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authorFranciszek Malinka <franciszek.malinka@gmail.com>2022-07-11 20:13:42 +0200
committerFranciszek Malinka <franciszek.malinka@gmail.com>2022-07-11 20:13:42 +0200
commitad63d0a98d8595d3267fd81196cf8c96361bd911 (patch)
treefc71ed3a1b0ce0f26e28d774fd3f160ce54b1ee4 /sections/conj_classes.tex
parentfa334cef8c04e50a45b366a3427db18e638fc992 (diff)
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@@ -232,22 +232,25 @@
set $A$.
\end{proof}
+ \begin{theorem}
+ \label{theorem:key-theorem}
+ Let $\cC$ be a Fraïssé class of finitely generated $L$-structures with WHP
+ and canonical amalgamation. Then $\Flim(\cC)$ has a generic automorphism.
+ \end{theorem}
+
+ \begin{proof}
+ It follows trivially from Corollary \ref{corollary:whp+canonical-iso}
+ and the above Theorem \ref{theorem:generic_aut_general}.
+ \end{proof}
+
\subsection{Properties of the generic automorphism}
- Let $\cC$ be a Fraïssé class in a finite relational language $L$ with
- weak Hrushovski property. Let $\cH$ be the Fraïssé class of the $L$-structures
- with additional automorphism symbol. Let $\Gamma = \Flim(\cC)$.
-
- % \begin{proposition}
- % Let $\sigma$ be the generic automorphism of the random graph $\FrGr$. Then
- % the graph induced by the set of the fixed points of $\sigma$ is isomorphic
- % to $\FrGr$.
- % \end{proposition}
- %
- % \begin{proof}
- % Let $F = \{v\in\FrGr\mid \sigma(v) = v\}$. It suffices to show that $F$ is
- % infinite and has the random graph property.
- % \end{proof}
+ Let $\cC$ be a Fraïssé class of finitely generated $L$-structures with
+ weak Hrushovski property and canonical amalgamation.
+ Let $\cD$ be the Fraïssé class (by the Theorem \ref{theorem:key-theorem}
+ of the structures of $\cC$ with additional automorphism of the strucutre.
+ Let $\Gamma = \Flim(\cC)$.
+
\begin{proposition}
\label{proposition:fixed_points}
Let $\sigma$ be the generic automorphism of $\Gamma$. Then the set
@@ -257,11 +260,11 @@
\begin{proof}
Let $S = \{x\in \Gamma\mid \sigma(x) = x\}$.
First we need to show that it is an infinite. By the theorem \ref{theorem:generic_aut_general}
- we know that $(\Gamma, \sigma)$ is the Fraïssé limit of $\cH$, thus we
+ we know that $(\Gamma, \sigma)$ is the Fraïssé limit of $\cD$, thus we
can embed finite $L$-structures of any size with identity as an
automorphism of the structure into $(\Gamma, \sigma)$. Thus $S$ has to be
infinite. Also, the same argument shows that the age of the structure is
exactly $\cC$. It is weakly ultrahomogeneous, also by the fact that
- $(\Gamma, \sigma)$ is in $\cH$.
+ $(\Gamma, \sigma)$ is in $\cD$.
\end{proof}
\end{document}