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Student[NumericalAnalysis]

 Steffensen
 numerically approximate the real roots of an expression using Steffensen's method

 Calling Sequence Steffensen(f, x=a, opts) Steffensen(f, a, opts)

Parameters

 f - algebraic; expression in the variable x representing a continuous function x - name; the independent variable of f a - numeric; the initial approximate root opts - (optional) equation(s) of the form keyword=value, where keyword is one of fixedpointiterator, functionoptions, lineoptions, maxiterations, output, pointoptions, showfunction, showlines, showpoints, stoppingcriterion, tickmarks, caption, tolerance, verticallineoptions, view; the options for approximating the roots of f

Options

 • fixedpointiterator = algebraic (optional)
 An expression that will be used to generate the fixed-point iteration sequence. If this option is specified, the first argument, f, must be omitted. See the Notes section for more details.
 • functionoptions = list
 A list of options for the plot of the expression f. By default, f is plotted as a solid red line.
 • lineoptions = list
 A list of options for the lines on the plot. By default the lines are solid blue.
 • maxiterations = posint
 The maximum number of iterations to to perform. The default value of maxiterations depends on which type of output is chosen:
 – output = value: default maxiterations = 100
 – output = sequence: default maxiterations = 10
 – output = information: default maxiterations = 10
 – output = plot: default maxiterations = 5
 – output = animation: default maxiterations = 10
 • output = value, sequence, plot, animation, or information
 The return value of the function. The default is value.
 – output = value returns the final numerical approximation of the root.
 – output = sequence returns an expression sequence ${p}_{k}$, $k$=$0..n$ that converges to the exact root for a sufficiently well-behaved function and initial approximation.
 – output = plot returns a plot of f with each iterative approximation shown and the relevant information about the numerical approximation displayed in the caption of the plot.
 – output = animation returns an animation showing the iterations of the root approximation process.
 – output = information returns detailed information about the iterative approximations of the root of f.
 • plotoptions = list
 The final plot options when output = plot or output = animation.
 • pointoptions = list
 A list of options for the points on the plot. By default, the points are plotted as green circles.
 • showfunction = truefalse
 Whether to display f on the plot or not.  By default, this option is set to true.
 • showlines = truefalse
 Whether to display lines that accentuate each approximate iteration when output = plot. By default, this option is set to true.
 • showpoints = truefalse
 Whether to display the points at each approximate iteration on the plot when output = plot. By default, this option is set to true.
 • stoppingcriterion = relative, absolute, or function_value
 The criterion that the approximations must meet before discontinuing the iterations. The following describes each criterion:
 – relative : $\frac{\left|{p}_{n}-{p}_{n-1}\right|}{\left|{p}_{n}\right|}$ < tolerance
 – absolute : $\left|{p}_{n}-{p}_{n-1}\right|$ < tolerance
 – function_value : $\left|f\left({p}_{n}\right)\right|$ < tolerance
 By default, stoppingcriterion = relative.
 • tickmarks = list
 The tickmarks when output = plot or output = animation. By default, tickmarks are placed at the initial and final approximations with the labels ${p}_{0}$ (or a and b for two initial approximates) and ${p}_{n}$, where $n$ is the total number of iterations used to reach the final approximation. See plot/tickmarks for more detail on specifying tickmarks.
 • caption = string
 A caption for the plot. The default caption contains general information concerning the approximation. For more information about specifying a caption, see plot/typesetting.
 • tolerance = positive
 The error tolerance of the approximation. The default value is $\frac{1}{10000}$.
 • verticallineoptions = list
 A list of options for the vertical lines on the plot. By default, the lines are dashed and blue.
 • view = [realcons..realcons, realcons..realcons]
 The plotview of the plot when output = plot.  See plot/options for more information.

Description

 • The Steffensen command numerically approximates the roots of an algebraic function, f, using fixed-point iteration coupled with a slightly modified version of Aitken's ${\mathrm{\Delta }}^{2}$ technique of accelerating sequential convergence.
 • Given an expression f and an initial approximate a, the Steffensen command computes a sequence ${p}_{k}$, $k$=$0..n$, of approximations to a root of f, where $n$ is the number of iterations taken to reach a stopping criterion. For sufficiently well-behaved functions and sufficiently good initial approximations, the convergence of ${p}_{k}$ toward the exact root is quadratic.
 • The first argument f may be substituted with an option of the form fixedpointiterator = fpexpr. See Notes.
 • The Steffensen command is a shortcut for calling the Roots command with the method=steffensen option.

Notes

 • This procedure first converts the problem of finding a root to the equation $f\left(x\right)=0$ to a problem of finding a fixed point for the function $g\left(x\right)$, where $g\left(x\right)=x-f\left(x\right)$ and $f\left(x\right)$ is specified by f and x.
 • The user can specify a custom iterator function $g\left(x\right)$ by omitting the first argument f and supplying the fixedpointiterator = g option. The right-hand side expression g specifies a function $g\left(x\right)$, and this procedure will aim to find a root to $f\left(x\right)$ = $x-g\left(x\right)=0$ by way of solving the fixed-point problem $g\left(x\right)=x$.
 When output = plot or output = animation is specified, both the function $f\left(x\right)$ and the fixed-point iterator function $g\left(x\right)$ will be plotted and correspondingly labelled.
 The tolerance option, when stoppingcriterion = function_value, applies to the function $f\left(x\right)$ in the root-finding form of the problem.

Examples

 > $\mathrm{with}\left(\mathrm{Student}\left[\mathrm{NumericalAnalysis}\right]\right):$
 > $f≔\frac{{x}^{2}}{3}-1:$
 > $\mathrm{Steffensen}\left(f,x=2.0,\mathrm{tolerance}={10}^{-2}\right)$
 ${1.732049797}$ (1)
 > $\mathrm{Steffensen}\left(f,x=2.0,\mathrm{tolerance}={10}^{-2},\mathrm{output}=\mathrm{sequence}\right)$
 ${2.0}{,}{1.727272728}{,}{1.732049797}$ (2)
 > $\mathrm{Steffensen}\left(f,x=2,\mathrm{output}=\mathrm{plot},\mathrm{stoppingcriterion}=\mathrm{function_value}\right)$
 > $\mathrm{Steffensen}\left(f,x=1.3,\mathrm{output}=\mathrm{animation},\mathrm{stoppingcriterion}=\mathrm{absolute}\right)$