# Bessel function

In mathematics, **Bessel functions**, first defined by the Swiss mathematician Daniel Bernoulli and named after Friedrich Bessel, are canonical solutions *y*(*x*) of Bessel's differential equation:

for an arbitrary real number α (the *order*). The most common and important special case is where α is an integer, *n*.

Although α and −α produce the same differential equation, it is conventional to define different Bessel functions for these two orders (e.g., so that the Bessel functions are mostly smooth functions of α).

## Applications

Bessel's equation arises when finding separable solutions to Laplace's equation and the Helmholtz equation in cylindrical or spherical coordinates, and Bessel functions are therefore especially important for many problems of wave propagation, static potentials, and so on. (For cylindrical problems, one obtains Bessel functions of integer order α = *n*; for spherical problems, one obtains half integer orders α = *n*+½.) For example:

- electromagnetic waves in a cylindrical waveguide
- heat conduction in a cylindrical object.
- modes of vibration of a thin circular (or annular) membrane.

Bessel functions also have useful properties for other problems, such as signal processing (e.g., see FM synthesis or Kaiser window).

## Definitions

Since this is a second-order differential equation, there must be two linearly independent solutions. Depending upon the circumstances, however, various formulations of these solutions are convenient, and the different variations are described below.

### Bessel functions of the first kind

Bessel functions of the first kind, denoted with *J*_{α}(*x*), are solutions of Bessel's differential equation which are finite at *x* = 0 for α an integer or α non-negative. The specific choice and normalization of *J*_{α} are defined by its properties below; another possibility is to define it by its Taylor series expansion around *x* = 0 (or a more general power series for non-integer α):

The graphs of Bessel functions look roughly like oscillating sine or cosine functions that decay proportionally to 1/√*x* (see also their asymptotic forms, below), although their roots are not generally periodic except asymptotically for large *x*.

Here is the plot of for :

If α is not an integer, the functions and are linearly independent and are therefore the two solutions of the differential equation. On the other hand, if the order is an integer, then the following relationship is valid:

This means that they are no longer linearly independent. The second linearly independent solution is then found to be the Bessel function of the second kind, as discussed below.

#### Bessel's integrals

Another definition of the Bessel function is possible using an integral equation:

This is the approach that Bessel used, and from this definition he derived several properties of the function. Another integral representation is:

#### Relation to hypergeometric series

The Bessel functions can be expressed in terms of the hypergeometric series as

### Bessel functions of the second kind

These are perhaps the most commonly used forms of the Bessel functions.

The Bessel functions of the second kind, denoted by *Y*_{α}(*x*), are solutions of the Bessel differential equation. They are singular (infinite) at *x* = 0.

*Y*_{α}(*x*) is sometimes also called the **Neumann function**, and is occasionally denoted instead by *N*_{α}(*x*). It is related to *J*_{α}(*x*) by:

where the case of integer α is handled by taking the limit.

When α is not an integer, the definition of *Y*_{α} is redundant (as is clear from its definition above). On the other hand, when α is an integer, *Y*_{α} is the second linearly independent solution of Bessel's equation; moreover, as was similarly the case for the functions of the first kind, the following relationship is valid:

Both *J*_{α}(*x*) and *Y*_{α}(*x*) are holomorphic functions of *x* on the complex plane cut along the negative real axis. When α is an integer, there is no branch point, and the Bessel functions are entire functions of *x*. If *x* is held fixed, then the Bessel functions are entire functions of α.

Here is the plot of for :

### Hankel functions

Another important formulation of the two linearly independent solutions to Bessel's equation are the **Hankel functions** *H*_{α}^{(1)}(*x*) and *H*_{α}^{(2)}(*x*), defined by:

where *i* is the imaginary unit. These linear combinations are also known as Bessel functions of the third kind; they are two linearly independent solutions of Bessel's differential equation. The Hankel functions express inward- and outward-propagating cylindrical wave solutions of the cylindrical wave equation. They are named for Hermann Hankel.

Using the previous relationships they can be expressed as:

if α is an integer, the limit has to be calculated. The following relationships are valid, whether α is an integer or not:

### Modified Bessel functions

The Bessel functions are valid even for complex arguments *x*, and an important special case is that of a purely imaginary argument. In this case, the solutions to the Bessel equation are called the **modified Bessel functions** of the first and second kind, and are defined by:

These are chosen to be real-valued for real arguments *x*. They are the two linearly independent solutions to the modified Bessel's equation:

Unlike the ordinary Bessel functions, which are oscillating, *I*_{α} and *K*_{α} are exponentially growing and decaying functions, respectively. Like the ordinary Bessel function *J*_{α}, the function *I*_{α} goes to zero at *x*=0 for α > 0 and is finite at *x*=0 for α=0. Analogously, *K*_{α} diverges at *x*=0.

### Spherical Bessel functions

When solving for separable solutions of Laplace's equation in spherical coordinates, the radial equation has the form:

The two linearly independent solutions to this equation are called the **spherical Bessel functions** *j*_{n} and *y*_{n} (also denoted *n*_{n}), and are related to the ordinary Bessel functions *J*_{n} and *Y*_{n} by:

The spherical Bessel functions can also be written as:

The first spherical Bessel function is also known as the sinc function. The first few spherical Bessel functions are:

and

There are also spherical analogues of the Hankel functions:

In fact, there are simple closed-form expressions for the Bessel functions of half-integer order in terms of the standard trigonometric functions, and therefore for the spherical Bessel functions. In particular, for non-negative integers *n*:

and *h*_{n}^{(2)} is the complex-conjugate of this (for real *x*). (!! is the double factorial.) It follows, for example, that *j*_{0}(*x*) = sin(*x*)/*x* and *y*_{0}(*x*) = -cos(*x*)/*x*, and so on.

### Riccati-Bessel functions

Riccati-Bessel functions only slightly differ from spherical Bessel functions:

They satisfy the differential equation:

This differential equation, and the Riccati-Bessel solutions, arises in the problem of scattering of electromagnetic waves by a sphere, known as Mie scattering after the first published solution by Mie (1908). See e.g. Du (2004) for recent developments and references.

Following Debye (1909), the notation is sometimes used instead of .

## Asymptotic forms

The Bessel functions have the following asymptotic forms. For small arguments 0 < *x* << 1, one obtains:

where α is non-negative and Γ denotes the gamma function. For large arguments *x* >> 1, they become:

Asymptotic forms for the other types of Bessel function follow straightforwardly from the above relations. For example, for large *x* >> 1, the modified Bessel functions become:

## Properties

For integer order α = *n*, *J*_{n} is often defined via a Laurent series for a generating function:

an approach used by P. A. Hansen in 1843. (This can be generalized to non-integer order by contour integration or other methods.) Another important relation for integer orders is the **Jacobi-Anger identity**:

which is used to expand a plane wave as a sum of cylindrical waves, or to find the Fourier series of a tone modulated FM signal.

The functions *J*_{α}, *Y*_{α}, *H*_{α}^{(1)}, and *H*_{α}^{(2)} all satisfy the recurrence relations:

where *Z* denotes *J*, *Y*, *H*^{(1)}, or *H*^{(2)}. (These two identities are often combined, e.g. added or subtracted, to yield various other relations.) In this way, for example, one can compute Bessel functions of higher orders (or higher derivatives) given the values at lower orders (or lower derivatives).

Because Bessel's equation becomes Hermitian (self-adjoint) if it is divided by *x*, the solutions must satisfy an orthogonality relationship for appropriate boundary conditions. In particular, it follows that:

where α > -1, δ_{m,n} is the Kronecker delta, and *u*_{α,m} is the *m*-th zero of *J*_{α}(*x*). This orthogonality relation can then be used to extract the coefficients in the *Fourier-Bessel series*, where a function is expanded in the basis of the functions *J*_{α}(*x* *u*_{α,m}) for fixed α and varying *m*. (An analogous relationship for the spherical Bessel functions follows immediately.)

Another orthogonality relation is the *closure equation*:

for α > -1/2 and where δ is the Dirac delta function. For the spherical Bessel functions the orthogonality relation is:

for α > 0.

Another important property of Bessel's equations, which follows from Abel's identity, involves the Wronskian of the solutions:

where *A*_{α} and *B*_{α} are any two solutions of Bessel's equation, and *C*_{α} is a constant independent of *x* (which depends on α and on the particular Bessel functions considered). For example, if *A*_{α} = *J*_{α} and *B*_{α} = *Y*_{α}, then *C*_{α} is 2/π. This also holds for the modified Bessel functions; for example, if *A*_{α} = *I*_{α} and *B*_{α} = *K*_{α}, then *C*_{α} is -1.

(There are a large number of other known integrals and identities that are not reproduced here, but which can be found in the references.)

## References

- Milton Abramowitz and Irene A. Stegun, eds.,
*Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables*(Dover: New York, 1972)- Chapter 9 Bessel Functions of integer order
- Section 9.1 J, Y (Weber) and H (Hankel)
- Section 9.6 Modified (I and K)
- Section 9.9 Kelvin functions

- Chapter 10 Bessel Functions of fractional order
- Section 10.1 Spherical Bessel Functions (j, y and h)
- Section 10.2 Modified Spherical Bessel functions (I and K)
- Section 10.3 Riccati-Bessel Functions
- Section 10.4 Airy functions

- Chapter 9 Bessel Functions of integer order
- George B. Arfken and Hans J. Weber,
*Mathematical Methods for Physicists*(Harcourt: San Diego, 2001). - Frank Bowman,
*Introduction to Bessel Functions*(Dover: New York, 1958) ISBN 0486604624. - G. N. Watson,
*A Treatise on the Theory of Bessel Functions, Second Edition*(Cambridge University Press, 1966). - G. Mie, \u201cBeiträge zur Optik trüber Medien, speziell kolloidaler Metallösungen,\u201d
*Ann. Phys. Leipzig***25**, 377\u2013445 (1908). - Hong Du, "Mie-scattering calculation,"
*Applied Optics***43**(9), 1951-1956 (2004).

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