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figures placement
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gavryushkin committed Jul 23, 2020
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Expand Up @@ -403,19 +403,19 @@ \section{$\findpath$ computes shortest paths in optimal time}
In this case, $C_k$ intersects $C$ and $D$ but not $A$ or $B$.
And we have the following two possibilities to consider.

\begin{enumerate}[label = \theenumi.\arabic*]
\item The ranks of $(C_k)_{T_1}$ and $(C_k)_R$ coincide.
In this case, the previous cluster $C_{k-1}$ of $R$ has to be $A \cup B$.
Since $A \cup B$ is not a cluster in $T'$, the first $\rnni$ move on $\fp(T', R)$ builds the cluster $A \cup B$ by swapping subtrees induced by cluster $B$ and $C$.
This move results in $T'_1 = T$ contradicting $|\fp(T',R)| < |\fp(T,R)| - 1$.

\begin{figure}[ht]
\centering
\includegraphics[width=0.4\textwidth]{thm_fp_nni3}
\caption{Comparison of paths $\fp(T, R)$ and $\fp(T', R)$ if there is an $\nni$ move between $T$ and $T'$ and a rank move on the interval above this edge follows on $\fp(T, R)$.}
\label{fig:thm_fp_nni3}
\end{figure}

\begin{enumerate}[label = \theenumi.\arabic*]
\item The ranks of $(C_k)_{T_1}$ and $(C_k)_R$ coincide.
In this case, the previous cluster $C_{k-1}$ of $R$ has to be $A \cup B$.
Since $A \cup B$ is not a cluster in $T'$, the first $\rnni$ move on $\fp(T', R)$ builds the cluster $A \cup B$ by swapping subtrees induced by cluster $B$ and $C$.
This move results in $T'_1 = T$ contradicting $|\fp(T',R)| < |\fp(T,R)| - 1$.

\item The rank of $(C_k)_{T_1}$ is strictly higher than that of $(C_k)_R$.
In this case, $\findpath$ decreases the rank of $(C_k)_{T_1}$ in the second step.
This results in the path from $T$ to the rightmost tree in Figure~\ref{fig:thm_fp_nni2a}.
Expand Down Expand Up @@ -546,20 +546,20 @@ \section{For what $\rho$ is $\decprob{\rho}$ polynomial?}
In the rest of this section, we show that the $\findpath$ algorithm substantially relies on the fact that the rank move and the $\nni$ move have the same weight in the $\rnni$ graph.
This suggests that a non-trivial algorithmic insight is necessary to extend our polynomial complexity result to other values of $\rho$.

\begin{proposition}
$\findpath$ does not compute shortest paths in $\rnni(\rho)$ for $\rho \neq 1$.
\label{prop:fp_only_rnni}
\end{proposition}

\proof

\begin{figure}[ht]
\centering
\includegraphics[width=0.6\textwidth]{fp_rho_greater_1_counterexample}
\caption{Path computed by $\findpath$ (top) and a shorter path (bottom) for $\rho > 1$.}
\label{fig:fp_rho_greater_1_counterexample}
\end{figure}

\begin{proposition}
$\findpath$ does not compute shortest paths in $\rnni(\rho)$ for $\rho \neq 1$.
\label{prop:fp_only_rnni}
\end{proposition}

\proof

For $\rho > 1$ a counterexample is given by the following trees (see Figure~\ref{fig:fp_rho_greater_1_counterexample})
\begin{align*}
T &= [\{a_1,a_2\},\{a_1,a_2,a_3\},\{a_1,a_2,a_3,a_4\}]\text{ and }\\
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