# Contraction mappings in $L^p$ and other function spaces

If $X$ is a complete metric space, then a contraction mapping is defined by $T:X\rightarrow X$ where $d(T(x),T(y))\leq cd(x,y)$ where $0\leq c < 1$ and $d$ is the appropriate metric/norm.

I know if we are considering spaces such as $[0,1]$ and the function $f$, it means the function $f$ has a fixed point, namely $f(x)=x$ where $x\in [0,1]$. How should interpret $T:L^p([a,b])\rightarrow L^p([a,b])$ via an operator $T(g)$? Does this mean there is a unique function $g$ in $L^p([a,b])$ where $T(g)=g$? In other words, the operator inputs a function from the domain space and outputs a function from the same space? I am struggling to get a clear intuition for this.

Next, could we prove the Fourier transform $\mathcal{F}:L^2([0,1])\rightarrow L^2([0,1])$ is a contraction mapping (on a suitable subset) via the following argument:

$(1)$ WTS $||\mathcal{F}f-\mathcal{F}g||_{L^2}\leq c||f-g||_{L^2}$

$(2)$ consider the closed unit ball in $L^2$, i.e. $B_{1}=\{f\in L^2([0,1]):||f||_{L^2}\leq 1\}$

Take the Fourier transform $\mathcal{F}:B_1\rightarrow B_1$ as $\mathcal{F}=\int_{0}^{1}f(x)e^{-2\pi i x} dx$ and show this is a contraction on $B_1$ via the definition.

$(3)$ conclude the Fourier transform maps $L^2$ functions to other $L^2$ functions on the unit ball.

Finally, what other interesting operators and contraction mappings can we consider for $L^p$ and other function spaces?

• It should be $0\le c<1$. – Chee Han May 7 '17 at 1:27
• @CheeHan edited. – Kernel_Dirichlet May 7 '17 at 1:29
• Interpreting an operator has nothing to do with fixed points. You should think of fixed points as an additional structure that an operator might have. – Chee Han May 7 '17 at 1:30
• That makes sense, but so then should I just view an operator (that has fixed points) as taking a function from one function space $X$ and outputting a function in the same space $X$? – Kernel_Dirichlet May 7 '17 at 1:35
• No, what you just said follows if you have an operator $T$ that maps from $X$ to $X$. If, in addition to this, $T$ has a fixed point $x\in X$, this means that $T$ maps this particular element $x$ to itself again. – Chee Han May 7 '17 at 1:39

## 1 Answer

Yes, an operator $T:L^p([a,b])\to L^p([a,b])$ takes a function $f$ and maps it into another function $T(f)$ in the same space. So having a fixed point, would mean that $T(f)=f$, that is, that the function $f$ is mapped into itself. For example, you could take $T(f)(x)=\int_a^x f(t)\,dt$ and so a fixed point for $T$ would be a function $f$ such that $f(x)=\int_a^x f(t)dt$ for every $x\in [a,b]$. The Fourier transform is defined for functions in $L^2(\mathbb{R})$ and not $L^2([0,1])$. Parseval identity gives $\Vert\mathcal{F(f)}\Vert_{L^2}=\Vert f\Vert_{L^2}$ and so the Fourier transform cannot be a contraction.

• Very helpful! So basically the Fourier transform is an operator on $L^2(\mathbb{R})$ and not a contraction because we have $c=1$ by Parseval's identity? – Kernel_Dirichlet May 7 '17 at 2:00
• correct. It is Lipschitz continuous with Lipschitz constant one and not a contraction, – Gio67 May 7 '17 at 2:01