diff --git a/inputs/lecture_20.tex b/inputs/lecture_20.tex index afe5663..8e0efde 100644 --- a/inputs/lecture_20.tex +++ b/inputs/lecture_20.tex @@ -135,6 +135,7 @@ coordinates. $x \mapsto x^{n}$. \end{definition} +\gist{% \begin{remark} \label{rem:l20:sigma} Note that for @@ -149,6 +150,7 @@ coordinates. (y, x_{d+1}) &\longmapsto & (\tau_d(y), \sigma(y) x_{d+1}). \end{IEEEeqnarray*} \end{remark} +}{} \begin{theorem} \label{thm:taudminimal:help} For every $d$ if $\tau_d$\footnote{more formally $((S^1)^d, \langle \tau_d \rangle)$} @@ -166,11 +168,14 @@ coordinates. $\tau$ is minimal. \end{corollary} \begin{proof} + \gist{% + We need to show that every orbit is dense. This follows from the definition of the product topology, since for a basic open set $U = U_1 \times \ldots \times U_d \times (S^1)^{\infty}$ it suffices to analyze the first $d$ coordinates. + }{We need to show that every orbit is dense. For this it suffices to analyze finitely many coordinates.} \end{proof} - +\gist{% \begin{refproof}{thm:taudminimal:help} Let $S \coloneqq \tau_d$, $T \coloneqq \tau_{d+1}$ and $Y \coloneqq (S^1)^d$. Consider @@ -193,8 +198,5 @@ coordinates. \phantom\qedhere \end{refproof} - - - - +}{} diff --git a/inputs/lecture_21.tex b/inputs/lecture_21.tex index 9bf9441..95a904b 100644 --- a/inputs/lecture_21.tex +++ b/inputs/lecture_21.tex @@ -1,6 +1,7 @@ \lecture{21}{2024-01-12}{Iterated Skew Shift} \begin{refproof}{thm:taudminimal:help} +\gist{% Suppose towards a contradiction that $Y \times S^1$ contains a proper minimal subflow $Z$. Consider the projection $\pi\colon Y \times S^1 \to Y$. @@ -77,6 +78,41 @@ so $[\phi \circ S \circ \gamma] = [\phi \circ \gamma]$. Thus $[x \mapsto (\sigma(\gamma(x))^n] = 0$, but that is a contradiction to (b) $\lightning$ +}{ + \begin{itemize} + \item $S \coloneqq \tau_d$, $T \coloneqq \tau_{d+1}$ + ($T(y, \xi) = (S(y), \sigma(y) + \xi)$), + $Y \coloneqq (S^1)^d$, + $\gamma: S^1 \to Y, x \mapsto (x,x,\ldots,x)$. + \item (a) $\gamma \simeq S \circ \gamma$. + \item (b) $\forall m \in \Z.~ [m \cdot \sigma \circ \gamma] = m$ + ($\sigma(\gamma(x)) = x$). + \item Suppose $Y \times S^1$ is not minimal. + Let $Z$ be a proper minimal subflow, + $\pi\colon Y \times S^1 \to Y$. + \item $\pi(Z) = Y$ ($Y$ minimal) + \item $Z + (0,\ldots,0,\theta)$ is minimal, so $\theta + Z = Z$ + or $\theta + Z \cap Z = \emptyset$. + \item $H \coloneqq \{\theta \in S^1 : \theta + Z = Z\}$ (closed subgroup of $S^1$). + \begin{itemize} + \item $H \neq S^1$ as $Z$ is a proper subflow. + \item $H = \langle \frac{1}{m} \rangle$. + \end{itemize} + \item $(y, \beta) \in Z$, then $(y, \beta + t) \in Z \iff m \cdot t = 0$. + Pick $\beta^{(y)}$ such that $(y, \xi) \in Z \iff \xi \in \{\beta^{(y)} + \frac{n}{m} : n \in \N\}$. + \item Consider $(y, \xi) \mapsto (y, m \cdot \xi)$. + Get + \begin{IEEEeqnarray*}{rCl} + \phi\colon Y &\longrightarrow & S^1 \\ + y &\longmapsto & m \beta^{(y)} + \end{IEEEeqnarray*} + continuous. + \item $\phi \circ S = m \cdot \phi \circ \sigma$: + $Z$ is $T$-invariant. + \item $[\phi \circ S \circ \gamma] = [\phi \circ \gamma] + [m \sigma \circ \gamma]$ + $\implies [m \sigma \circ \gamma] = 0 \lightning$. + \end{itemize} +} \end{refproof} Let $X_n \coloneqq (S^1)^n$ and $X \coloneqq (S^1)^{\N}$. diff --git a/inputs/lecture_22.tex b/inputs/lecture_22.tex index 6032904..2b99068 100644 --- a/inputs/lecture_22.tex +++ b/inputs/lecture_22.tex @@ -103,7 +103,7 @@ For this we define \end{pmatrix*} \end{IEEEeqnarray*} - +\gist{% \begin{example} Consider the following flow: \begin{IEEEeqnarray*}{rCl} @@ -121,10 +121,12 @@ For this we define \end{IEEEeqnarray*} we can write this as $\tau(x,y,z) = (x \cdot f_1 \circ \pi_1(x,y,z), y \cdot f_2\pi_2(x,y,z), z \cdot f_3\pi_3(x,y,z))$ \end{example} -% \begin{example} -% The skew shift can be written in this form as well: -% TODO -% \end{example} +\begin{example} + The skew shift can be written in this form as well. + Consider $f_1\colon x \mapsto \alpha$ + and $f_n\colon (x_0,\ldots, x_{n-2}) \mapsto x_{n-2}$. +\end{example} +}{} \begin{theorem}[Beleznay Foreman] \label{thm:distalminimalofallranks} @@ -143,9 +145,8 @@ For this we define For all $\overline{f} \in \mathbb{K}_I$, the flow $E_I \overline{f}$ is distal. This is the same as for iterated skew shifts. - % TODO since for $\overline{x}, \overline{y} \in \mathbb{K}^I$, - % $d(x_\alpha, y_\alpha) = d((f(\overline{x})_\alpha, (f(\overline{y})_\alpha))$. - \item Minimality:% + + \item Minimality: \notexaminable{% Let $\langle E_n : n < \omega \rangle$ be an enumeration of a countable basis for $\mathbb{K}^I$. @@ -156,7 +157,7 @@ For this we define \] where $f = E_I \overline{f}$ and $\overline{1} = (1,1,1,\ldots)$. - Beleznay and Foreman showed that $U_n$ is open + \textsc{Beleznay} and \textsc{Foreman} showed that $U_n$ is open and dense for all $n$. So if $\overline{f} \in \bigcap_{n} U_n$, then $\overline{1}$ @@ -165,7 +166,7 @@ For this we define that one orbit is dense (cf.~\yaref{thm:distalflowpartition}). } - \item The order of the flow is $\eta$:% + \item The order of the flow is $\eta$: \notexaminable{% Let $\overline{f} = (f_i)_{i \in I} \in \mathbb{K}_I$. Consider the flows we get from $(f_i)_{i < j}$