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authorFranciszek Malinka <franciszek.malinka@gmail.com>2022-03-26 15:25:46 +0100
committerFranciszek Malinka <franciszek.malinka@gmail.com>2022-03-26 15:25:46 +0100
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% $\Flim(\bK)$ is $\aleph_0$-categorical and has quantifier elimination.
% \end{fact}
- \section{Conjugacy classes in automorphism groups}
-
- \subsection{Prototype: pure set}
- In this section, $M=(M,=)$ is an infinite countable set (with no structure
- beyond equality).
- \begin{proposition}
- If $f_1,f_2\in \Aut(M)$, then $f_1$ and $f_2$ are conjugate if and only
- if for each $n\in \bN\cup \{\aleph_0\}$, $f_1$ and $f_2$ have the same
- number of orbits of size $n$.
- \end{proposition}
-
- \begin{proposition} The conjugacy class of $f\in \Aut(M)$ is dense if
- and only if... \end{proposition} \begin{proposition} If $f\in
- \Aut(M)$ has an infinite orbit, then the conjugacy class of $f$ is
- meagre.
- \end{proposition}
-
- % \begin{proposition}
- % An automorphism $f$ of $M$ is generic if and only if...
- % \end{proposition}
-
- % \begin{proof}
-
- % \end{proof}
-
- \subsection{More general structures}
-
- \begin{fact}
- Suppose $M$ is an arbitrary structure and $f_1,f_2\in \Aut(M)$.
- Then $f_1$ and $f_2$ are conjugate if and only if $(M,f_1)\cong
- (M,f_2)$ as structures with one additional unary relation that is
- an automorphism.
- \end{fact}
-
- \begin{proof}
- Suppose that $f_1 = g^{-1}f_2g$ for some $g\in \Aut(M)$.
- Then $g$ is the automorphism we're looking for. On the other hand if
- $g\colon (M, f_1)\to (M, f_2)$ is an isomorphism, then
- $g\circ f_1 = f_2 \circ g$ which exactly means that $f_1, f_2$ conjugate.
- \end{proof}
-
- \begin{definition} We say that a Fraïssé class $\bK$ has \emph{weak
- Hrushovski property} (\emph{WHP}) if for every $A\in \bK$ and an isomorphism
- of substructures of $A$ $p\colon A\to A$, there is some $B\in \bK$ such
- that $p$ can be extended to an automorphism of $B$, i.e.\ there is an
- embedding $i\colon A\to B$ and a $\bar p\in \Aut(B)$ such that the following
- diagram commutes:
- \begin{center}
- \begin{tikzcd}
- B\ar[r,dashed,"\bar p"]&B\\
- A\ar[u,dashed,"i"]\ar[r,"p"]&A\ar[u,dashed,"i"]
- \end{tikzcd}
- \end{center}
- \end{definition}
-
- % \begin{proposition} Suppose $\cC$ is a Fraïssé class in a relational
- % language with WHP. Then generically, for an $f\in \Aut(\Flim(\cC))$, all
- % orbits of $f$ are finite. \end{proposition} \begin{proposition} Suppose
- % $\cC$ is a Fraïssé class in an arbitrary countable language with WHP.
- % Then generically, for an $f\in \Aut(\Flim(\cC))$ ... \end{proposition}
-
\subsection{Random graph}
In this section we'll take a closer look on a class of finite graphs, which
@@ -605,7 +544,69 @@
It may be there some day, but it may not!
\end{proof}
- % \begin{proposition} Generically, the set of fixed points of $f\in
+ \section{Conjugacy classes in automorphism groups}
+
+ \subsection{Prototype: pure set}
+ In this section, $M=(M,=)$ is an infinite countable set (with no structure
+ beyond equality).
+ \begin{proposition}
+ If $f_1,f_2\in \Aut(M)$, then $f_1$ and $f_2$ are conjugate if and only
+ if for each $n\in \bN\cup \{\aleph_0\}$, $f_1$ and $f_2$ have the same
+ number of orbits of size $n$.
+ \end{proposition}
+
+ \begin{proposition} The conjugacy class of $f\in \Aut(M)$ is dense if
+ and only if... \end{proposition} \begin{proposition} If $f\in
+ \Aut(M)$ has an infinite orbit, then the conjugacy class of $f$ is
+ meagre.
+ \end{proposition}
+
+ % \begin{proposition}
+ % An automorphism $f$ of $M$ is generic if and only if...
+ % \end{proposition}
+
+ % \begin{proof}
+
+ % \end{proof}
+
+ \subsection{More general structures}
+
+ \begin{fact}
+ Suppose $M$ is an arbitrary structure and $f_1,f_2\in \Aut(M)$.
+ Then $f_1$ and $f_2$ are conjugate if and only if $(M,f_1)\cong
+ (M,f_2)$ as structures with one additional unary relation that is
+ an automorphism.
+ \end{fact}
+
+ \begin{proof}
+ Suppose that $f_1 = g^{-1}f_2g$ for some $g\in \Aut(M)$.
+ Then $g$ is the automorphism we're looking for. On the other hand if
+ $g\colon (M, f_1)\to (M, f_2)$ is an isomorphism, then
+ $g\circ f_1 = f_2 \circ g$ which exactly means that $f_1, f_2$ conjugate.
+ \end{proof}
+
+ \begin{definition} We say that a Fraïssé class $\bK$ has \emph{weak
+ Hrushovski property} (\emph{WHP}) if for every $A\in \bK$ and an isomorphism
+ of substructures of $A$ $p\colon A\to A$, there is some $B\in \bK$ such
+ that $p$ can be extended to an automorphism of $B$, i.e.\ there is an
+ embedding $i\colon A\to B$ and a $\bar p\in \Aut(B)$ such that the following
+ diagram commutes:
+ \begin{center}
+ \begin{tikzcd}
+ B\ar[r,dashed,"\bar p"]&B\\
+ A\ar[u,dashed,"i"]\ar[r,"p"]&A\ar[u,dashed,"i"]
+ \end{tikzcd}
+ \end{center}
+ \end{definition}
+
+ % \begin{proposition} Suppose $\cC$ is a Fraïssé class in a relational
+ % language with WHP. Then generically, for an $f\in \Aut(\Flim(\cC))$, all
+ % orbits of $f$ are finite. \end{proposition} \begin{proposition} Suppose
+ % $\cC$ is a Fraïssé class in an arbitrary countable language with WHP.
+ % Then generically, for an $f\in \Aut(\Flim(\cC))$ ... \end{proposition}
+
+ % \begin{proposition} Generically, the set of fixed points of $f\in
% \Aut(M)$ is isomorphic to $M$ (as a graph). \end{proposition}
+
\printbibliography
\end{document}