Edmond Halley was an English mathematician who introduced the method now called by his name. Halley's method is a numerical algorithm for solving the nonlinear equation f(x) = 0. In this case, the function f has to be a function of one real variable. The method consists of a sequence of iterations:
If f is a three times continuously differentiable function and a is a zero of f but not of its derivative, then, in a neighborhood of a, the iterates xn satisfy:
This means that the iterates converge to the zero if the initial guess is sufficiently close, and that the convergence is cubic.
The following alternative formulation shows the similarity between Halley's method and Newton's method. The expression is computed only once, and it is particularly useful when can be simplified:
When the second derivative is very close to zero, the Halley's method iteration is almost the same as the Newton's method iteration.
Consider the function
Any root of f which is not a root of its derivative is a root of g; and any root r of g must be a root of f provided the derivative of f at r is not infinite. Applying Newton's method to g gives
and the result follows. Notice that if f′ (c) = 0, then one cannot apply this at c because g(c) would be undefined.
Suppose a is a root of f but not of its derivative. And suppose that the third derivative of f exists and is continuous in a neighborhood of a and xn is in that neighborhood. Then Taylor's theorem implies:
where ξ and η are numbers lying between a and xn. Multiply the first equation by and subtract from it the second equation times to give:
Canceling and re-organizing terms yields:
Put the second term on the left side and divide through by
The limit of the coefficient on the right side as xn → a is:
If we take K to be a little larger than the absolute value of this, we can take absolute values of both sides of the formula and replace the absolute value of coefficient by its upper bound near a to get:
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^Proinov, Petko D.; Ivanov, Stoil I. (2015). "On the convergence of Halley's method for simultaneous computation of polynomial zeros". J. Numer. Math. 23 (4): 379–394. doi:10.1515/jnma-2015-0026. S2CID 10356202.