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'''Sidi's method''' is a [[root-finding algorithm]], that is, a [[numerical method]] for solving [[equations]] of the form <math>f(x)=0</math> . The method was published
by Avraham Sidi.<ref>
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
</ref><ref>
The home page of Avraham Sidi at the Israel Institute of Technology is at [http://www.cs.technion.ac.il/people/asidi/ Avraham Sidi]
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:<math> s^{k+1} - s^k - s^{k-1} - \dots - s - 1 </math>
We have e.g. <math>\psi_1 = (1+\sqrt{5})/2</math> ≈ 1.6180, <math>\psi_2</math> ≈ 1.8393 and <math>\psi_3</math> ≈ 1.9276. The order approaches 2 from below if ''k'' becomes large: <math> \lim\limits_{k \to \infty} \psi_k = 2</math>
<ref name="muller">
Muller, David E., "A Method for Solving Algebraic Equations Using an Automatic Computer," Mathematical Tables and Other Aids to Computation '''10''' (1956), 208-215
</ref>
Sidi's method has also been discussed
<ref>
Soleymani F. and Hosseinabadi V., "New Third- and Sixth-Order Derivative-Free Techniques for Nonlinear Equations", Journal of Mathematics Research '''3''' (2011), 107-112
</ref>
in the context of an "efficiency index". This index weighs the convergence order of a method against the number of function evaluations per iterative step, the number being 1 in Sidi's case.
== Related algorithms ==
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This is the [[Newton's method|Newton-Raphson method]]. It starts off with a single estimate <math>x_1</math> so we can take ''k'' = 0 in {{EquationNote|2}}. It does not require an interpolating polynomial but instead one has to evaluate the derivative <math>f'</math> in each iteration. Depending on the nature of <math>f</math> this may not be possible or practical.
Once the interpolating polynomial <math>p_{n,k} (x)</math> has been calculated, one can also calculate the next estimate <math>x_{n+k+1}</math> as a solution of <math>p_{n,k} (x)=0</math> instead of using {{EquationNote|1}}. For ''k'' = 1 these two methods are identical: it is the [[secant method]]. For ''k'' = 2 this method is known as [[Muller's method]] <ref name="muller"/>. For ''k'' = 3 this approach involves finding the roots of a [[cubic function]], which is unattractively complicated. This problem becomes worse for even larger values of ''k''. An additional complication is that the equation <math>p_{n,k} (x)=0</math> will in general have [[Properties of polynomial roots|multiple solutions]] and a prescription has to be given which of these solutions is the next estimate <math>x_{n+k+1}</math>. Muller does this for the case ''k'' = 2 but no such prescriptions appear to exist for k > 2.
== References ==
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