I wish to use the Brouwer's fixed point theorem to show that continuous, bounded function $f$ from $[0, \infty) \times [0, \infty)$ to $[0, \infty) \times [0, \infty)$ has a fixed point.
However, my version of the Brouwer's fixed point theorem (BFPT) states that,
Let $A \subset \mathbb{R}^2$ be a nonempty convex compact set, then every continuous
function $f: A \to A$ has a fixed point in $A$.
I need to use Brouwer's fixed point theorem for this problem (not homework by the way). The main difficulty is that the sets are closed but not compact.
In 1D, we have, suppose that $f$ is bounded, then let $M$ be the upper bound of $f$, this means $|f| \leq M, \forall x \in [0, \infty)$. So we can restrict the range of $f$ to $[0, M]$ which is compact. Then $f: [0, M] \to [0,M]$ has a fixed point by BFPT.
I am not sure how to upgrade this to 2D. I think the idea is the similar. But the notion of upper bound is strange. Can someone please help?
Best Answer
Let $f=(f_1,f_2)$. Then $\|f(x)\| \leq M$ for all $x$ implies that $|f_1(x)| \leq M$ and $|f_2(x)| \leq M$. Hence $f$ maps $[0,M]\times [0,M]$ into itself. Since the rectangle $[0,M]\times [0,M]$ is compact and convex there exists $(x,y)$ in it with $f(x,y)=(x,y)$.