poly-roots

Find all roots of a polynomial using the Jenkins-Traub method. In other words, it factorizes the polynomial over the complex numbers.

N.B.: I fear I strayed too far toward translating cpoly while trying to understand the algorithm. It's similar enough to likely be covered under the original ACM Software License Agreement. Sorry.

Introduction

This module factors a polynomial of the form

.

It uses the Jenkins-Traub method, and more specifically it's very nearly a line-by-line translation of the tried and true cpoly.f90. No really, it's almost a direct translation, taking some leeway in reworking goto statements into javascript. I started off with a pretty naive implementation of the original paper, A three-stage variable shift iteration for polynomial zeros and its relation to generalized Ralyeigh iteration by M. A. Jenkins and J. F. Traub, 1968, but there are some serious shortcuts and simplifications you can take if you stop and think about what you're doing. So I gave up cleaning up and refactoring my own version and reworked an existing implementation into JavaScript.

The good:

• It's reasonably fast
• It's numerically stable
• Memory usage is linear
• It benefits from the experimentation of the people who originally sat down and came up with a great implementation
• No dependencies

The bad:

• It's been translated by hand.
• The convergence criteria need a bit of work. I glossed over a couple subroutines that juggle some operations in order to prevent underflow errors, so I suspect the error estimates relative to machine epsilon aren't stricly accurate.
• It can maybe be translated better and more effectively via f2c + emscripten.
• The speed can be cut in half for polynomials with real coefficients by using the rpoly.f90 variant

You can go do some research about good root-finders, but for a quick rundown of what you have to work with if you want to stick with JavaScript, see a quick benchmark.

Usage

require('poly-roots')( real_coeffs [, imag_coeffs] )

Computes the roots of a polynomial given the coefficients in descending order.

• real_coeffs the real coefficients of the polynomial arranged in order of decreasing degree
• imag_coeffs (optional) the imaginary coefficients of the polynomial. If not specified, assumed to be zero

Returns: A pair of vectors representing the real and imaginary parts of the roots of the polynomial

Example

var roots = require('poly-roots');
 
// Roots of x^2 + 2x - 3:
var r1 = roots([1,2,-3]);
 
// Roots of z^3 - (4 + i)z^2 + (1 + i)z + (6 + 2i):
var r2 = roots([1,-4,1,6],[0,-1,1,2]);

See also

For the companion roots version that determines roots by solution of an eigenvalue problem (via numeric.js), see companion-roots. For a blazing fast variant that might struggle in corner cases (like closely-spaced roots), see durand-kerner.

Credits

Since this inherits a lot from cpoly.f90 and cpoly.f90 in turn is an update of the original code from CACM 419, I'm afraid that it may be subject to the ACM Software License Agreement which, in short, grants to you a royalty-free, nonexclusive right to execute, copy, modify and distribute both the binary and source code solely for academic, research and other similar noncommercial uses. :(