Bessel function
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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:
 <math>x^2 \frac{d^2 y}{dx^2} + x \frac{dy}{dx} + (x^2  \alpha^2)y = 0<math>
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).
Contents 
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 secondorder 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 and second kind
These are perhaps the most commonly used forms of the Bessel functions.
 Bessel functions of the first kind, J_{α}(x), are solutions of Bessel's differential equation which are finite at x = 0 for α an integer or α nonnegative. 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 noninteger α):
 <math> J_\alpha(x) = \sum_{m=0}^\infty \frac{(1)^m}{m! \Gamma(m+\alpha+1)} {\left({\frac{x}{2}}\right)}^{2m+\alpha}. <math>
 Bessel functions of the second kind, Y_{α}(x), are solutions which 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:
 <math>Y_\alpha(x) = \frac{J_\alpha(x) \cos(\alpha\pi)  J_{\alpha}(x)}{\sin(\alpha\pi)},<math>
where the case of integer α is handled by taking the limit.
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.
For integer order n, J_{n} and J_{n} are not linearly independent:
 <math>J_{n}(x) = (1)^n J_n(x)<math>
 <math>Y_{n}(x) = (1)^n Y_n(x)<math>
in which case Y_{n} is needed to provide the second linearly independent solution of Bessel's equation. In contrast, for noninteger order, J_{α} and J_{α} are linearly independent, and Y_{α} is redundant (as is clear from its definition above).
The graphs of Bessel functions look roughly like oscillating sine or cosine functions that decay proportional to 1/√x (see also their asymptotic forms, below), although their roots are not generally periodic except asymptotically for large x.
Relation to hypergeometric series
The Bessel functions can be expressed in terms of the hypergeometric series as
 <math>J_\nu(z)=\frac{(z/2)^\nu}{\Gamma(\nu+1)} \;_0F_1 (\nu+1; z^2/4).<math>
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:
 <math>H_\alpha^{(1)}(x) = J_\alpha(x) + i Y_\alpha(x)<math>
 <math>H_\alpha^{(2)}(x) = J_\alpha(x)  i Y_\alpha(x)<math>
where i is the imaginary unit. These linear combinations are also known as Bessel functions of the third kind. (The Hankel functions express inward and outwardpropagating cylindrical wave solutions of the cylindrical wave equation.) They are named for Hermann Hankel.
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:
 <math>I_\alpha(x) = i^{\alpha} J_\alpha(ix)<math>
 <math>K_\alpha(x) = \frac{\pi}{2} i^{\alpha+1} H_\alpha^{(1)}(ix).<math>
These are chosen to be realvalued for real arguments x. They are the two linearly independent solutions to the modified Bessel's equation:
 <math>x^2 \frac{d^2 y}{dx^2} + x \frac{dy}{dx}  (x^2 + \alpha^2)y = 0.<math>
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.
ModifiedBessel.png
Plot of some modified Bessel functions
Plot of six modified Bessel functions. In solid line K_{0}, K_{1}, and K_{2}. In dashed line : I_{0}, I_{1}, and I_{2}.
Spherical Bessel functions
When solving for separable solutions of Laplace's equation in spherical coordinates, the radial equation has the form:
 <math>x^2 \frac{d^2 y}{dx^2} + 2x \frac{dy}{dx} + [x^2  n(n+1)]y = 0.<math>
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_{α} and Y_{α} by:
 <math>j_n(x) = \sqrt{\frac{\pi}{2x}} J_{n+1/2}(x),<math>
 <math>y_n(x) = \sqrt{\frac{\pi}{2x}} Y_{n+1/2}(x) = (1)^{n+1} \sqrt{\frac{\pi}{2x}} J_{n1/2}(x).<math>
The spherical Bessel functions can also be written as:
 <math>j_n(x) = (x)^n \left(\frac{1}{x}\frac{d}{dx}\right)^n\,\frac{\sin x}{x} ,<math>
 <math>y_n(x) = (x)^n \left(\frac{1}{x}\frac{d}{dx}\right)^n\,\frac{\cos x}{x}.<math>
The first spherical Bessel function <math>j_0(x)<math> is also known as the sinc function. The first few spherical Bessel functions are:
 <math>j_0(x)=\frac{\sin x} {x}<math>
 <math>j_1(x)=\frac{\sin x} {x^2} \frac{\cos x} {x}<math>
 <math>j_2(x)=\left(\frac{3}{x^2}\frac{1}{x}\right) \sin x  \frac{3} {x^2} \cos x<math>
and
 <math>y_0(x)=j_{1}(x)=\,\frac{\cos x} {x}<math>
 <math>y_1(x)=j_{2}(x)=\,\frac{\cos x} {x^2} \frac{\sin x} {x}<math>
 <math>y_2(x)=j_{3}(x)=\left(\,\frac{3}{x^2}+\frac{1}{x}\right) \cos x  \frac{3} {x^2} \sin x.<math>
There are also spherical analogues of the Hankel functions:
 <math>h_n^{(1)}(x) = j_n(x) + i y_n(x)<math>
 <math>h_n^{(2)}(x) = j_n(x)  i y_n(x).<math>
In fact, there are simple closedform expressions for the Bessel functions of halfinteger order in terms of the standard trigonometric functions, and therefore for the spherical Bessel functions. In particular, for nonnegative integers n:
 <math>h_n^{(1)}(x) = (i)^{n+1} \frac{e^{ix}}{x} \sum_{m=0}^n \frac{i^m}{m!(2x)^m} \frac{(n+m)!!}{(nm)!!}<math>
and h_{n}^{(2)} is the complexconjugate 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.
Asymptotic forms
The Bessel functions have the following asymptotic forms. For small arguments 0 < x << 1, one obtains:
 <math>J_\alpha(x) \rightarrow \frac{1}{\Gamma(\alpha+1)} \left( \frac{x}{2} \right) ^\alpha <math>
 <math>Y_\alpha(x) \rightarrow \left\{ \begin{matrix}
\frac{2}{\pi} \ln (x/2) & \mbox{if } \alpha=0 \\ \\ \frac{\Gamma(\alpha)}{\pi} \left( \frac{2}{x} \right) ^\alpha & \mbox{if } \alpha > 0
\end{matrix} \right.<math>
where α is nonnegative and Γ denotes the gamma function. For large arguments x >> 1, they become:
 <math>J_\alpha(x) \rightarrow \sqrt{\frac{2}{\pi x}}
\cos \left( x\frac{\alpha\pi}{2}  \frac{\pi}{4} \right)<math>
 <math>Y_\alpha(x) \rightarrow \sqrt{\frac{2}{\pi x}}
\sin \left( x\frac{\alpha\pi}{2}  \frac{\pi}{4} \right).<math>
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:
 <math>I_\alpha(x) \rightarrow \frac{1}{\sqrt{2\pi x}} e^x,<math>
 <math>K_\alpha(x) \rightarrow \sqrt{\frac{\pi}{2x}} e^{x}.<math>
Properties
For integer order α = n, J_{n} is often defined via a Laurent series for a generating function:
 <math>e^{(x/2)(t1/t)} = \sum_{n=\infty}^\infty J_n(x) t^n,<math>
an approach used by P. A. Hansen in 1843. (This can be generalized to noninteger order by contour integration or other methods.) Another important relation for integer orders is the JacobiAnger identity:
 <math>e^{iz \cos \phi} = \sum_{n=\infty}^\infty i^n J_n(z) e^{in\phi},<math>
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:
 <math>Z_{\alpha1}(x) + Z_{\alpha+1}(x) = \frac{2\alpha}{x} Z_\alpha(x)<math>
 <math>Z_{\alpha1}(x)  Z_{\alpha+1}(x) = 2\frac{dZ_\alpha}{dx}<math>
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 (selfadjoint) if it is divided by x, the solutions must satisfy an orthogonality relationship for appropriate boundary conditions. In particular, it follows that:
 <math>\int_0^1 x J_\alpha(x u_{\alpha,m}) J_\alpha(x u_{\alpha,n}) dx = \frac{\delta_{m,n}}{2} J_{\alpha+1}(u_{\alpha,m})^2,<math>
where α > 1, δ_{m,n} is the Kronecker delta, and u_{α,m} is the mth zero of J_{α}(x). This orthogonality relation can then be used to extract the coefficients in the FourierBessel 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:
 <math>\int_0^\infty x J_\alpha(ux) J_\alpha(vx) dx = \frac{1}{u} \delta(u  v)<math>
for α > 1/2 and where δ is the Dirac delta function.
Another important property of Bessel's equations, which follows from Abel's identity, involves the Wronskian of the solutions:
 <math>A_\alpha(x) \frac{dB_\alpha}{dx}  \frac{dA_\alpha}{dx} B_\alpha(x) = \frac{C_\alpha}{x},<math>
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 (http://www.math.sfu.ca/~cbm/aands/page_355.htm) Bessel Functions of integer order
 Section 9.1 (http://www.math.sfu.ca/~cbm/aands/page_358.htm) J, Y (Weber) and H (Hankel)
 Section 9.6 (http://www.math.sfu.ca/~cbm/aands/page_374.htm) Modified (I and K)
 Section 9.9 (http://www.math.sfu.ca/~cbm/aands/page_379.htm) Kelvin functions
 Chapter 10 (http://www.math.sfu.ca/~cbm/aands/page_435.htm) Bessel Functions of fractional order
 Section 10.1 (http://www.math.sfu.ca/~cbm/aands/page_437.htm) Spherical Bessel Functions (j, y and h)
 Section 10.2 (http://www.math.sfu.ca/~cbm/aands/page_443.htm) Modified Spherical Bessel functions (I and K)
 Section 10.3 (http://www.math.sfu.ca/~cbm/aands/page_445.htm) RiccatiBessel Functions
 Section 10.4 (http://www.math.sfu.ca/~cbm/aands/page_446.htm) Airy functions
 Chapter 9 (http://www.math.sfu.ca/~cbm/aands/page_355.htm) 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).de:Besselsche Differentialgleichung
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