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ZoƩ Christoff edited untitled.tex
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Let us first recall some vocabulary.
Let $G = \tuple{\N, R}$ be a graph and $R^*$ be the transitive and symmetric closure of $R$.
\begin{itemize}
\item[]A \emph{path} is a sequence of nodes $$, such that, for all $l\in\{1,\dots,k\}$,
$i_lRi_{l+1}$. $i_lRi_{l+1}$;
\item[]A \emph{cycle} is a path of length $k$ such that
$i_1=i_k$. $i_1=i_k$;
\item[]A set of nodes $S\subseteq \N$ is said to be:
\begin{itemize}
\item[]\emph{connected} if for any $i,j \in \S$: $iR^*j$,
\item[]\emph{strongly connected} if for any $i,j \in \S$: $iRj$,
\item[]\emph{closed} if for any $i\in S$, $j\notin S$, it is not the case that
$iRj$ $iRj$,
\item[]a \emph{connected component} if for any $i,j \in \N$: $iR^*j$ if and only if $i,j\in
C$. C$,
\item[]\emph{aperiodic} if the greatest common divisor of its cycles is $1$.
\end{itemize}
\end{itemize}
We note Note that
a graph which is the graphs we are interested in (finite, serial and
functional contains exactly one cycle in functional) come with a special shape: each of
its finite their connected
components, components contains exactly one cycle, and this cycle
has to be forms the ``tail" of the component:
\begin{fact} \label{fact:connected}
Let $G$ be an influence graph and $C$ be a connected component of $G$.
Then $C$ contains exactly one
cycle, and cycle.
%and this cycle is a closed set.
\end{fact}
\begin{proof}
Assume that $C$ does not contain any cycle.
Then Then, since $\N$ is finite and no path
repeats any node, any path in $C$
can contain twice is finite. Let $i$ be the
same node. last element of the longuest path in $C$. Then $i$ does not have any successor, which contradicts seriality.
Assume that $C$ contains
a cycle which is not closed. Then more than one cycle. Let $i,j\in C$ be such that $i$ is a member of a cycle, and $j$ a member of a different cycle. By connectedness, there is a path from $i$ to $j$, so there exists a node in one of the cycles which has a successor outside of
the this cycle,
but that which contradicts functionality.
Therefore, $C$ contains exactly one cycle. If the cycle were not closed,
Now assume that $C$ contains no cycle.
\end{proof}
\subsection{Backdrop: extra graph theoretical remark (WE DON'T NEED THIS ONE)}