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Sidi's generalized secant method
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Sidi's generalized secant method : ウィキペディア英語版
Sidi's generalized secant method
Sidi's generalized secant method is a root-finding algorithm, that is, a numerical method for solving equations of the form f(x)=0 . The method was published
by Avram Sidi.〔
Sidi, Avram, "Generalization Of The Secant Method For Nonlinear Equations", Applied Mathematics E-notes 8 (2008), 115–123, http://www.math.nthu.edu.tw/~amen/2008/070227-1.pdf

The method is a generalization of the secant method. Like the secant method, it is an iterative method which requires one evaluation of f in each iteration and no derivatives of f. The method can converge much faster though, with an order which approaches 2 provided that f satisfies the regularity conditions described below.
== Algorithm ==

We call \alpha the root of f, that is, f(\alpha)=0. Sidi's method is an iterative method which generates a sequence \ of approximations of \alpha. Starting with ''k'' + 1 initial approximations x_1 , \dots , x_, the approximation x_ is calculated in the first iteration, the approximation x_ is calculated in the second iteration, etc. Each iteration takes as input the last ''k'' + 1 approximations and the value of f at those approximations. Hence the ''n''th iteration takes as input the approximations x_n , \dots , x_ and the values f(x_n) , \dots , f(x_).
The number ''k'' must be 1 or larger: ''k'' = 1, 2, 3, .... It remains fixed during the execution of the algorithm. In order to obtain the starting approximations x_1 , \dots , x_ one could carry out a few initializing iterations with a lower value of ''k''.
The approximation x_ is calculated as follows in the ''n''th iteration. A polynomial of interpolation p_ (x) of degree ''k'' is fitted to the ''k'' + 1 points (x_n, f(x_n)), \dots (x_, f(x_)). With this polynomial, the next approximation x_ of \alpha is calculated as
with p_'(x_) the derivative of p_ at x_. Having calculated x_ one calculates f(x_) and the algorithm can continue with the (''n'' + 1)th iteration. Clearly, this method requires the function f to be evaluated only once per iteration; it requires no derivatives of f.
The iterative cycle is stopped if an appropriate stop-criterion is met. Typically the criterion is that the last calculated approximation is close enough to the sought-after root \alpha.
To execute the algorithm effectively, Sidi's method calculates the interpolating polynomial p_ (x) in its Newton form.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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