Cylinder functions

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2020 Mathematics Subject Classification: Primary: 33C10 [MSN][ZBL]

Also generally called Bessel functions. Solutions $Z_\nu$ of the Bessel differential equation \begin{equation}\label{e:Bessel} z^2 \frac{d^2 Z}{dz^2} + z \frac{dZ}{dz} + (z^2-\nu^2) Z = 0 \end{equation} where $\nu$ is an arbitrary real or complex number. The cylinder functions are functions of one complex variable.

Cylinder functions of arbitrary order.

If $\nu$ is not an integer, then the general solution of equation \eqref{e:Bessel} has the form \[ c_1 J_\nu + c_{-1} J_{-\nu} \] where $J_{\pm \nu}$ are the so-called cylinder functions of the first kind or Bessel functions of the first kind. They have the expansion \begin{equation}\label{e:Bessel_f} J_\nu (z) = \left(\frac{z}{2}\right)^\nu \sum_{m=0}^\infty \frac{(-1)^m}{m! \Gamma (\nu +m +1)} \left(\frac{z}{2}\right)^{2m} \, , \end{equation} where $\Gamma$ is the Gamma-function (i.e. the standard extension of the factorial to a meromorphic function on $\mathbb C$) and $z^\nu$ is given by ${\rm exp}\, (\log z)$, with the convention of having chosen the determination of $\log z$ such that $-\pi < {\rm Im}\, (\log z) \leq \pi$. The formula \eqref{e:Bessel_f} still makes sense when $\nu$ is a nonnegative integer. For $n\in \mathbb N$ the function $J_{-n}$ is defined to be $(-1)^n J_n$. The Bessel functions of integer order are usually defined as the coefficients of the following Laurent series: \begin{equation}\label{e:Anger-Jacobi} e^{\frac{z}{2} \left(t - \frac{1}{t}\right)} = \sum_{n=-\infty}^\infty t^n J_n (z)\, . \end{equation} The series on the right-hand side of \eqref{e:Bessel_f} converges absolutely and uniformly on any compact subset of $\mathbb C$. The functions $J_\nu$ are holomorphic on $\mathbb C \setminus \{0\}$ (in fact they are entire functions when $\nu$ is an integer). $\infty$ is a singular point, in the sense of the theory of analytic functions.

If $\nu$ is an integer, then $J_{\pm \nu}$ are linearly dependent and the linear combinations of them no longer yield the general solutions of \eqref{e:Bessel}. Therefore, apart from cylinder functions of the first kind one introduces cylinder functions of the second kind, commonly called Bessel functions of the second kind (or Neumann functions, Weber functions, cf. Neumann functions): \begin{equation}\label{e:Bessel_f2} Y_\nu (z) = \frac{J_\nu (z) \cos \nu \pi - J_{-\nu} (z)}{\sin \nu \pi} \qquad \mbox{for}\; \nu\not\in \mathbb Z \end{equation} \[ Y_n (z) = \lim_{\nu\to n} Y_\nu (z) \qquad \mbox{for}\; n\in \mathbb Z\, \] Another, less common notation, for the Bessel functions of the second kind is $N_\nu$. By means of these functions the general solution of equation \eqref{e:Bessel} can be written in the form \[ Z_\nu = c_1 J_\nu + c_2 Y_\nu\, . \] For applications, a third class of solutions of \eqref{e:Bessel}, called cylinder functions of the third kind or Hankel functions are rather important They are denoted by $H^{(1)}_\nu$ and $H^{(2)}_\nu$ and defined as \begin{align} &H^{(1)}_\nu = J_\nu + i Y_\nu\\ &H^{(2)}_\nu = J_\nu - i Y_\nu\\ \end{align} In particular, when $\nu\not\in \mathbb Z$, we have the expressions \begin{align} &H^{(1)}_\nu (z) = \frac{J_{-\nu} (z) - e^{-\nu \pi i} J_\nu (z)}{i\sin \nu\pi}\\ &H^{(2)}_\nu (z) = \frac{J_{-\nu} (z) - e^{\nu \pi i} J_\nu (z)}{-i\sin \nu\pi i}\, , \end{align} whereas for integer values $n$ of $\nu$ analogous formulas hold if we replace the right hand sides with their limits as $\nu\to n$. When $\nu\in \mathbb R$, if we restrict the domain of the Hankel functions to the real axis, we get a pair of complex conjugate solutions of \eqref{e:Bessel}, of which the functions $J_\nu$ give the real part and the functions $Y_\nu$ give the imaginary part.

Each pair of functions $\{J_\nu, J_{-\nu}\}$, $\{Y_\nu, Y_{-\nu}\}$ and $\{H^{(1)}_\nu, H^{(2)}_\nu\}$ forms, when $\nu\not\in \mathbb Z$, a fundamental system of solutions of \eqref{e:Bessel}. The pairs $\{J_\nu, Y_\nu\}$ and $\{H^{(1)}_\nu, H^{(2)}_\nu\}$ form each a fundamental system of solutions for every value of $\nu$.

Some notable identities

We have the following important identities relating cylinder functions with their derivatives \begin{align} & \pi\, z \left( J_\nu (z) J'_{-\nu} (z) - J'_\nu (z) J_{-\nu} (z)\right) = - 2 \sin \nu\pi\\ & \pi\, z \left( J_\nu (z) Y'_{\nu} (z) - J'_\nu (z) Y_{\nu} (z)\right) = -2\\ & \pi\, z \left( H^{(1)}_\nu (z) (H^{(2)}_\nu)' (z)- (H^{(1)}_\nu)' (z) H^{(2)}_{\nu} (z) \right) = -4i \end{align} Similar formulas hold for expressions of the form $J_\nu Y''_\nu - J''_\nu Y_\nu$, $J'_\nu Y''_\nu - J''_\nu Y'_\nu$ and so on (see Section 3.63 of [Wa]).

Moreover, all cylinder functions $Z_\nu$ satisfy the recurrence formulas \begin{align} &z Z_{\nu-1} (z) + z Z_{\nu+1} (z) = 2\nu Z_\nu (z)\label{e:rec1}\\ & Z_{\nu-1} (z) - Z_{\nu+1} (z) = 2 Z'_\nu (z)\, . \end{align}

Modified cylinder functions

Modified cylinder functions are cylinder functions of an imaginary argument. For instance the function $I_\nu$ is defined as \[ I_\nu (z) = \left\{\begin{array}{l} &e^{-\frac{1}{2}\nu \pi i} J_\nu \left(z e^{\frac{1}{2}\pi i}\right) \qquad \mbox{if } -\pi < {\rm arg}\, z \leq \frac{\pi}{2}\\ &e^{\frac{3}{2}\nu \pi i} J_\nu \left(z e^{-\frac{3}{2}\pi i}\right) \qquad \mbox{if } \frac{\pi}{2} < {\rm arg}\, z \leq \pi \end{array}\right. \] and the Macdonald function $K_\nu$ as \[ K_\nu (z) = \frac{\pi i}{2} e^{\frac{1}{2}\nu \pi i} H^{(1)}_\nu (iz) = \frac{\pi i}{2} e^{-\frac{1}{2}\nu \pi i} H^{(1)}_{-\nu} (iz) \] (in particular observe that $K_{-\nu} = K_\nu$). Obviously these functions are solutions of the differential equation \[ z^2 \frac{d^2 Z}{dz^2} + z \frac{dZ}{dz} - (\nu^2 + z^2) Z = 0 \] and in fact $\{I_\nu, K_\nu\}$ forms a fundamental system of solutions (similar to what happens for the usual cylinder functions, the pair $\{I_\nu, I_{-\nu}\}$ forms a fundamental system only when $\nu$ is not an integer).

Moreover, the modified cylinder functions satisfy the recurrence relation \begin{align} &z (I_{\nu-1} (z) + I_{\nu+1} (z)) = 2 \nu I_\nu (z)\label{e:rec2}\\ &z (K_{\nu-1} (z) + K_{\nu+1} (z)) = 2 \nu K_\nu (z)\, . \end{align}

Cylinder functions of integral and half-integral orders.

If $\nu=n\in \mathbb Z$ the Bessel functions $J_n$ of the first kind satisfy \eqref{e:Anger-Jacobi}, which is called Anger-Jacobi formula and which can be usefully rewritten as the identity \[ e^{\pm i z\sin \theta} = J_0 (z) + 2 \sum_{j=1}^\infty \left( J_{2n} (z) \sin 2n \theta \pm J_{2n+1} (z) \sin\, (2n+1)\theta\right)\, . \] Moreover $J_{-n} = (-1)^n J_n$, whereas the series \eqref{e:Bessel_f} still makes sense for $n \geq 0$. In particular this shows that $J_n$ is an entire function of the argument $z$. $J_n$ is in fact [[Transcendental function|transcendental and when $z$ is a nonzero algebraic number, $J_n (z)$ is a transcendental number, and $J_n (z)\neq J_m (z)$ whenever $n\geq m$.

As already mentioned the pair $\{J_\nu, Y_\nu\}$ is a fundamental system of solution of Bessel's equation $z^2 Z'' (z) + z Z' (z) + (z^2-n^2) Z(z) =0$. The following expressions are valid for $Y_n$: \begin{align} Y_n (z) &= \lim_{\nu\to n} Y_\nu (z) = \lim_{\nu\to n} \frac{J_\nu (z) \cos \nu \pi - J_{-\nu} (z)}{\sin \nu\pi}\\ &=\frac{1}{\pi} \lim_{\nu\to n} \frac{J_\nu (z) - (-1)^n J_{-\nu} (z)}{\nu -n} =\frac{1}{\pi} \left[\left.\frac{\partial J_\nu}{\partial \nu}\right|_{\nu =n} (z) - (-1)^n \left.\frac{\partial J_{-\nu}}{\partial \nu}\right|_{\nu =n} (z)\right]\\ &= \frac{2}{\pi} \left(\gamma + \log \frac{z}{2}\right) J_n (z) - \frac{2^n}{\pi z^n} \sum_{m=0}^{n-1} \frac{(-1)^m}{m! (n+m)!} \left(\frac{z}{2}\right)^{n+2m} - \frac{z^n}{\pi 2^n} \sum_{m=0}^\infty \frac{(-1)^m}{m! (n+m)!} \left(\frac{z}{2}\right)^m \left(\sum_{l=1}^m \frac{1}{l} + \sum_{l=1}^{m+n} \frac{1}{l}\right)\, .\label{e:expansion_of_Y} \end{align} where $\gamma$ is the Euler constant and the latter expansion is valid for $n\in \mathbb N$ (for $n \in \mathbb Z\setminus \mathbb N$ a corresponding identity can be derived from the relation $Y_{-n} = (-1)^n Y_n$.

Half-integral orders

Cylinder functions turn into elementary functions if and only if the index $\nu$ is an half integer, i.e. of the form $\nu = \frac{1}{2} + n$ with $n \in \mathbb N$. In particular he following formulas hold for all $n\in \mathbb N$: \begin{align} J_{n+\frac{1}{2}} (z) & = (-1)^n \sqrt{\frac{2}{\pi}} z^{n+\frac{1}{2}} \left(\frac{1}{z} \frac{d}{dz}\right)^n \frac{\sin z}{z}\, ,\\ J_{-n+\frac{1}{2}} (z) &= \sqrt{\frac{2}{\pi}} z^{n+\frac{1}{2}} \left(\frac{1}{z}\frac{d}{dz}\right)^n \frac{\cos z}{z}\, ,\\ H^{(1)}_{-n-\frac{1}{2}} (z) &= i (-1)^n H^{(1)}_{n+\frac{1}{2}} (z)\, ,\\ H^{(2)}_{-n-\frac{1}{2}} (z) &= - i (-1)^n H^{(2)}_{n+\frac{1}{2}} (z)\, ,\\ Y_{n+\frac{1}{2}} (z) &= (-1)^{n+1} J_{-n-\frac{1}{2}} (z)\, ,\\ Y_{-n-\frac{1}{2}} (z) &= (-1)^{n+1} J_{n+\frac{1}{2}} (z)\, ,\\ K_{n+\frac{1}{2}} (z) = K_{-n-\frac{1}{2}} (z) &= (-1)^n \sqrt{\frac{2}{\pi}} z^{n+\frac{1}{2}} \left(\frac{1}{z} \frac{d}{dz}\right)^n \frac{e^{-z}}{z}\, . \end{align}

Integral representations

Integer order

When $\nu = n\in \mathbb N$ there are the Bessel's integral representations \begin{align} J_n (z) &= \frac{1}{\pi} \int_0^\pi \cos (z\sin \phi - n \phi)\, d\phi\\ Y_n (z) &= \frac{1}{\pi} \int_0^\pi \cos (z\sin \phi - n \phi)\, d\phi - \frac{2}{\pi} e^{- i n \frac{\pi}{2}} \int_0^\infty e^{-z\sinh t} {\rm cotanh}\, n \left(t + \frac{i\pi}{2}\right)\, dt\, , \qquad {\rm Re}\, z>0\, . \end{align}

General order

For ${\rm Re}\, \nu > -\frac{1}{2}$ and ${\rm Re}\, z > 0$ there are the Poisson's integral representations \begin{align} J_\nu (z) &= \frac{2 \left(\frac{z}{2}\right)^\nu}{\sqrt{\pi}\, \Gamma \left(\nu + \frac{1}{2}\right)} \int_0^{\frac{\pi}{2}} \cos (z\sin \phi) (\cos \phi)^{2\nu}\, d\phi\, ,\\ I_\nu (z) &= \frac{2 \left(\frac{z}{2}\right)^\nu}{\sqrt{\pi}\, \Gamma \left(\nu + \frac{1}{2}\right)} \int_{-1}^1 e^{-zt} (1-t^2)^{\nu-\frac{1}{2}}\, dt\, ,\\ H^{(k)}_\nu (z) &= (-1)^k i \frac{2 \left(\frac{z}{2}\right)^\nu}{\sqrt{\pi}\, \Gamma \left(\nu + \frac{1}{2}\right)} \int_0^{\frac{\pi}{2}} \frac{(\cos \phi)^{\nu-\frac{1}{2}}}{(\sin \phi)^{2\nu+1}} {\rm exp}\, \left((-1)^{k-1} i \left(z-\nu\pi+\frac{\phi}{2}\right) - 2 z {\rm cotan}\, \phi\right)\, d\phi\, ,\\ K_\nu (z) &= \frac{\sqrt{\pi} \left(\frac{z}{2}\right)^\nu}{\Gamma \left(\nu + \frac{1}{2}\right)} \int_0^\infty e^{-z \cosh t} (\sinh t)^{2\nu}\, dt\, . \end{align} Apart from these there are many other integral representations, in particular in the form of contour integrals (see [AS], [BE] and [GR]).

Asymptotic behaviour

For $|z|\ll 1$ and $\nu = n + \eta$ with $n\in \mathbb N$ and $\eta\in [0,1[$ we have \begin{align} J_\nu (z) &\sim \frac{1}{\Gamma (\nu+1)} \left(\frac{z}{2}\right)^\nu\\ J_{-\nu} (z) &\sim (-1)^n \Gamma (\nu) \frac{\sin (\pi \eta)}{\eta} \left(\frac{2}{z}\right)^\nu\, . \end{align} Under the same assumptions for $z=x\in \mathbb R$: \begin{align} Y_0 (x) &\sim \frac{2}{\pi} \left(\log x + C\right)\\ Y_\nu (x) &\sim -\frac{\Gamma (\nu)}{\pi} \left(\frac{2}{x}\right)^\nu\qquad \qquad n >0\, . \end{align} Next denote by $(\nu, m)$ the Hankel symbol: \[ (\nu, m) = \frac{\Gamma \left(\nu + m + \frac{1}{2}\right)}{m!\Gamma \left(\nu - m + \frac{1}{2}\right)}\, . \] For $|z| \gg \max \{|\nu|^2, 1\}$ and $-\pi < {\rm arg}\, z < \pi$ we have the following estimates: \begin{align} J_\nu (z) = \sqrt{\frac{2}{\pi z}} \left\{ \cos \left(z-\frac{\nu \pi}{2} - \frac{\pi}{4}\right) \left(\sum_{m=0}^{M-1} (-1)^m \frac{(\nu, 2m)}{(2z)^{2m}} + O \left(|z|^{-2M}\right)\right) - \sin \left(z-\frac{\nu \pi}{2} - \frac{\pi}{4}\right) \left(\sum_{m=0}^{M-1} (-1)^m \frac{(\nu, 2m+1)}{(2z)^{2m+1}} + O \left(|z|^{-2M-1}\right)\right)\right\}\\ Y_\nu (z) = \sqrt{\frac{2}{\pi z}} \left\{ \sin \left(z-\frac{\nu \pi}{2} - \frac{\pi}{4}\right) \left(\sum_{m=0}^{M-1} (-1)^m \frac{(\nu, 2m)}{(2z)^{2m}} + O \left(|z|^{-2M}\right)\right) + \cos \left(z-\frac{\nu \pi}{2} - \frac{\pi}{4}\right) \left(\sum_{m=0}^{M-1} (-1)^m \frac{(\nu, 2m+1)}{(2z)^{2m+1}} + O \left(|z|^{-2M-1}\right)\right)\right\}\, . \end{align} For $|z| \gg \max \{|\nu|^2, 1\}$ and $-\pi < {\rm arg}\, z < 2\pi$: \begin{equation}\label{e:Hankel_1_as} H^{(1)}_\nu (z) = \sqrt{\frac{2}{\pi z}} e^{-i (z-\frac{\nu\pi}{2} - \frac{\pi}{4})} \left(\sum_{m=0}^{M-1} \frac{(\nu, m)}{(-2iz)^{m}} + O \left(|z|^{-M}\right)\right)\, . \end{equation} For $|z| \gg \max \{|\nu|^2, 1\}$ and $-2\pi < {\rm arg}\, z < \pi$: \begin{equation}\label{e:Hankel_2_as} H^{(2)}_\nu (z) = \sqrt{\frac{2}{\pi z}} e^{-i (z-\frac{\nu\pi}{2} - \frac{\pi}{4})} \left(\sum_{m=0}^{M-1} \frac{(\nu, m)}{(2iz)^{m}} + O \left(|z|^{-M}\right)\right)\, . \end{equation} For $|z| \gg \max \{|\nu|^2, 1\}$ and $-\frac{3\pi}{2} < {\rm arg}\, z < \frac{\pi}{2}$: \[ I_\nu (z) = \frac{\cos \nu\pi}{\pi\, \sqrt{2\pi z}} \left\{ \sum_{m=0}^{M-1} \left(e^z - i (-1)^m e^{-i\pi \nu -z}\right) \Gamma \left(m+\frac{1}{2}-\nu\right) \Gamma \left(m+\frac{2}{2}+\nu\right) \frac{(2z)^{-m}}{m!} + e^z O \left(|z|^{-M}\right)\right\}\, . \] For $|z| \gg \max \{|\nu|^2, 1\}$ and $-\frac{3\pi}{2} < {\rm arg}\, z < \frac{3\pi}{2}$: \[ K_\nu (z) = \sqrt{\frac{\pi}{2z}} e^{-z} \left(\sum_{m=0}^{M-1} \frac{(\nu,m)}{(2z)^m} + O \left(|z|^{-M}\right)\right)\, . \] For $\nu - \frac{1}{2} = n \in \mathbb N$ the series \eqref{e:Hankel_1_as} and \eqref{e:Hankel_2_as} terminate. The Hankel functions are the only cylinder functions that tend to $0$ for complex values of the variable $z$ as $|z|\to\infty$ (and this is their merit in applications): \begin{align} &\lim_{|z|\to\infty} H^{(1)}_\nu (z) = 0 \qquad 0\leq {\rm arg}\, z\leq \pi\\ &\lim_{|z|\to\infty} H^{(2)}_\nu (z) = 0 \qquad -\pi \leq {\rm arg}\, z\leq 0 \end{align} For large values of $|z|$ and $|\nu|$ (see [AS], [BE], [L] and [Wa]).


The zeros of an arbitrary cylinder function are simple except for $z=0$. If $a$, $b$ and $\nu$ are real, then between two real zeros of $J_\nu$ lies one real zero of $aJ_\nu + bY_\nu$. For real $\nu$, $J_\nu$ has infinitely many real zeros (see Bessel functions for pictures of the graphs of $J_0$ and $J_1$) and if $\nu \in \mathbb R$ is larger than $-1$ all zeros of $J_\nu$ are real.

Still assuming that $\nu \in \mathbb R$, if $0< j_{\nu,1} < j_{\nu, 2} < \ldots$ is the ordering of the positive zeros of $J_\nu$, then we have the following ``interlacing inequalities: \[ 0 < j_{\nu,1} < j_{\nu+1, 1} < j_{\nu, 2} < j_{\nu+1, 2} < j_{\nu, 3} < \ldots \, . \] No pairs of functions $J_n, J_{m}, J_{n+\frac{1}{2}}, J_{m+\frac{1}{2}}$ with $m>n \in \mathbb N$ have no common zeros except for the origin. If \[ \nu = - (2n+1) - \eta\, , \qquad n\in \mathbb N\quad \eta\in ]0,1[ \] then $J_\nu$ has exactly $4n +2$ complex (non real) zeros , two of which are pure imaginary; if \[ \nu = - 2n - \eta\, , \qquad n\in \mathbb N\setminus \{0\} \quad \eta\in ]0,1[ \] then $J_\nu$ has exactly $4n$ complex (non real) zeros.

Addition theorems

The following addition is due to Neumann. If $r_1, r_2, \phi\in \mathbb R$ with $r_2>r_1$ and we set \[ R = \sqrt{r_1^2 + r_2^2 - 2 r_1 r_2 \cos \phi}\, , \] then \[ J_0 (R) = \sum_{m=-\infty}^\infty J_m (r_1) J_m (r_2) e^{i m\phi}\, . \] The formula generalizes to complex values $\nu$ after introducing the angle $\psi$, which is related to the unknowns by the formula \[ \sin \psi = \frac{r_1}{R} \sin \phi\, . \] Then, denoting by $Z_\nu$ an arbitrary cylinder function we have the vectorial identities \begin{align} Z_\nu (R) \left(\begin{array}{l} \cos \nu\psi\\ \sin \nu\psi \end{array}\right) &= \sum_{m=-\infty}^\infty J_m (r_1)\, Z_{\nu+m} (r_2)\, \left(\begin{array}{l} \cos m\phi\\ \sin m\phi \end{array}\right)\, ,\\ I_\nu (R) \left(\begin{array}{l} \cos \nu\psi\\ \sin \nu\psi \end{array}\right) &= \sum_{m=-\infty}^\infty (-1)^m I_m (r_1)\, I_{\nu+m} (r_2)\, \left(\begin{array}{l} \cos m\phi\\ \sin m\phi \end{array}\right)\, ,\\ K_\nu (R) \left(\begin{array}{l} \cos \nu\psi\\ \sin \nu\psi \end{array}\right) &= \sum_{m=-\infty}^\infty (-1)^m I_m (r_1)\, K_{\nu+m} (r_2)\, \left(\begin{array}{l} \cos m\phi\\ \sin m\phi \end{array}\right)\, . \end{align} After introducing the ultraspherical polynomials \[ C^\nu_n (x) = \sum_{k=0}^{\lfloor m/2\rfloor} (-1)^k 2^{m-2k} \frac{\Gamma (\nu+m-k)}{\Gamma (\nu) k! (m-2k)!} x^{m-2k} \] (which are the $n$-th coefficient of the power series expansion of $y \mapsto (1-2yx + y^2)^{-\nu}$) we also have the formulas \begin{align} Z_\nu (R) R^{-\nu} &= 2^\nu \Gamma (\nu) \sum_{m=0}^\infty (\nu+m) \frac{J_{\nu+m} (r_1) Z_{\nu+m} (r_2)}{r_1^\nu r_2^\nu} C_m^\nu (\cos \phi)\, ,\\ I_\nu (R) R^{-\nu} &= 2^\nu \Gamma (\nu) \sum_{m=0}^\infty (-1)^m (\nu+m) \frac{I_{\nu+m} (r_1) I_{\nu+m} (r_2)}{r_1^\nu r_2^\nu} C_m^\nu (\cos \phi)\, ,\\ K_\nu (R) R^{-\nu} &= 2^\nu \Gamma (\nu) \sum_{m=0}^\infty (-1)^m (\nu+m) \frac{I_{\nu+m} (r_1) K_{\nu+m} (r_2)}{r_1^\nu r_2^\nu} C_m^\nu (\cos \phi)\, . \end{align} Indeed the above identitities can be extended to complex values of all the parameters involved, see Chapter XI of [Wa].

In expansions of cylinder functions one uses Lommel polynomials, Neumann series, Fourier–Bessel series, and Dirichlet series.

Relations to spherical functions

Connected with Spherical functions are the Anger functions, the Struve functions, the Lommel functions, as well as the Kelvin functions and the Airy functions.

Cylinder functions can be defined as limit functions of spherical functions in the following way: \begin{align} &\lim_{n\to\infty} P_n \left(\cos \frac{z}{n}\right) = J_0 (z)\\ &\lim_{n\to\infty} n^m P_n^{-m} \left(\cos \frac{x}{n}\right) = J_m (x)\qquad 0<x<\pi\, ,\\ &\lim_{n\to\infty} P_n^{-m} \left(\cosh \frac{z}{n}\right) = I_m (z)\\ &\lim_{n\to\infty} \frac{n^{-m} \sin n\pi}{\sin (m+n)\pi} Q_n^m \left(\cosh \frac{z}{n}\right) = K_m (z)\, . \end{align} Here, asymptotic representations of spherical functions are connected with cylinder functions and vice versa, for example, as in Hilb's formula \[ P_n (\cos \theta) = \sqrt{\frac{\theta}{\sin \theta}} J_0 \left(\left(n+\frac{1}{2}\right)\theta\right) + O \left(n^{-\frac{3}{2}}\right)\, , \] and in the expansions of Macdonald, Watson, Tricomi, and others (see [BE], [GR], [Wa].

Numerical approximations

For the numerical evaluation of the functions $J_0, J_1, Y_0, Y_1, I_0, I_1, K_0, K_1$ approximations by polynomials and rational functions are convenient (see [AS]). For expansions with respect to Chebyshev polynomials see [Cl]. For the numerical calculation of functions of large integral order one uses the recurrence relations \eqref{e:rec1} and \eqref{e:rec2} (see [AS]).


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How to Cite This Entry:
Cylinder functions. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by L.N. KarmazinaA.P. Prudnikov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article