bessel

Pure-JS implementation of Bessel functions J,Y,I,K (for the browser and nodejs).
Emphasis on correctness and performance for integer order.

The standard notation is used here:

- J is the Bessel function of the first kind
- Y is the Bessel function of the second kind
- I is the modified Bessel function of the first kind
- K is the modified Bessel function of the second kind

Installation

With npm:

``bash
$ npm install bessel
`

In the browser:

`html

`

The browser exposes a variable BESSEL

The script will manipulate module.exports if available (e.g. in a CommonJS
require context). This is not always desirable. To prevent the behavior,
define DO_NOT_EXPORT_BESSEL

Usage

In all cases, the relevant function takes two arguments (value, order).

The return value is a JS number. NaN signals an error in calculation.

- BESSEL.besselj(x, n) computes J_{n}(x)

- BESSEL.bessely(x, n) computes Y_{n}(x)

- BESSEL.besseli(x, n) computes I_{n}(x)

- BESSEL.besselk(x, n) computes K_{n}(x)

For example:

`js
// var BESSEL = require('bessel'); // uncomment this line if in node
BESSEL.besselj(1.5,0) // 0.5118276712499389
BESSEL.bessely(1.5,0) // 0.38244892476502895
BESSEL.besseli(1.5,0) // 1.6467232021476754
BESSEL.besselk(1.5,0) // 0.2138055693236539

var Y = BESSEL.bessely
Y(Math.PI, 5) + Y(Math.PI, 3) - (2 4 / Math.PI) Y(Math.PI, 4) // 0
`

Testing

make test will run the nodejs-based test.

To generate the excel.tsv test cases, make a 6-column Excel Sheet:

| x | n | BESSELI | BESSELJ | BESSELK | BESSELY |
|---|---|:--------------:|:--------------:|:--------------:|:--------------:|
| x | n |BESSELI(A1,B1)|BESSELJ(A1,B1)|BESSELK(A1,B1)|BESSELY(A1,B1)|

To generate the mma.tsv test cases, use the Mathematica Bessel Functions:

`mathematica
( Bessel_[value, order] )
F[x_,n_]:={x/2,n,BesselI[n,x/2], BesselJ[n,x/2], BesselK[n,x/2], BesselY[n,x/2]}
Do[Print[ExportString[N[F[x,n],10],"csv"]], {n,1,3}, {x,1,20} ]
`

Note: Each function follows Excel semantics (value, order)`. Other platforms
like Mathematica reverse the argument order.

License

Please consult the attached LICENSE file for details. All rights not explicitly
granted by the Apache 2.0 License are reserved by the Original Author.

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