brents-method

Brent's Method

Description

Brent's Method is a novel, highly efficient method for finding the roots of a function within given bounds - that is, where the function returns 0 (or very nearly 0), also known as an x-intercept.

Wikipedia's entry reads:

In numerical analysis, Brent's method is a root-finding algorithm combining the bisection method, the secant method and inverse quadratic interpolation. It has the reliability of bisection but it can be as quick as some of the less-reliable methods. The algorithm tries to use the potentially fast-converging secant method or inverse quadratic interpolation if possible, but it falls back to the more robust bisection method if necessary.

Example

const brentMethod = require('brents-method')

brentsMethod(
  x => (x + 3) * Math.pow(x - 1, 2),
  -4,
  4 / 3,
) // -3

Parameters

Param	Type	Default	Description
f	function		The function for which roots are desired.
lowerLimit	Number		The lower value used to bracket root finding into the function.
upperLimit	Number		The upper value used to bracket root finding into the function.
options	Object		An optional arguments object.
[options.errorTolerance]	Number	1e-7	Used to determine how close the bounds bracketing root finding must converge before quitting.
[options.maxIterations]	Number	50	The maximum number of iterations before root finding will fail.

Specifics

In my implementation, the inverse quadratic interpolation is applied directly. In other cases, the Lagrange polynomial is reduced by defining the variables p, q, r, s, and t, and the x value is not overwritten with the bisection method, but modified. For simplicity's sake, the inverse quadratic interpolation is applied directly in this implementation and the new guess is overwritten if needed.

This is based in part on a number of other implementations and references.

Useful references: https://www.youtube.com/watch?v=-bLSRiokgFk https://en.wikipedia.org/wiki/Brent%27s_method#Brent's_method https://en.wikipedia.org/wiki?title=Talk:Brent%27s_method#Removed_code

Gotchas

Several implementations I have seen contain a common error, which I carefully corrected. The bug isn't fatal, but can cause the algorithm to run extra iterations. Two of these are listed below, both as useful references and as a warning to avoid this error.

One in Julia And one in C++

Testing

npm test

If you have suggestions for particularly trying functions, or better tests, please hit me up 😁

TODO:

• More thorough description than a wiki link
• Usage stats

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License

ISC, see license for details.