) represents the modified Bessel functions of the first kind:


The Bessel functions are defined for complex arguments v and z.

A floating-point value is returned if either of the arguments is a floating-point number and the other argument is numerical. For most exact arguments the Bessel functions return an unevaluated function call. Special values at index v = 0 and/or argument z = 0 are implemented. Explicit symbolic expressions are returned, when the index v is a half integer. See Example 2.

For nonnegative integer indices v some of the Bessel functions have a branch cut along the negative real axis. A jump occurs when crossing this cut. See Example 3.

Environment Interactions

When called with floating-point arguments, these functions are sensitive to the environment variable DIGITS which determines the numerical working precision.


Example 1

Unevaluated calls are returned for exact or symbolic arguments:

besselI(2, 1 + I), besselI(0, x), besselI(v, x)

Floating point values are returned for floating-point arguments:

besselI(2, 5.0), besselI(3.2 + I, 10000.0)

Example 2

Bessel functions can be expressed in terms of elementary functions if the index is an odd integer multiple of :

besselI(1/2, x), besselI(3/2, x)

besselI(7/2, x), besselI(-7/2, x)

Example 3

The negative real axis is a branch cut of the Bessel functions for non-integer indices v. A jump occurs when crossing this cut:

besselI(-3/4, -1.2),
besselI(-3/4, -1.2 + I/10^10),
besselI(-3/4, -1.2 - I/10^10)

Example 4

The functions diff, float, limit, and series handle expressions involving the Bessel functions:

diff(besselI(0, x), x, x), float(ln(3 + besselI(17, sqrt(PI))))

limit(1/besselI(2, x^2 + 1)*sqrt(x), x = infinity)

series(besselI(3, x)/x, x = infinity, 3)


v, z

arithmetical expressions

Return Values

Arithmetical expression.

Overloaded By



The modified Bessel functions Iv(z) and Kv(z) satisfy the modified Bessel equation:


When the index v is an integer, the modified Bessel functions of the first kind are governed by reflection formulas:


See Also

MuPAD Functions

Was this topic helpful?