Bessel functions
Cylinder functions of the first kind. A Bessel function of order
can be defined as the series
 | (*) |
which converges throughout the plane. A Bessel function of order
is the solution of the corresponding
Bessel equation. If the argument and the order
are real numbers, the Bessel function is real, and its graph has the
form of a damped vibration (Fig.); if the order is even, the Bessel
function is even, if odd, it is odd.
Figure: b015840a
Graphs of the functions
and
.
The behaviour of a Bessel function in a neighbourhood of zero is given by the first term of the series (*); for large
, the asymptotic representation
holds. The zeros of a Bessel function (i.e. the roots of the equation
) are simple, and the zeros of
are situated between the zeros of
. Bessel functions of "half-integral" order
are expressible by trigonometric functions; in particular
The Bessel functions
(where
are the positive zeros of
,
) form an orthogonal system with weight
in the interval
. Under certain conditions the following expansion is valid:
In an infinite interval this expansion is replaced by the Fourier–Bessel integral
The following formulas play an important role in the theory of Bessel functions and their applications:
1) the integral representation
2) the generating function
3) the addition theorem for Bessel functions of order zero
4) the recurrence formulas
For references, see
Cylinder functions.
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