# Error Bound of the Fixed Point of Contraction Mappings

This afternoon, I read the proof of Banach fixed-point theorem in Wikipedia.1 It’s said that

\begin{equation} d(x^*,x_n) \le \frac{q^n}{1 - q} d(x_1,x_0). \label{eq:inf_err} \end{equation}

In the proofs for the lemmas, I could only find something like $x_k$ inside the brackets, but not $x^$. Thus, I *couldn’t figure out how one can derive inequality \eqref{eq:inf_err} from an inequality derived in the proof of Lemma 2.

\begin{equation} d(x_m,x_n) \le \frac{q^n}{1 - q} d(x_1,x_0), \text{ where } m > n. \label{eq:finite_err} \end{equation}

I googled for some notes, and found one which told me to take the limit of the L.H.S. of inequality \eqref{eq:finite_err} as $m \to \infty$.2 After looking at Corollary 2.4 in the PDF file in footnote #2 for a while, I know what I’ve missed.

If $\left\{ p_k \right\}$ converges to $p$,

\begin{equation} \lim_{k \to \infty} d(p_k,q) = d(p,q) = d\left(\lim_{k \to \infty} p_k,q \right) \label{eq:dist_limit} \end{equation}

That’s why I wrote the previous post.

With equation \eqref{eq:dist_limit}, I can now derive \eqref{eq:inf_err} from \eqref{eq:finite_err}.

$\because \lim\limits_{k \to \infty} x_k = x_*$

\begin{aligned} d(x^*,x_n) =& d\left( \lim_{k \to \infty} x_k,x_n\right) \\ =& \lim_{k \to \infty} d(x_k,x_n) \\ \le& \lim_{k \to \infty} \frac{q^n}{1 - q} d(x_1,x_0) \qquad \text{(by \eqref{eq:finite_err})} \\ =& \frac{q^n}{1 - q} d(x_1,x_0) \end{aligned}

1. Banach fixed-point theorem. (2014, July 15). In Wikipedia, The Free Encyclopedia. Retrieved 17:34, August 10, 2014, from http://en.wikipedia.org/w/index.php?title=Banach_fixed-point_theorem&oldid=617083697

2. Conrad, K. (2014). The contraction mapping theorem. Expository paper. University of Connecticut, College of Liberal Arts and Sciences, Department of Mathematics. Retrieved August 10,2014, from http://www.math.uconn.edu/~kconrad/blurbs/analysis/contraction.pdf