...one of the most highly
regarded and expertly designed C++ library projects in the
world.

— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards

Bessel Functions are solutions to Bessel's ordinary differential equation:

where ν is the *order* of the equation, and may be an
arbitrary real or complex number, although integer orders are the most
common occurrence.

This library supports either integer or real orders.

Since this is a second order differential equation, there must be two linearly
independent solutions, the first of these is denoted J_{v}
and known as a Bessel
function of the first kind:

This function is implemented in this library as cyl_bessel_j.

The second solution is denoted either Y_{v} or N_{v}
and is known as either a Bessel
Function of the second kind, or as a Neumann function:

This function is implemented in this library as cyl_neumann.

The Bessel functions satisfy the recurrence relations:

Have the derivatives:

Have the Wronskian relation:

and the reflection formulae:

The Bessel functions are valid for complex argument *x*,
and an important special case is the situation where *x*
is purely imaginary: giving a real valued result. In this case the functions
are the two linearly independent solutions to the modified Bessel equation:

The solutions are known as the modified Bessel functions of the first and
second kind (or occasionally as the hyperbolic Bessel functions of the
first and second kind). They are denoted I_{v} and K_{v}
respectively:

These functions are implemented in this library as cyl_bessel_i and cyl_bessel_k respectively.

The modified Bessel functions satisfy the recurrence relations:

Have the derivatives:

Have the Wronskian relation:

and the reflection formulae:

When solving the Helmholtz equation in spherical coordinates by separation of variables, 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} , and are related to the ordinary Bessel
functions J_{n} and Y_{n} by:

The spherical Bessel function of the second kind y_{n}
is also known as the
spherical Neumann function n_{n}.

These functions are implemented in this library as sph_bessel and sph_neumann.