Continuous decreasing function has a fixed point 
Let $f$ be continuous and decreasing everywhere on $\mathbb{R}$. Show that:
1) $f$ has a unique fixed point
2) $f\circ f$ has either an infinite number of fixed points or an odd number of fixed points.

The first part is easy and I am sure it is available on this website. The basic idea is to use the fact that a decreasing function satisfies $\lim_{x \to -\infty}f(x) = A\text{ or }\infty$ and $\lim_{x \to \infty}f(x) = B\text{ or }-\infty$ and in each case apply the intermediate value theorem on $g(x) = f(x) - x$. The uniqueness of the fixed point is also easy to understand as $f(a) - a = 0 = f(b) - b$ would imply $b - a = f(b) - f(a)$. If $b \neq a$ then this goes against the decreasing nature of $f$.
It is the second part of the problem which is bit troublesome. Let $h(x) = f(f(x))$ let $c$ be the unique fixed point of $f$ so that $f(c) = c$. This means that $f(f(c)) = f(c) = c$ so that $c$ is also a fixed point of $h = f \circ f$. But counting the number of fixed points of $h$ seems tricky. Any hints are welcome!

Hint:


*

*Let $d = f(f(d))$.

*If $d$ is a fix-point of $f$, then there is only one such $d$.

*Otherwise $d \neq f(d)$; let $\hat{d} = f(d)$, then $$f(f(\hat{d})) = f(f(f(d))) = f(d) = \hat{d},$$ that is, $\hat{d}$ is another fix-point of $(f\circ f)$.


I hope this helps $\ddot\smile$
