# Bessel function

Bessel functions, first defined by the mathematician Daniel Bernoulli and generalized by Friedrich Bessel, are the canonical solutions y(x) of Bessel's differential equation

${\displaystyle x^{2}{\frac {d^{2}y}{dx^{2}}}+x{\frac {dy}{dx}}+(x^{2}-\alpha ^{2})y=0}$

for an arbitrary complex number α (the order of the Bessel function). Although α and −α produce the same differential equation for real α, it is conventional to define different Bessel functions for these two values in such a way that the Bessel functions are mostly smooth functions of α.

The most important cases are for α an integer or half-integer. Bessel functions for integer α are also known as cylinder functions or the cylindrical harmonics because they appear in the solution to Laplace's equation in cylindrical coordinates. Spherical Bessel functions with half-integer α are obtained when the Helmholtz equation is solved in spherical coordinates.

## Applications of Bessel functions

Bessel's equation arises when finding separable solutions to Laplace's equation and the Helmholtz equation in cylindrical or spherical coordinates. Bessel functions are therefore especially important for many problems of wave propagation and static potentials. In solving problems in cylindrical coordinate systems, one obtains Bessel functions of integer order (α = n); in spherical problems, one obtains half-integer orders (α = n+1/2). For example:

Bessel functions also appear in other problems, such as signal processing (e.g., see FM synthesis, Kaiser window, or Bessel filter).

## Definitions

Because 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. Different variations are described below.

### Bessel functions of the first kind : Jα

Bessel functions of the first kind, denoted as Jα(x), are solutions of Bessel's differential equation that are finite at the origin (x = 0) for integer or positive α, and diverge as x approaches zero for negative non-integer α. It is possible to define the function by its Taylor series expansion around x = 0, which can be found by applying the Frobenius method to Bessel's equation:[1]

${\displaystyle J_{\alpha }(x)=\sum _{m=0}^{\infty }{\frac {(-1)^{m}}{m!\,\Gamma (m+\alpha +1)}}{\left({\frac {x}{2}}\right)}^{2m+\alpha }}$

where Γ(z) is the gamma function, a shifted generalization of the factorial function to non-integer values. The Bessel function of the first kind is an entire function if α is an integer, otherwise it is a multivalued function with singularity at zero. 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. (The Taylor series indicates that −J1(x) is the derivative of J0(x), much like −sin(x) is the derivative of cos(x); more generally, the derivative of Jn(x) can be expressed in terms of Jn±1(x) by the identities below.)

Plot of Bessel function of the first kind, Jα(x), for integer orders α = 0, 1, 2

For non-integer α, the functions Jα(x) and J−α(x) are linearly independent, and are therefore the two solutions of the differential equation. On the other hand, for integer order α, the following relationship is valid (note that the Gamma function has simple poles at each of the non-positive integers):[2]

${\displaystyle J_{-n}(x)=(-1)^{n}J_{n}(x).\,}$

This means that the two solutions are no longer linearly independent. In this case, 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, for integer values of n, is possible using an integral representation:[3]

${\displaystyle J_{n}(x)={\frac {1}{\pi }}\int _{0}^{\pi }\cos(n\tau -x\sin(\tau ))\,d\tau .}$

Another integral representation is:[3]

${\displaystyle J_{n}(x)={\frac {1}{2\pi }}\int _{-\pi }^{\pi }e^{i(n\tau -x\sin(\tau ))}\,d\tau .}$

This was the approach that Bessel used, and from this definition he derived several properties of the function. The definition may be extended to non-integer orders by (for Re(x) > 0), one of Schläfli's integrals:[3]

${\displaystyle J_{\alpha }(x)={\frac {1}{\pi }}\int _{0}^{\pi }\cos(\alpha \tau -x\sin \tau )\,d\tau -{\frac {\sin(\alpha \pi )}{\pi }}\int _{0}^{\infty }e^{-x\sinh(t)-\alpha t}\,dt.}$ [4][5][6][7]

#### Relation to hypergeometric series

The Bessel functions can be expressed in terms of the generalized hypergeometric series as[8]

${\displaystyle J_{\alpha }(x)={\frac {({\frac {x}{2}})^{\alpha }}{\Gamma (\alpha +1)}}\;_{0}F_{1}(\alpha +1;-{\tfrac {x^{2}}{4}}).}$

This expression is related to the development of Bessel functions in terms of the Bessel–Clifford function.

#### Relation to Laguerre polynomials

In terms of the Laguerre polynomials Lk and arbitrarily chosen parameter t, the Bessel function can be expressed as[9]

${\displaystyle {\frac {J_{\alpha }(x)}{\left({\frac {x}{2}}\right)^{\alpha }}}={\frac {e^{-t}}{\Gamma (\alpha +1)}}\sum _{k=0}^{\infty }{\frac {L_{k}^{(\alpha )}\left({\frac {x^{2}}{4t}}\right)}{k+\alpha \choose k}}{\frac {t^{k}}{k!}}.}$

### Bessel functions of the second kind : Yα

The Bessel functions of the second kind, denoted by Yα(x), occasionally denoted instead by Nα(x), are solutions of the Bessel differential equation that have a singularity at the origin (x = 0) and are multivalued. These are sometimes called Weber functions after Heinrich Martin Weber, and also Neumann functions after Carl Neumann.

Plot of Bessel function of the second kind, Yα(x), for integer orders α = 0, 1, 2.

For non-integer α, Yα(x) is related to Jα(x) by:

${\displaystyle Y_{\alpha }(x)={\frac {J_{\alpha }(x)\cos(\alpha \pi )-J_{-\alpha }(x)}{\sin(\alpha \pi )}}.}$

In the case of integer order n, the function is defined by taking the limit as a non-integer α tends to n:

${\displaystyle Y_{n}(x)=\lim _{\alpha \to n}Y_{\alpha }(x).}$

There is also a corresponding integral formula (for Re(x) > 0),[10]

${\displaystyle Y_{n}(x)={\frac {1}{\pi }}\int _{0}^{\pi }\sin(x\sin \theta -n\theta )\,d\theta -{\frac {1}{\pi }}\int _{0}^{\infty }\left[e^{nt}+(-1)^{n}e^{-nt}\right]e^{-x\sinh t}\,dt.}$

Yα(x) is necessary as the second linearly independent solution of the Bessel's equation when α is an integer. But Yα(x) has more meaning than that. It can be considered as a 'natural' partner of Jα(x). See also the subsection on Hankel functions below.

When α is an integer, moreover, as was similarly the case for the functions of the first kind, the following relationship is valid:

${\displaystyle Y_{-n}(x)=(-1)^{n}Y_{n}(x).\,}$

Both Jα(x) and Yα(x) are holomorphic functions of x on the complex plane cut along the negative real axis. When α is an integer, the Bessel functions J are entire functions of x. If x is held fixed at a non-zero value, then the Bessel functions are entire functions of α.

The Bessel functions of the second kind when α is an integer is an example of the second kind of solution in Fuchs's theorem.

### {{safesubst:#invoke:anchor|main}}Hankel functions: Hα(1), Hα(2)

Another important formulation of the two linearly independent solutions to Bessel's equation are the Hankel functions Hα(1)(x) and Hα(2)(x), defined by:[11]

${\displaystyle H_{\alpha }^{(1)}(x)=J_{\alpha }(x)+iY_{\alpha }(x)}$
${\displaystyle H_{\alpha }^{(2)}(x)=J_{\alpha }(x)-iY_{\alpha }(x)}$

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. They are named after Hermann Hankel.

The importance of Hankel functions of the first and second kind lies more in theoretical development rather than in application. These forms of linear combination satisfy numerous simple-looking properties, like asymptotic formulae or integral representations. Here, 'simple' means an appearance of the factor of the form eif(x). The Bessel function of the second kind then can be thought to naturally appear as the imaginary part of the Hankel functions.

The Hankel functions are used to express outward- and inward-propagating cylindrical wave solutions of the cylindrical wave equation, respectively (or vice versa, depending on the sign convention for the frequency).

Using the previous relationships they can be expressed as:

${\displaystyle H_{\alpha }^{(1)}(x)={\frac {J_{-\alpha }(x)-e^{-\alpha \pi i}J_{\alpha }(x)}{i\sin(\alpha \pi )}}}$
${\displaystyle H_{\alpha }^{(2)}(x)={\frac {J_{-\alpha }(x)-e^{\alpha \pi i}J_{\alpha }(x)}{-i\sin(\alpha \pi )}}.}$

If α is an integer, the limit has to be calculated. The following relationships are valid, whether α is an integer or not:[12]

${\displaystyle H_{-\alpha }^{(1)}(x)=e^{\alpha \pi i}H_{\alpha }^{(1)}(x)}$
${\displaystyle H_{-\alpha }^{(2)}(x)=e^{-\alpha \pi i}H_{\alpha }^{(2)}(x).}$

In particular, if α = m + 1/2 with m a nonnegative integer, the above relations imply directly that

${\displaystyle J_{-(m+{\frac {1}{2}})}(x)=(-1)^{m+1}Y_{m+{\frac {1}{2}}}(x)}$
${\displaystyle Y_{-(m+{\frac {1}{2}})}(x)=(-1)^{m}J_{m+{\frac {1}{2}}}(x).}$

These are useful in developing the spherical Bessel functions (below).

The Hankel functions admit the following integral representations for Re(x) > 0:[13]

${\displaystyle H_{\alpha }^{(1)}(x)={\frac {1}{\pi i}}\int _{-\infty }^{+\infty +i\pi }e^{x\sinh t-\alpha t}\,dt,}$
${\displaystyle H_{\alpha }^{(2)}(x)=-{\frac {1}{\pi i}}\int _{-\infty }^{+\infty -i\pi }e^{x\sinh t-\alpha t}\,dt,}$

where the integration limits indicate integration along a contour that can be chosen as follows: from −∞ to 0 along the negative real axis, from 0 to ±iπ along the imaginary axis, and from ±iπ to +∞±iπ along a contour parallel to the real axis.[14]

### Modified Bessel functions : Iα, Kα

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 (or occasionally the hyperbolic Bessel functions) of the first and second kind, and are defined by:[15]

${\displaystyle I_{\alpha }(x)=i^{-\alpha }J_{\alpha }(ix)=\sum _{m=0}^{\infty }{\frac {1}{m!\,\Gamma (m+\alpha +1)}}\left({\frac {x}{2}}\right)^{2m+\alpha }}$
${\displaystyle K_{\alpha }(x)={\frac {\pi }{2}}{\frac {I_{-\alpha }(x)-I_{\alpha }(x)}{\sin(\alpha \pi )}},}$

when α is not an integer; when α is an integer, then the limit is used. These are chosen to be real-valued for real and positive arguments x. The series expansion for Iα(x) is thus similar to that for Jα(x), but without the alternating (−1)m factor.

If −π < arg(x) ≤ π/2, Kα(x) can be expressed as a Hankel function of the first kind:

${\displaystyle K_{\alpha }(x)={\frac {\pi }{2}}i^{\alpha +1}H_{\alpha }^{(1)}(ix),}$

and if π/2 < arg(x) ≤ π, it can be expressed as a Hankel function of the second kind:

${\displaystyle K_{\alpha }(x)={\frac {\pi }{2}}(-i)^{\alpha +1}H_{\alpha }^{(2)}(-ix).}$

We can express the first and second Bessel functions in terms of the modified Bessel functions (these are valid if −π < arg(z) ≤ π/2):

{\displaystyle {\begin{aligned}J_{\alpha }(iz)&=e^{\frac {\alpha i\pi }{2}}I_{\alpha }(z)\\Y_{\alpha }(iz)&=e^{\frac {(\alpha +1)i\pi }{2}}I_{\alpha }(z)-{\frac {2}{\pi }}e^{-{\frac {\alpha i\pi }{2}}}K_{\alpha }(z).\end{aligned}}}

Iα(x) and Kα(x) are the two linearly independent solutions to the modified Bessel's equation:[16]

${\displaystyle x^{2}{\frac {d^{2}y}{dx^{2}}}+x{\frac {dy}{dx}}-(x^{2}+\alpha ^{2})y=0.}$

Unlike the ordinary Bessel functions, which are oscillating as functions of a real argument, Iα and Kα are exponentially growing and decaying functions, respectively. Like the ordinary Bessel function Jα, the function Iα goes to zero at x = 0 for α > 0 and is finite at x = 0 for α = 0. Analogously, Kα diverges at x = 0.

 Modified Bessel functions of 1st kind, Iα(x), for α = 0, 1, 2, 3 Modified Bessel functions of 2nd kind, Kα(x), for α = 0, 1, 2, 3

Two integral formulas for the modified Bessel functions are (for Re(x) > 0):[17]

${\displaystyle I_{\alpha }(x)={\frac {1}{\pi }}\int _{0}^{\pi }\exp(x\cos(\theta ))\cos(\alpha \theta )\,d\theta -{\frac {\sin(\alpha \pi )}{\pi }}\int _{0}^{\infty }\exp(-x\cosh t-\alpha t)\,dt,}$
${\displaystyle K_{\alpha }(x)=\int _{0}^{\infty }\exp(-x\cosh t)\cosh(\alpha t)\,dt.}$

Modified Bessel functions K1/3 and K2/3 can be represented in terms of rapidly converged integrals[18]

{\displaystyle {\begin{aligned}K_{\frac {1}{3}}(\xi )&={\sqrt {3}}\,\int _{0}^{\infty }\,\exp \left[-\xi \left(1+{\frac {4x^{2}}{3}}\right){\sqrt {1+{\frac {x^{2}}{3}}}}\,\right]\,dx\\K_{\frac {2}{3}}(\xi )&={\frac {1}{\sqrt {3}}}\,\int _{0}^{\infty }\,{\frac {3+2x^{2}}{\sqrt {1+{\frac {x^{2}}{3}}}}}\exp \left[-\xi \left(1+{\frac {4x^{2}}{3}}\right){\sqrt {1+{\frac {x^{2}}{3}}}}\,\right]\,dx.\end{aligned}}}

The modified Bessel function of the second kind has also been called by the now-rare names:

### Spherical Bessel functions: jn, yn

Spherical Bessel functions of 1st kind, jn(x), for n = 0, 1, 2
Spherical Bessel functions of 2nd kind, yn(x), for n = 0, 1, 2

When solving the Helmholtz equation in spherical coordinates by separation of variables, the radial equation has the form:

${\displaystyle x^{2}{\frac {d^{2}y}{dx^{2}}}+2x{\frac {dy}{dx}}+[x^{2}-n(n+1)]y=0.}$

The two linearly independent solutions to this equation are called the spherical Bessel functions jn and yn, and are related to the ordinary Bessel functions Jn and Yn by:[21]

${\displaystyle j_{n}(x)={\sqrt {\frac {\pi }{2x}}}J_{n+{\frac {1}{2}}}(x),}$
${\displaystyle y_{n}(x)={\sqrt {\frac {\pi }{2x}}}Y_{n+{\frac {1}{2}}}(x)=(-1)^{n+1}{\sqrt {\frac {\pi }{2x}}}J_{-n-{\frac {1}{2}}}(x).}$

yn is also denoted nn or ηn; some authors call these functions the spherical Neumann functions.

The spherical Bessel functions can also be written as (Rayleigh's formulas):[22]

${\displaystyle j_{n}(x)=(-x)^{n}\left({\frac {1}{x}}{\frac {d}{dx}}\right)^{n}\,{\frac {\sin(x)}{x}},}$
${\displaystyle y_{n}(x)=-(-x)^{n}\left({\frac {1}{x}}{\frac {d}{dx}}\right)^{n}\,{\frac {\cos(x)}{x}}.}$

The first spherical Bessel function j0(x) is also known as the (unnormalized) sinc function. The first few spherical Bessel functions are:

${\displaystyle j_{0}(x)={\frac {\sin(x)}{x}}}$
${\displaystyle j_{1}(x)={\frac {\sin(x)}{x^{2}}}-{\frac {\cos(x)}{x}}}$
${\displaystyle j_{2}(x)=\left({\frac {3}{x^{2}}}-1\right){\frac {\sin(x)}{x}}-{\frac {3\cos(x)}{x^{2}}}}$[23]
${\displaystyle j_{3}(x)=\left({\frac {15}{x^{3}}}-{\frac {6}{x}}\right){\frac {\sin(x)}{x}}-\left({\frac {15}{x^{2}}}-1\right){\frac {\cos(x)}{x}},}$

and

${\displaystyle y_{0}(x)=-j_{-1}(x)=-\,{\frac {\cos(x)}{x}}}$
${\displaystyle y_{1}(x)=j_{-2}(x)=-\,{\frac {\cos(x)}{x^{2}}}-{\frac {\sin(x)}{x}}}$
${\displaystyle y_{2}(x)=-j_{-3}(x)=\left(-\,{\frac {3}{x^{2}}}+1\right){\frac {\cos(x)}{x}}-{\frac {3\sin(x)}{x^{2}}}}$[24]
${\displaystyle y_{3}\left(x\right)=j_{-4}(x)=\left(-{\frac {15}{x^{3}}}+{\frac {6}{x}}\right){\frac {\cos(x)}{x}}-\left({\frac {15}{x^{2}}}-1\right){\frac {\sin(x)}{x}}.}$

#### Generating function

The spherical Bessel functions have the generating functions [25]

${\displaystyle {\frac {1}{z}}\cos \left({\sqrt {z^{2}-2zt}}\right)=\sum _{n=0}^{\infty }{\frac {t^{n}}{n!}}j_{n-1}(z),}$
${\displaystyle {\frac {1}{z}}\sin \left({\sqrt {z^{2}+2zt}}\right)=\sum _{n=0}^{\infty }{\frac {(-t)^{n}}{n!}}y_{n-1}(z).}$

#### Differential relations

${\displaystyle \left({\frac {1}{z}}{\frac {d}{dz}}\right)^{m}\left(z^{n+1}f_{n}(z)\right)=z^{n-m+1}f_{n-m}(z),}$
${\displaystyle \left({\frac {1}{z}}{\frac {d}{dz}}\right)^{m}\left(z^{-n}f_{n}(z)\right)=(-1)^{m}z^{-n-m}f_{n+m}(z).}$

### Spherical Hankel functions: hn(1), hn(2)

There are also spherical analogues of the Hankel functions:

${\displaystyle h_{n}^{(1)}(x)=j_{n}(x)+iy_{n}(x)\,}$
${\displaystyle h_{n}^{(2)}(x)=j_{n}(x)-iy_{n}(x).\,}$

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:

${\displaystyle h_{n}^{(1)}(x)=(-i)^{n+1}{\frac {e^{ix}}{x}}\sum _{m=0}^{n}{\frac {i^{m}}{m!(2x)^{m}}}{\frac {(n+m)!}{(n-m)!}}}$

and ${\displaystyle h_{n}^{(2)}}$ is the complex-conjugate of this (for real x). It follows, for example, that ${\displaystyle j_{0}(x)=\sin(x)/x}$ and ${\displaystyle y_{0}(x)=-\cos(x)/x}$, and so on.

The spherical Hankel functions appear in problems involving spherical wave propagation, for example in the multipole expansion of the electromagnetic field.

### Riccati–Bessel functions: Sn, Cn, ξn, ζn

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

${\displaystyle S_{n}(x)=xj_{n}(x)={\sqrt {\frac {\pi x}{2}}}\,J_{n+{\frac {1}{2}}}(x)}$
${\displaystyle C_{n}(x)=-xy_{n}(x)=-{\sqrt {\frac {\pi x}{2}}}\,Y_{n+{\frac {1}{2}}}(x)}$
${\displaystyle \xi _{n}(x)=xh_{n}^{(1)}(x)={\sqrt {\frac {\pi x}{2}}}\,H_{n+{\frac {1}{2}}}^{(1)}(x)=S_{n}(x)-iC_{n}(x)}$
${\displaystyle \zeta _{n}(x)=xh_{n}^{(2)}(x)={\sqrt {\frac {\pi x}{2}}}\,H_{n+{\frac {1}{2}}}^{(2)}(x)=S_{n}(x)+iC_{n}(x).}$

They satisfy the differential equation:

${\displaystyle x^{2}{\frac {d^{2}y}{dx^{2}}}+[x^{2}-n(n+1)]y=0.}$

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)[27] for recent developments and references.

Following Debye (1909), the notation ${\displaystyle \psi _{n},\chi _{n}}$ is sometimes used instead of ${\displaystyle S_{n},C_{n}}$.

## Asymptotic forms

The Bessel functions have the following asymptotic forms. For small arguments[1] ${\displaystyle 0, one obtains, when α is not a negative integer:

${\displaystyle J_{\alpha }(z)\sim {\frac {1}{\Gamma (\alpha +1)}}\left({\frac {z}{2}}\right)^{\alpha }}$

When α is a negative integer, we have:

${\displaystyle J_{\alpha }(z)\sim {\frac {(-1)^{\alpha }}{(-\alpha )!}}\left({\frac {2}{z}}\right)^{\alpha }}$

For the Bessel function of the second kind we have three cases:

${\displaystyle Y_{\alpha }(z)\sim {\begin{cases}{\frac {2}{\pi }}\left(\ln \left({\frac {z}{2}}\right)+\gamma \right)&{\text{if }}\alpha =0\\\\-{\frac {\Gamma (\alpha )}{\pi }}\left({\frac {2}{z}}\right)^{\alpha }+{\frac {1}{\Gamma (\alpha +1)}}\left({\frac {z}{2}}\right)^{\alpha }\cot(\alpha \pi )&{\text{if }}\alpha {\text{ is not a non-positive integer (one term dominates unless }}\alpha {\text{ is imaginary)}}\\\\-{\frac {(-1)^{\alpha }\Gamma (-\alpha )}{\pi }}\left({\frac {z}{2}}\right)^{\alpha }&{\text{if }}\alpha {\text{ is a negative integer}}\end{cases}}}$

where γ is the Euler–Mascheroni constant (0.5772...).

For large real arguments ${\displaystyle x\gg \left|\alpha ^{2}-{\tfrac {1}{4}}\right|}$, one cannot write a true asymptotic form for Bessel functions of the first and second kind (unless α is half-integer) because they have zeros all the way out to infinity which would have to be matched exactly by any asymptotic expansion. However, for a given value of arg(z) one can write an equation containing a term of order |z|−1:[28]

{\displaystyle {\begin{aligned}J_{\alpha }(z)&={\sqrt {\frac {2}{\pi z}}}\left(\cos \left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)+e^{|\operatorname {Im} (z)|}O(|z|^{-1})\right)&&{\text{ for }}|\arg z|<\pi \\Y_{\alpha }(z)&={\sqrt {\frac {2}{\pi z}}}\left(\sin \left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)+e^{|\operatorname {Im} (z)|}O(|z|^{-1})\right)&&{\text{ for }}|\arg z|<\pi .\end{aligned}}}

(For α = 1/2 the last terms in these formulas drop out completely; see the spherical Bessel functions above.) Even though these equations are true, better approximations may be available for complex z. For example, J0(z) when z is near the negative real line is approximated better by

${\displaystyle J_{0}(z)\approx {\sqrt {\frac {-2}{\pi z}}}\cos \left(z+{\frac {\pi }{4}}\right)}$

than by

${\displaystyle J_{0}(z)\approx {\sqrt {\frac {2}{\pi z}}}\cos \left(z-{\frac {\pi }{4}}\right).}$

The asymptotic forms for the Hankel functions are:

{\displaystyle {\begin{aligned}H_{\alpha }^{(1)}(z)&\sim {\sqrt {\frac {2}{\pi z}}}\exp \left(i\left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)\right)&&{\text{ for }}-\pi <\arg z<2\pi \\H_{\alpha }^{(2)}(z)&\sim {\sqrt {\frac {2}{\pi z}}}\exp \left(-i\left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)\right)&&{\text{ for }}-2\pi <\arg z<\pi \end{aligned}}}

These can be extended to other values of arg(z) using equations relating ${\displaystyle H_{\alpha }^{(1)}(ze^{im\pi })}$ and ${\displaystyle H_{\alpha }^{(2)}(ze^{im\pi })}$ to Hα(1)(z) and Hα(2)(z).[29] It is interesting that although the Bessel function of the first kind is the average of the two Hankel functions, Jα(z) is not asymptotic to the average of these two asymptotic forms when z is negative (because one or the other will not be correct there, depending on the arg(z) used). But the asymptotic forms for the Hankel functions permit us to write asymptotic forms for the Bessel functions of first and second kinds for complex (non-real) z so long as |z| goes to infinity at a constant phase angle arg z (using the square root having positive real part):

{\displaystyle {\begin{aligned}J_{\alpha }(z)&\sim {\frac {1}{\sqrt {2\pi z}}}\exp \left(i\left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)\right)&&{\text{ for }}-\pi <\arg z<0\\J_{\alpha }(z)&\sim {\frac {1}{\sqrt {2\pi z}}}\exp \left(-i\left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)\right)&&{\text{ for }}0<\arg z<\pi \\Y_{\alpha }(z)&\sim -i{\frac {1}{\sqrt {2\pi z}}}\exp \left(i\left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)\right)&&{\text{ for }}-\pi <\arg z<0\\Y_{\alpha }(z)&\sim -i{\frac {1}{\sqrt {2\pi z}}}\exp \left(-i\left(z-{\frac {\alpha \pi }{2}}-{\frac {\pi }{4}}\right)\right)&&{\text{ for }}0<\arg z<\pi \end{aligned}}}

For the modified Bessel functions, Hankel developed asymptotic expansions as well:

${\displaystyle I_{\alpha }(z)\sim {\frac {e^{z}}{\sqrt {2\pi z}}}\left(1-{\frac {4\alpha ^{2}-1}{8z}}+{\frac {(4\alpha ^{2}-1)(4\alpha ^{2}-9)}{2!(8z)^{2}}}-{\frac {(4\alpha ^{2}-1)(4\alpha ^{2}-9)(4\alpha ^{2}-25)}{3!(8z)^{3}}}+\cdots \right){\text{ for }}|\arg z|<{\tfrac {\pi }{2}},}$[30]