Changeset - 9b0f04598ca9
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Andras Gilyen - 8 years ago 2017-09-07 16:40:09
gilyen@clayoquot.swat.cwi.nl
nicer proof
1 file changed with 11 insertions and 11 deletions:
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@@ -640,107 +640,107 @@ The intuition of the following lemma is that the far right can only affect the z
 
 		&=\!\!\!\sum_{\ell, r\in [k]: \ell<r-1}\!\!\!\P^{[k]}([\ell],[r,k]\in\mathcal{P})
 
 		+\bigO{p^{k}} \tag*{$\left(\P^{[k]}([k]\in\mathcal{P})=\bigO{p^{k}}\right)$}\\	
 
 		&=\!\!\!\sum_{\ell, r\in [k]: \ell<r-1}\!\!\!
 
 		\P^{[\ell+1]}_{b_{\ell+1}=1}([\ell]\in\mathcal{P})
 
 		\P^{[\ell+1,r-1]}(\NZ{\ell+1}\cap \NZ{r-1})
 
 		\P^{[r-1,k]}_{b_{r-1}=1}([r,k]\in\mathcal{P})
 
 		+\bigO{p^{k}} \tag{by Lemma~\ref{lemma:eventindependenceNew}}\\
 
 		&=\!\!\!\sum_{\ell, r\in [k]: \ell<r-1}\!\!\!
 
 		\P^{[\ell+1]}_{b_{\ell+1}=1}([\ell]\in\mathcal{P})
 
 		\left(\P^{[\ell+1,r-1]}(\NZ{\ell+1})
 
		\P^{[\ell+1,r-1]}(\NZ{r-1})\right)
 
 		\P^{[r-1,k]}_{b_{r-1}=1}([r,k]\in\mathcal{P})
 
 		+\bigO{p^{k}} \tag{by induction}\\
 
 		&=\!\!\!\sum_{\ell, r\in [k]: \ell<r-1}\!\!\!
 
 		\P^{[\ell+1]}_{b_{\ell+1}=1}([\ell]\in\mathcal{P})
 
 		\left(\P^{[\ell+1,k]}(\NZ{\ell+1})
 
 		\P^{[1,r-1]}_{b_{r-1}=1}(\NZ{r-1})\right)
 
 		\P^{[r-1,k]}([r,k]\in\mathcal{P})
 
 		+\bigO{p^{k}} \tag{by Corrolary~\ref{cor:probIndepNew}}\\
 
 		&=\!\!\!\sum_{\ell, r\in [k]: \ell<r-1}\!\!\!
 
 		\P^{[k]}([\ell]\in\mathcal{P})
 
 		\P^{[k]}([r,k]\in\mathcal{P})
 
 		+\bigO{p^{k}} \tag{by Lemma~\ref{lemma:eventindependenceNew}}\\
 
 		&=\left(\sum_{\ell\in [k]}\P^{[k]}([\ell]\in\mathcal{P})\right)
 
 		\left(\sum_{r\in [k]}\P^{[k]}([r,k]\in\mathcal{P})\right)
 
 		+\bigO{p^{k}} \tag*{$\left(\P^{[k]}([\ell]\in\mathcal{P})=\bigO{p^{\ell}}\right)$}\\	
 
 		&=\P^{[k]}(\Z{1})\P^{[k]}(\Z{k})
 
 		+\bigO{p^{k}}.	
 
 		\end{align*}
 
 	\end{proof}
 
 	
 
	\begin{theorem}
 
		$R^{(n)}-R^{(m)}=\bigO{p^{\min(n,m)}}$.
 
	\end{theorem}
 
	\begin{proof}
 
        Some notation: let $P$ be an interval $[a,b]$. We say $P$ is a \emph{patch} when the $\Z{i}$ event holds for all $i \in [a,b]$ and $\NZ{a-1}$ and $\NZ{b+1}$ holds. We denote this event by $P\in\mathcal{P}$, so
 
        \begin{align*}
 
            P\in\mathcal{P} \equiv \NZ{a-1} \cap \Z{a} \cap \Z{a+1} \cap \cdots \cap \Z{b-1} \cap \Z{b} \cap \NZ{b+1} .
 
        \end{align*}
 
        Note that we have the following partition of the event $\Z{v}$ for any vertex $v\in[n]$:
 
        \begin{align*}
 
            \Z{v} = \dot\bigcup_{P : v\in P} (P\in\mathcal{P})
 
        \end{align*}
 
		Let $N\geq \max(2n,2m)$, then
 
		\begin{align*}
 
			R^{(n)}
 
			&= \E^{(n)}(\Res{1}) \tag{by translation invariance}\\
 
			&= \sum_{k=1}^{\infty}\P^{(n)}(\Res{1}\geq 1) \\
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}\P^{(n)}(\Res{1}\geq 1\& P\in\mathcal{P}) \tag{partition}\\
 
			&= \frac{1}{n}\sum_{v\in[n]}\sum_{t=1}^{\infty}\sum_{P\text{ patch}}t\cdot\P^{(n)}(v \text{ is resampled }t\text{ times and }v\in P | P\in\mathcal{P}) \; \P^{(n)}(P\in\mathcal{P})\\
 
			&= \frac{1}{n}\sum_{P\text{ patch}}\E^{(n)}(\# \text{ resamples in }P|P\in \mathcal{P}) \; \P^{(n)}(P\in\mathcal{P})\\
 
			&= \sum_{s=1}^{n-1}\E^{(n)}(\# \text{ resamples in }[s] \;|\; [s]\in \mathcal{P}) \; \P([s]\in\mathcal{P}) +\bigO{p^{n}}
 
			\tag{by translation symmetry}\\
 
			&= ???? \\
 
			&= \sum_{s=1}^{n-1}\E^{[0,s+1]}(\# \text{ resamples in }[s]|[s]\in \mathcal{P})\P^{[s+1,n]}(\NZ{s+1}\cap\NZ{n})/(1+p)^2+\bigO{p^{n}} \tag{by Lemma~\ref{lemma:eventindependenceNew}}\\   
 
			&= \sum_{s=1}^{n-1}\E^{[0,s+1]}(\# \text{ resamples in }[s]|[s]\in \mathcal{P})\left(\P^{[s+1,n]}(\NZ{s+1})\right)^2/(1+p)^2+\bigO{p^{n}} \tag{by Lemma~\ref{lemma:independenetSidesNew}}\\   
 
			&= \sum_{s=1}^{n-1}\E^{[0,s+1]}(\# \text{ resamples in }[s]|[s]\in \mathcal{P})\left(\P^{[s+1,N]}(\NZ{s+1})\right)^2/(1+p)^2+\bigO{p^{n}} \tag{by Corollary~\ref{cor:probIndepNew}}\\   			
 
			&= \sum_{s=1}^{n-1}\E^{[-N,N]}(\# \text{ resamples in }[s]|[s]\in \mathcal{P})+\bigO{p^{n}} \tag{by Lemma~\ref{lemma:eventindependenceNew}, Corollary~\ref{cor:probIndepNew}}\\   	
 
			&= \sum_{s=1}^{N}\E^{[-N,N]}(\# \text{ resamples in }[s]|[s]\in \mathcal{P})+\bigO{p^{n}}.
 
			%&= \sum_{k=1}^{\infty}\sum_{\underset{\ell\geq r-1}{\ell,r\in[n]}}\P^{(n)}(\Res{1}\geq 1\,\&\, [\ell+1,r-1]\in\mathcal{P}) \tag{partition}\\
 
			%&= \sum_{k=1}^{\infty}\sum_{\underset{\ell\geq r}{\ell,r\in[n]}}\P^{(n)}(\Res{1}\geq 1\,\&\, [\ell+1,r-1]\in\mathcal{P})  +\bigO{p^{n}} \\	
 
			%&= \sum_{k=1}^{\infty}\sum_{\underset{\ell\geq r}{\ell,r\in[n]}}\P^{[l,r]}_{b_{\ell}=b_{r}=1}(\Res{1}\geq 1\,\&\, [\ell+1,r-1]\in\mathcal{P}) \P^{[r,\ell]}(\NZ{\ell,r}) +\bigO{p^{n}} \tag{by Lemma~\ref{lemma:eventindependenceNew}}\\				
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}\P^{(n)}(\Res{1}\geq 1\,\&\, P\in\mathcal{P}) \tag{partition}\\
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}^{|P|<n}\P^{(n)}(\Res{1}\geq 1\,\&\, P\in\mathcal{P}) +\bigO{p^{n}}\\
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}^{|P|<n}\P^{[P\cup \partial P]}_{b_{\partial P}=1}(\Res{1}\geq 1\,\&\, P\in\mathcal{P}) \P^{[\overline{P}]}(\NZ{\partial P}) +\bigO{p^{n}} \tag{by Lemma~\ref{lemma:eventindependenceNew}}\\
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}^{|P|<n}\P^{[P\cup \partial P]}_{b_{\partial P}=1}(\Res{1}\geq 1\,\&\, P\in\mathcal{P}) \left(\left(\P^{[|\overline{P}|]}(\NZ{1})\right)^2+\bigO{p^{|\overline{P}|}}\right) +\bigO{p^{n}} \tag{by Lemma~\ref{lemma:independenetSidesNew}}\\
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}^{|P|<n}\P^{[P\cup \partial P]}_{b_{\partial P}=1}(\Res{1}\geq 1\,\&\, P\in\mathcal{P}) \left(\left(\P^{[N]}(\NZ{1})\right)^2+\bigO{p^{|\overline{P}|}}\right) +\bigO{p^{n}} \tag{by Corollary~\ref{cor:probIndepNew}}\\
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}^{|P|<n}\P^{[-N,N]}(\Res{1}\geq 1\,\&\, P\in\mathcal{P}) +\bigO{p^{n}} \tag{by Lemma~\ref{lemma:eventindependenceNew}}\\
 
			&= \sum_{k=1}^{\infty}\sum_{P\text{ patch}:1\in P}\P^{[-N,N]}(\Res{1}\geq 1\,\&\, P\in\mathcal{P}) +\bigO{p^{n}} \tag{by Lemma~\ref{lemma:eventindependenceNew}}\\
 
			&= \E^{[-N,N]}(\Res{1})+\bigO{p^{n}}.
 
		\end{align*}		
 
		
 
		Repeating the same calculation with $m$, and comparing the two expressions completes the proof.
 
	\end{proof} 	
 

	
 
Old:
 

	
 
The intuition of the following lemma is that the far right can only affect the zero vertex if there is an interaction chain forming, which means that every vertex should get resampled to $0$ at least once.
 
\begin{lemma}\label{lemma:probIndep}
 
	Suppose we have a finite set $I\subset\mathbb{N}_+$ of vertices.
 
    Let $I_{\max}:=\max(I)$ and $I':=I\setminus\{I_{\max}\}$, and similarly let $I_{\min}:=\min(I)$. These definitions are illustraded in Figure \ref{fig:lemmaillustration}.
 
	Then $\P^\infty_{I}(\Z{0})-\P^\infty_{I'}(\Z{0}) = O(p^{I_{\max}-|I'|})$.
 
\end{lemma}
 
\begin{proof}
 
\begin{figure}
 
	\begin{center}
 
    	\includegraphics{diagram_proborders.pdf}
 
    \end{center}
 
    \caption{\label{fig:lemmaillustration} Illustration of setup of Lemma \ref{lemma:probIndep}.}
 
\end{figure}
 
	The proof uses induction on $|I|$. For $|I|=1$ the statement is easy, since every resample sequence that resamples vertex $0$ to zero must produce at least $I_{\max}$ zeroes in-between.
 
	
 
    Induction step: For an event $A$ and $k>0$ let us denote $A_k = A\cap\left(\cap_{j=0}^{k-1} \mathrm{Z}^{(j)}\right)\cap \NZ{(k)}$, i.e. $A_k$ is the event $A$ \emph{and} ``Each vertex in $0,1,2,\ldots, k-1$ becomes $0$ at some point before termination (either by resampling or initialisation), but vertex $k$ does not''. Observe that these events form a partition, so $\Z{(0)}=\dot{\bigcup}_{k=1}^{\infty}\Z{(0)}_k$.
 
    Let $I_{<k}:=I\cap[1,k-1]$ and similarly $I_{>k}:=I\setminus[1,k]$, finally let $I_{><}:=\{I_{\min}+1,I_{\max}-1]\}\setminus I$ (note that $I_{><} = \gaps{I}$ as shown in Figure \ref{fig:diametergap}). Suppose we have proven the claim up to $|I|-1$, then the induction step can be shown by
 
	\begin{align*}
 
		\P^\infty_{I}(\Z{(0)})
 
		&=\sum_{k=1}^{\infty}\P^\infty(\Z{(0)}_k) \tag{the events are a partition}\\
 
        &=\sum_{k\in \mathbb{N}\setminus I}\P^\infty(\Z{(0)}_k) \tag{$\mathbb{\P^\infty}(A_k)=0$ for $k\in I$}\\
 
        &=\sum_{k\in\mathbb{N}\setminus I}\P^\infty_{I_{<k}}(\Z{(0)}_k)\cdot \P^\infty_{I_{>k}}(\NZ{(k)}) \tag{by Claim~\ref{claim:eventindependence}}\\
 
        &=\sum_{k\in I_{><}}\P^\infty_{I_{<k}}(\Z{(0)}_k)\cdot \P^\infty_{I_{>k}}(\NZ{(k)})+\mathcal{O}(p^{I_{\max}+1-|I|})
 
		\tag{$k<I_{\min}\Rightarrow \P^\infty_{I_{<k}}(\Z{(0)}_k)=0$}\\
 
        &=\sum_{k\in I_{><}}\P^\infty_{I'_{<k}}(\Z{(0)}_k)\cdot \P^\infty_{I_{>k}}(\NZ{(k)})+\mathcal{O}(p^{I_{\max}+1-|I|})	
 
		\tag{$k< I_{\max}\Rightarrow I_{<k}=I'_{<k}$}\\
 
		&=\sum_{k\in I_{><}}\P^\infty_{I'_{<k}}(\Z{(0)}_k)\cdot
 
        \left(\P^\infty_{I'_{>k}}(\NZ{(k)})+\mathcal{O}(p^{I_{\max}-k+1-|I_{>k}|})\right) +\mathcal{O}(p^{I_{\max}+1-|I|})	\tag{by induction, since for $k>I_{\min}$ we have $|I_{<k}|<|I|$}\\
 
		&=\sum_{k\in I_{><}}\P^\infty_{I'_{<k}}(\Z{(0)}_k)\cdot
 
        \P^\infty_{I'_{>k}}(\NZ{(k)}) +\mathcal{O}(p^{I_{\max}+1-|I|})	
 
		\tag{as $\P^\infty_{I'_{<k}}(\Z{(0)}_k)=\mathcal{O}(p^{k-|I'_{<k}|})$}\\
 
		&=\sum_{k\in\mathbb{N}\setminus I}\P^\infty_{I'_{<k}}(\Z{(0)}_k)\cdot
 
        \P^\infty_{I'_{>k}}(\NZ{(k)}) +\mathcal{O}(p^{I_{\max}+1-|I|})\\
 
		&=\sum_{k\in\mathbb{N}\setminus I'}\P^\infty_{I'_{<k}}(\Z{(0)}_k)\cdot
 
        \P^\infty_{I'_{>k}}(\NZ{(k)}) +\mathcal{O}(p^{I_{\max}+1-|I|})	\tag{$k=I_{\max}\Rightarrow \P^\infty_{I'_{<k}}(\Z{(0)}_k)=\mathcal{O}(p^{I_{\max}-|I'|})=\mathcal{O}(p^{I_{\max}+1-|I|})$}\\
 
		&=\P^\infty_{I'}(\Z{(0)}) +\mathcal{O}(p^{I_{\max}-|I'|})	\tag{analogously to the beginning}			
 
	\end{align*}
 
\end{proof}
 
\begin{corollary}\label{cor:probIndep}
 
	Suppose $I,J\subset\mathbb{N}_+$ are finite sets of vertices, and let $m=\min(\Delta(I,J))$.
 
	Then $\P^\infty_{I}(\Z{0})-\P^\infty_{J}(\Z{0}) = O(p^{|[m]\cap I\cap J|})$.
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