The difference between non-wandering points and recurrent points in dynamical systems

dynamical systemsgeneral-topologytopological-dynamics

I didn't understand what is the difference between non-wandering points and recurrent points on dynamical systems.

An example would be very appreciated.

Best Answer

In general any recurrent point is a non-wandering point, but not vice versa.

For an example consider the expanding map $f:\mathbb{T}\to \mathbb{T}, x\mapsto 2x$ and the point $x_0=\dfrac{1}{2}$. Then $f^n(x_0)=0$ for $n\in\mathbb{Z}_{\geq1}$, so that $x_0$ is not an $f$-recurrent point (the orbit of $x_0$ does not approximate $x_0$ arbitrarily well).

However it is $f$-non-wandering, since if $U\ni x_0$ is an open arc containing $x_0$, $f^n(U)$ will have length $2^n$ times the length of $U$, so that for some $n\in\mathbb{Z}_{\geq1}$, $f^n(U)$ will cover the whole circle; in particular $f^n(U)\cap U\neq \emptyset$.