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I'm trying to find out how to use the given interval on which fixed-point iteration will converge, for $f(x) = x^3-2x-5$, with interval $[2,3]$.

Also, how many iterations are necessary to obtain approximations to $10^{-5}$, and displaying the calculations.

What I note is that if we need to solve for:

$x^3 = 2x+5$, and then we can get a $g(x) = \sqrt[3]{2x+5}$, and what is $p_0$?


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For this particular way (there are others) of solving the equation by fixed point iteration, things will be OK as long as we can make sure that the absolute value of the derivative starts and stays below $k$, where $k<1$. In that case the iterates are sucked towards the root. Note that there is, by Intermediate Value Theorem, a root between $2$ and $3$. Starting reasonably close is good. We have $g'(x)=(2/3)(2x+5)^{-2/3}$, and this is adequately small in our target interval, and indeed well beyond. – André Nicolas Jan 28 '12 at 22:30
@Buddy Holly: (More) In your interval, the derivative has absolute value less than $k$, where $k\approx 0.154$. So as long as you make sure we stay in the interval, each iteration multiplies the error by something less than $m$. If the original error $e$ is small, it doesn't take many iterations $i$ to get $em^i$ below $10^{-5}$. – André Nicolas Jan 28 '12 at 22:46
up vote 3 down vote accepted

The post below answers your question, in perhaps too much detail.

The equation $x^3-2x-5=0$ has a solution in the interval $[2,3]$. For let $f(x)=x^3-2x-5$ as you did. Then $f(2)=-1<0$, and $f(3)=16>0$. So by the Intermediate Value Theorem, there is a solution of $f(x)=0$ somewhere between $x=2$ and $x=3$. More informally, the curve $y=f(x)$ is below the $x$-axis when $x=2$, above the $x$-axis when $x=3$, so since $f(x)$ is continuous, the curve must cross the $x$-axis somewhere between $2$ and $3$.

Since $f(2)$ is much closer to $0$ than $f(3)$, it is reasonable to expect that there is a root much closer to $2$ than to $3$. For fun, let's calculate $f(2.1)$. It is about $0.061$, already positive, so there is a root between $2$ and $2.1$.

The equation only has one root. There are various ways to show this. For example, note that $f'(x)=3x^2-2$. So our function is increasing until $x=-\sqrt{2/3}$, then decreasing until $x=\sqrt{2/3}$, then increasing. Since $f(-\sqrt{2/3}<0$, it follows that there is at most one root, and it is $>\sqrt{2/3}$.

But we can get all this, at least informally, by asking a program to graph the curve.

Now to the question you are really asking, about fixed point iteration. There are many ways to use fixed point iteration to solve the equation $f(x)=0$. For instance, the famous Newton Method is actually a form of fixed point iteration.

Your choice of using equivalent equation $g(x)=x$, where $g(x)=\sqrt[3]{2x+5}$, is good. We will soon see why.

Note that $$g'(x)=\frac{2}{3}(2x+5)^{-2/3}.$$ Informally, we can be sure that the fixed point iteration $x_{k+1}=g(x_k)$ converges to a root if there is a non-negative constant $c<1$ such that
for all $i$, $|g'(x_i)|<c$. For then the distance of $x_{k+1}$ from the root is less than $c$ times the distance of $x_k$ from the root.

So if our initial estimate $x_0$ has error $e$, then the error in the estimate $x_k$ is less than $ec^k$.

Let's not work too hard in finding an initial estimate. The choice of $x_0=2$ is fine, and the choice of $x_0=2.1$ is much better. Let's make the not so good choice $x_0=2$. And let's be sloppy about the error estimate. The root is between $2$ and $3$, so the initial error is less than $1$.

In the interval $[2,3]$, $g'(x)$ reaches a maximum at $x=2$. The value of $g'(2)$ is $(2/3)(9^{-2/3})$, which is a little less than $0.155$. We will be sucked towards the root, so $g'(x_k)$ will be positive and less than $0.155$ throughout the calculation.

Now calculate. I get $x_0=2$, $x_1=g(x_0)=2.0800838$, $x_2=2.0923507$, $x_3=2.094217$, $x_4=2.0945007$, $x_5=2.0945438$, $x_6=2.0945503$, $x_6=2.0945513$.

The error of $x_k$ is less (by quite a bit) than $(1)(0.155)^k$. That estimate shows that $k=7$ does the job. (We can do this by experimentation, or by solving $(0.155)^k<10^{-5}$ using logarithms.) But the error in $x_5$ is already less than $10^{-5}$. and if we had started with $x_0=2.10$, we would have gotten close enough with quite a bit less work.

Remark: Note that $g'(x)<0.155$ for all $x\ge 2$. So if we had started with a truly awful first estimate, like $x_0=100$, the procedure would still converge.

There is somewhat less leeway with bad first estimates that are smaller than $2$. We get that $g'(x)=1$ a bit to the left of $x=-2.2278$, so $g'(x)$ is positive and less than $1$ for $x>-2.2278$. So even if we make a bad initial estimate like $x_0=-1$, the fixed point iteration converges to the root.

But remember, much work is saved if our initial estimate is close, so it is always worthwhile to invest some effort to choose $x_0$ well!

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Given an attracting fixed point $p$ of a continuous function $g$, let $(a,b)$ be the immediate basin of attraction of $p$, i.e. the largest interval consisting of points attracted to $p$. There are the following possibilities with $a$ and $b$ finite:

1) $g(a) = a$, $g(b)=b$ : $a$ and $b$ are non-attracting fixed points.

2) $g(a) = a$, $g(b)=a$: $a$ is a non-attracting fixed point, $b$ is a point mapped to $a$.

3) $g(a) = b$, $g(b)=b$: $b$ is a non-attracting fixed point, $a$ is a point mapped to $b$.

4) $g(a) = b$, $g(b) = a$: $a$ and $b$ form a non-attracting 2-cycle.

There are three more possibilities with $a$ or $b$ infinite:

5) $g(a) = a$, $b = +\infty$: $a$ is a non-attracting fixed point.

6) $a = -\infty$, $g(b) = b$: $b$ is a non-attracting fixed point.

7) $a = -\infty$, $b = +\infty$.

In your case $g(x) = (2 x + 5)^{1/3}$ is an increasing function, so only cases (1), (5), (6) and (7) are possible. But $g(x)$ has only one fixed point because $f(x)$ has only one real root, so the interval is $(-\infty, \infty)$ i.e. the iteration converges to the solution no matter what (real) initial point is chosen.

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