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Find zeros of a function using the Modified Newton-Raphson method

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The Newton-Raphson method uses the tangent of a curve to iteratively approximate a zero of a function, f(x). The Modified Newton-Raphson [1][2] method uses the fact that f(x) and u(x) := f(x)/f'(x) have the same zeros and instead approximates a zero of u(x). That is, by defining

u(x) = \frac{f(x)}{f'(x)},

the Newton-Raphson update for u(x),

x_{i + 1} = x_i -\frac{u(x_i)}{u'(x_i)},


x_{i+1} = x_i - \frac{f(x_i)f'(x_i)}{(f'(x_i))^2 - f(x_i)f''(x_i)}.

In other words, by effectively estimating the order of convergence, it overrelaxes or underrelaxes the update, in particular converging much more quickly for roots with multiplicity greater than 1.


Consider the zero of (x + 2) * (x - 1)^4 at x = 1. Due to its multiplicity, Newton-Raphson is likely to reach the maximum number of iterations before converging. Modified Newton-Raphson computes the multiplicity and overrelaxes, converging quickly using either provided derivatives or numerical differentiation.

var mnr = require('modified-newton-raphson');
function f   (x) { return Math.pow(- 1, 4) * (+ 2); }
function fp  (x) { return 4 * Math.pow(- 1, 3) * (+ 2) + Math.pow(- 1, 4); }
function fpp (x) { return 12 * Math.pow(- 1, 2) * (+ 2) + 8 * Math.pow(- 1, 3); }
// Using first and second derivatives:
mnr(f, fp, fpp, 2)
// => 1.0000000000000000 (4 iterations)
// Using numerical second derivative:
mnr(f, fp, 2)
// => 1.000000000003979 (4 iterations)
// Using numerical first and second derivatives:
mnr(f, 2)
// => 0.9999999902458561 (4 iterations)


$ npm install modified-newton-raphson


require('modified-newton-raphson')(f[, fp[, fpp]], x0[, options])

Given a real-valued function of one variable, iteratively improves and returns a guess of a zero.


  • f: The numerical function of one variable of which to compute the zero.
  • fp (optional): The first derivative of f. If not provided, is computed numerically using a fourth order central difference with step size h.
  • fpp (optional): The second derivative of f. If both fp and fpp are not provided, is computed numerically using a fourth order central difference with step size h. If fp is provided and fpp is not, then is computed using a fourth order first central difference of fp.
  • x0: A number representing the intial guess of the zero.
  • options (optional): An object permitting the following options:
    • tolerance (default: 1e-7): The tolerance by which convergence is measured. Convergence is met if |x[n+1] - x[n]| <= tolerance * |x[n+1]|.
    • epsilon (default: 2.220446049250313e-16 (double-precision epsilon)): A threshold against which the first derivative is tested. Algorithm fails if |y'| < epsilon * |y|.
    • maxIter (default: 20): Maximum permitted iterations.
    • h (default: 1e-4): Step size for numerical differentiation.
    • verbose (default: false): Output additional information about guesses, convergence, and failure.

Returns: If convergence is achieved, returns an approximation of the zero. If the algorithm fails, returns false.

See Also


[1] Wu, X., Roots of Equations, Course notes.
[2] Mathews, J., The Accelerated and Modified Newton Methods, Course notes.


© 2016 Scijs Authors. MIT License.


Ricky Reusser

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