I was reading this post and wondered. Does there exist a topologically transitive (TT) map $f:\mathbb{R}^n\to\mathbb{R}^n$ when $n\geq 2$? I know that post asks for compactness and topological mixing but if we relax the requirement to only TT is it possible?

Note: If $\mathbb{R}^n$ is replaced by an infinite-dimensional Hilbert space, then the Ansari-Bernal theorem guarantees such a map exists; moreover it can be linear... So maybe it can exist in the finite-dimensional case?

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    $\begingroup$ It's overkill, but the answer to the post you cited in your question says that there is a Baire-generic subset of topologically mixing diffeomorphisms in the space of volume-preserving diffeomorphisms of any closed manifold of dimension greater than $1$. You can use this in the $n+1$-dimensional sphere. If you pick any diffeomorphism of this sphere with a hyperbolic fixed point, then arbitrarily close to it there is a topologically mixing diffeomorphism which still has a fixed point. Removing this point you get a topologically mixing (in particular transitive) diffeomorphism of $\mathbb{R}^n$ $\endgroup$ – Andres Koropecki Jun 10 '20 at 15:12

To close this question, let me present a more explicit answer than the one given in the comments.

First, let $w:[0,+\infty)\to [0,+\infty)$ be constructed as follows. On $[1,2]$ define $w$ to be the tent map, with $w(1)=w(2)=0$, and $w(3/2)=4$. Then copy-paste scaled versions of this tent infinitely in both sides so that the graph consists of a sequence of congruent triangles.

It is proven in Silverman - On Maps with Dense Orbits and the Definition of Chaos, p 360, that this map has a property that for every open non-empty $U\subset [0,+\infty)$ there is
$k\in\mathbb{Z}$ and $m\in\mathbb{N}$ such that $[0,2^{k+2r}]\subset w^{2r}(U)$, for $r>m$.

Now consider $v:[0,+\infty)^n\to [0,+\infty)^n$ defined by $v(x_1,...,x_n)=(w(x_1),...,w(x_n))$. Let $U\subset [0,+\infty)^n$ be open and non-empty. There are $U_i\subset [0,+\infty)$, such that $U_1\times...\times U_n\subset U$. From the property of $w$ there are $k_1,...,k_n\in\mathbb{Z}$ and $m\in\mathbb{N}$ such that $[0,2^{k_1+2r}]\times...\times[0,2^{k_m+2r}] \subset v^{2r}(U_1\times...\times U_n)\subset v^{2r}(U)$, for $r>m$.

Now observe that $[0,+\infty)^n$ is homeomorphic to $W=[0,+\infty)\times(-\infty,+\infty) ^{n-1}$ with the boundary corresponding to the boundary. Below I will view $v$ as a map on $W$. Then the set $\{(x_1,...,x_n), x_1=0\}$ is invariant with respect to $v$, and also if $U\subset W$ is open and nonempty we have $\bigcup v^{r}(U)= W$.

Let $h:\mathbb{R}^n\to \mathbb{R}^n$ be the reflection $f(x_1,...,x_n)=(-x_1,x_2,...,x_n)$. Define $f:\mathbb{R}^n\to \mathbb{R}^n$ by

$$f\left(x_1,...,x_n\right)=\left\{\begin{array}{ll} h(v(x_1,...,x_n)) & x_1\ge 0 \\ v(h(x_1,...,x_n)) & x_1\le 0 \end{array}\right.$$

Note that when $x_1=0$, $h$ does nothing, but also since $v$ maps boundary into the boundary, the first coordinate of $v(x_1,...,x_n)$ is $0$, from where the map is well-defined, and therefore continuous. Observe that $f^2|_{W}=v^2$, and so if $U\subset \mathbb{R}^n$ is open and nonempty one can show that $\bigcup f^{r}(U)= \mathbb{R}^n$, which implies TT.

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    $\begingroup$ The last couple steps you use to modify the n-fold tent map is very nice. $\endgroup$ – BLBA Jun 14 '20 at 6:52
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    $\begingroup$ @AnnieLeKatsu thanks, but for the most part it's just a modification of the Silverman's construction $\endgroup$ – erz Jun 14 '20 at 8:16

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