Changeset - d3ea27296b0f
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András Gilyén - 8 years ago 2017-09-06 06:11:37
gilyen@cwi.nl
cycle lemma
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@@ -733,192 +733,250 @@ Note by Tom: So $A^{(\mathcal{P})}$ is the event that the set of all patches is
 
	$$R_I:=\mathbb{E}[\#\{\text{resamplings when started from inital state }I\}].$$
 
	For a patch set $\mathcal{P}$ and some $P\in\mathcal{P}$ we define
 
	$$R^{(\mathcal{P})}_I:=\mathbb{E}[\#\{\text{resamplings when started from inital state }I\}|A^{(\mathcal{P})}]$$	
 
	and 
 
	$$R^{(P,\mathcal{P})}_I:=\mathbb{E}[\#\{\text{resamplings inside }P\text{ when started from inital state }I\}|A^{(\mathcal{P})}]$$		
 
	finally
 
	$$R^{(P)}_I:=\mathbb{E}[\#\{\text{resamplings inside }P\text{ when started from inital state }I\}|A^{(P)}].$$	
 
\end{definition} 
 

	
 
    Similarly to Mario's proof I use the observation that 
 
    \begin{align*}
 
    R^{(n)} &= \frac{1}{n}\sum_{b\in\{0,1,1'\}^{n}} \rho_b \; R_{\bar{b}}(p)\\
 
    &= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{f\in\{0,1'\}^{|S|}}\rho_{S(f)} R_{S(f)}\\
 
    &= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{f\in\{0,1'\}^{|S|}}\rho_{S(f)}
 
    \sum_{\mathcal{P}\text{ patches}} \mathbb{P}_{S(f)}(A^{(\mathcal{P})}) R^{(\mathcal{P})}_{S(f)} \\
 
    &= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{f\in\{0,1'\}^{|S|}}\rho_{S(f)}
 
    \sum_{\mathcal{P}\text{ patches}} \mathbb{P}_{S(f)}(A^{\mathcal{P}}) \sum_{P\in\mathcal{P}} R^{(P,\mathcal{P})}_{S(f)}\\
 
    &= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{f\in\{0,1'\}^{|S|}}\rho_{S(f)} 
 
    \sum_{\mathcal{P}\text{ patches}} \mathbb{P}_{S(f)}(A^{\mathcal{P}}) \sum_{P\in\mathcal{P}} R^{(P)}_{S(f)\cap P}\tag{by Claim~\ref{claim:eventindependence}}\\ 
 
    &= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{f\in\{0,1'\}^{|S|}}\rho_{S(f)} 
 
    \sum_{P\text{ patch}} R^{(P)}_{S(f)\cap P}\sum_{\mathcal{P}:P\in\mathcal{P}}\mathbb{P}_{S(f)}(A^{\mathcal{P}})\\     
 
    &= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{P\text{ patch}}\sum_{f\in\{0,1'\}^{|S|}}
 
     \rho_{S(f)} R^{(P)}_{S(f)\cap P}\mathbb{P}_{S(f)}(A^{(P)}) \tag{by definition}\\        
 
    &= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{P\text{ patch}}\sum_{f\in\{0,1'\}^{|S|}}
 
    \rho_{S(f)} R^{(P)}_{S(f)\cap P}\mathbb{P}_{S(f)\cap P}(A^{(P)})\mathbb{P}_{S(f)\cap \overline{P}}(\overline{Z^{(P_{\min}-1)}}\cap\overline{Z^{(P_{\max}+1)}}) \tag{remember Definition~\ref{def:visitingResamplings} and use Claim~\ref{claim:eventindependence}}\\    
 
    &= \frac{1}{n}\sum_{S\subseteq [n]} \sum_{P\text{ patch}} \sum_{f_P\in\{0,1'\}^{|S\cap P|}}
 
    \rho_{S(f_P)} R^{(P)}_{S(f_P)} \mathbb{P}_{S(f_P)}(A^{(P)})
 
    \sum_{f_{\overline{P}}\in\{0,1'\}^{|S\cap \overline{P}|}} \rho_{S(f_{\overline{P}})} \mathbb{P}_{S(f_{\overline{P}})}(\overline{Z^{(P_{\min}-1)}}\cap\overline{Z^{(P_{\max}+1)}}) \\   
 
	&= \frac{1}{n}\sum_{S\subseteq [n]}\sum_{P\text{ patch}}\sum_{f_P\in\{0,1'\}^{|S\cap P|}}
 
	\rho_{S(f_P)}
 
        \sum_{f_{\overline{P}}\in\{0,1'\}^{|S\cap \overline{P}|}}\rho_{S(f_{\overline{P}})}\mathcal{O}(p^{|S_{><}|}) \tag{see below} \\
 
	&= \frac{1}{n}\sum_{S\subseteq [n]}\mathcal{O}(p^{|S|+|S_{><}|}).
 
    \end{align*}
 
\begin{figure}
 
	\begin{center}
 
    	\includegraphics{diagram_patches.pdf}
 
    \end{center}
 
    \caption{\label{fig:patches} Illustration of last steps of the proof.}
 
\end{figure}
 
    The penultimate inequality can be seen by case separation as follows: If $S\subseteq P$ then there is no splitting into $S\cap P$ and $S\setminus P$, and we already have $\mathbb{P}_{S(f_P)}(A^{(P)})=\mathcal{O}(p^{|S_{><}|})$ simply because the patch $P$ must be filled with zeroes that were not yet in $S$, so this is at least $|S_{><}|$ resampled zeroes. For the more general case, assume that $S$ is larger than $P$ on both sides of $P$. This is illustrated in Figure \ref{fig:patches}. We will focus on the following sum that was in the above equations:
 
    \begin{align*}
 
        \sum_{f_{\overline{P}}\in\{0,1'\}^{|S \cap \overline{P}|}} \rho_{S(f_{\overline{P}})} \mathbb{P}_{S(f_{\overline{P}})}(\overline{Z^{(P_{\min}-1)}}\cap\overline{Z^{(P_{\max}+1)}})
 
    \end{align*}
 
    By Lemma \ref{lemma:eventindependence} we can split this sum into two parts: the part to the left of $P$ and the part to the right of $P$. Define $S_\mathrm{left}=S\cap[S_\mathrm{min},P_{\mathrm{min}}-1]$ and $S_\mathrm{right}=S\cap[P_{\mathrm{max}}+1,S_\mathrm{max}]$, so that $S\cap\overline{P} = S_\mathrm{left} \cup S_\mathrm{right}$. These are also illustrated in Figure \ref{fig:patches}. Then we have
 
    \begin{align*}
 
        \mathbb{P}_{S(f_{\overline{P}})}(\overline{Z^{(P_{\min}-1)}}\cap\overline{Z^{(P_{\max}+1)}})
 
        &= \mathbb{P}_{S(f_{\mathrm{left}})}(\overline{Z^{(P_{\min}-1)}}) \;\cdot\; \mathbb{P}_{S(f_{\mathrm{right}})}(\overline{Z^{(P_{\max}+1)}})
 
    \end{align*}
 
    and hence we can split the sum. Without loss of generality we now only consider the `right' part of the sum:
 
    \begin{align*}
 
        \sum_{f\in\{0,1'\}^{|S_\mathrm{right}|}} \rho_{S_\mathrm{right}(f)} \mathbb{P}_{S_\mathrm{right}(f)}(\overline{Z^{(P_{\max}+1)}})
 
    \end{align*}
 
    Now further split this sum over the value of $f$ at position $S_\mathrm{max}$:
 
    \begin{align*}
 
        \sum_{f\in\{0,1'\}^{|S_\mathrm{right}\setminus\{S_\mathrm{max}\}|}} \sum_{f'\in\{0,1'\}}
 
        \rho_{S_\mathrm{right}(f\,f')} \mathbb{P}_{S_\mathrm{right}(f\,f')}(\overline{Z^{(P_{\max}+1)}})
 
    \end{align*}
 
    and we use the definition of $\rho$ for the sum over $f'$:
 
    \begin{align*}
 
         \sum_{f\in\{0,1'\}^{|S_\mathrm{right}\setminus\{S_\mathrm{max}\}|}}
 
        \rho_{S_\mathrm{right}(f)} \left(p \mathbb{P}_{S_\mathrm{right}(f\, 0)}(\overline{Z^{(P_{\max}+1)}}) + (-p) \mathbb{P}_{S_\mathrm{right}(f\, 1)}(\overline{Z^{(P_{\max}+1)}}) \right)
 
    \end{align*}
 
    Now we recognize the setup of Lemma~\ref{lemma:probIndep} with $I=S_\mathrm{right}(f\,0)$ and $I'=S_\mathrm{right}(f\,1)$. The lemma yields
 
    \begin{align*}
 
        \mathbb{P}_{S_\mathrm{right}(f\, 0)}(\overline{Z^{(P_{\max}+1)}}) &= \mathbb{P}_{S_\mathrm{right}(f\, 1)}(\overline{Z^{(P_{\max}+1)}}) + \mathcal{O}(p^{S_\mathrm{max}-(P_{\mathrm{max}}+1)+1-|S_\mathrm{right}|}) \\
 
        &= \mathbb{P}_{S_\mathrm{right}(f\, 1)}(\overline{Z^{(P_{\max}+1)}}) + \mathcal{O}(p^{S_\mathrm{max}-P_{\mathrm{max}}-|S_\mathrm{right}|}) .
 
    \end{align*}
 
    Entering this back into the sum gives
 
    \begin{align*}
 
         \sum_{f\in\{0,1'\}^{|S_\mathrm{right}\setminus\{S_\mathrm{max}\}|}}
 
        \rho_{S_\mathrm{right}(f)} \mathcal{O}(p^{S_\mathrm{max}-P_{\mathrm{max}}-|S_\mathrm{right}|+1})
 
         = \sum_{f\in\{0,1'\}^{|S_\mathrm{right}|}}
 
        \rho_{S_\mathrm{right}(f)} \mathcal{O}(p^{S_\mathrm{max}-P_{\mathrm{max}}-|S_\mathrm{right}|})
 
    \end{align*}
 
    One can do the same for the `left' part, which gives a term $\mathcal{O}(p^{P_\mathrm{min}-S_{\mathrm{min}}-|S_\mathrm{left}|})$. The part of $S$ that was within $P$ gives $\mathbb{P}_{S(f_P)}(A^{(P)})=\mathcal{O}(p^{P_\mathrm{max}-P_\mathrm{min}+1-|S\cap P|})$. Combining these three factors yields
 
    \begin{align*}
 
        (\textrm{left part})(P\textrm{ part})(\textrm{right part}) &=
 
\mathcal{O}(p^{P_\mathrm{min}-S_{\mathrm{min}}-|S_\mathrm{left}|}) \cdot \mathcal{O}(p^{P_\mathrm{max}-P_\mathrm{min}+1-|S\cap P|}) \cdot \mathcal{O}(p^{S_\mathrm{max}-P_{\mathrm{max}}-|S_\mathrm{right}|}) \\
 
        &= \mathcal{O}(p^{S_\mathrm{max}-S_\mathrm{min}+1-|S_\mathrm{left}\cup S_\mathrm{right}\cup (S\cap P)|})\\
 
        &= \mathcal{O}(p^{S_\mathrm{max}-S_\mathrm{min}+1-|S|})
 
        = \mathcal{O}(p^{|S_{><}|})
 
    \end{align*}
 
    as required. This finishes the proof.
 

	
 
    ~
 

	
 
	I think the same arguments would translate to the torus and other translationally invariant spaces, so we could go higher dimensional as Mario suggested. Then I think one would need to replace $|S_{><}|$ by the minimal number $k$ such that there is a $C$ set for which $S\cup C$ is connected. I am not entirely sure how to generalise Lemma~\ref{lemma:probIndep} though, which has key importance in the present proof.
 
    
 
    Questions:
 
    \begin{itemize}
 
    	\item Is this proof finally flawless?
 
    	\item In view of this proof, can we better characterise $a_k^{(k+1)}$?
 
    	\item Why did Mario's and Tom's simulation show that for fixed $C$ the contribution coefficients have constant sign? Is it relevant for proving \ref{it:pos}-\ref{it:geq}?
 
    	\item Can we prove the conjectured formula for $a_k^{(3)}$?		
 
    \end{itemize} 
 
    
 
	\begin{lemma}On the infinite chain
 
		$$\P_I(Z^{0}\cap Z^{k})=\P_I(Z^{0})\P_I(Z^{k})+\mathcal{O}(p^{k-|b|+1})$$
 
	\end{lemma}   
 
    Note that using De Morgan's law and the inclusion-exclusion formula we can see that this is equivalent to saying:
 
    $$\P_I(NZ^{0}\cap NZ^{k})=\P_I(NZ^{0})\P_I(NZ^{k})+\mathcal{O}(p^{k-|b|+1})$$
 
    \begin{proof}
 
    We proceed by induction on $|I|$. For $|I|=0,1$ the statement is trivial.
 
    
 
    Now observe that:
 
    $$\P_I(Z^{0})=\sum_{P\text{ patch}\,:\,0\in P}\P_I(P\in\mathcal{P})$$
 
    $$\P_I(Z^{k})=\sum_{P\text{ patch}\,:\,k\in P}\P_I(P\in\mathcal{P})$$
 
    
 
    Suppose $|I|\geq 2$, then we proceed using induction similarly to the above
 
    \begin{align*}
 
    &\P_I(Z^{0}\cap Z^{k})\\
 
    &=\sum_{\underset{P_l^{\max}+1<P_r^{\min}}{P_l,P_r\text{ patches}\,:\,0\in P_l,k\in P_r}}
 
    \P_I(P_l,P_r\in\mathcal{P})
 
    +\sum_{P\text{ patch}\,:\,0,k\in P}\P_I(P\in\mathcal{P})\\
 
    &=\sum_{\underset{P_l^{\max}+1<P_r^{\min}}{P_l,P_r\text{ patches}\,:\,0\in P_l,k\in P_r}}
 
    \P_I(P_l,P_r\in\mathcal{P})
 
    +\mathcal{O}(p^{k+1})\\
 
    &\overset{Lemma~\ref{claim:eventindependence}}{=}\sum_{\underset{P_l^{\max}+1<P_r^{\min}}{P_l,P_r\text{ patches}\,:\,0\in P_l,k\in P_r}}
 
    \P_{I\cap P_l}(P_l\in\mathcal{P})
 
    \P_{> I <}(NZ^{P_l^{\max}+1}\cap NZ^{P_r^{\min}-1})
 
    \P_{I\cap P_r}(P_r\in\mathcal{P})
 
    +\mathcal{O}(p^{k+1})\\
 
    &\overset{\text{induction}}{=}\sum_{\underset{P_l^{\max}+1<P_r^{\min}}{P_l,P_r\text{ patches}\,:\,0\in P_l,k\in P_r}}
 
    \P_{I\cap P_l}(P_l\in\mathcal{P})
 
    \P_{> I <}(NZ^{P_l^{\max}+1})\P_{> I <}(NZ^{P_r^{\min}-1})
 
    \P_{I\cap P_r}(P_r\in\mathcal{P})
 
    +\mathcal{O}(p^{k-|I|+1})\\
 
    &\overset{Lemma~\ref{lemma:probIndep}}{=}\sum_{\underset{P_l^{\max}+1<P_r^{\min}}{P_l,P_r\text{ patches}\,:\,0\in P_l,k\in P_r}}
 
    \P_{I\cap P_l}(P_l\in\mathcal{P})
 
    \P_{I\setminus P_l}(NZ^{P_l^{\max}+1})\P_{I\setminus P_r}(NZ^{P_r^{\min}-1})
 
    \P_{I\cap P_r}(P_r\in\mathcal{P})
 
    +\mathcal{O}(p^{k-|I|+1})\\
 
    &\overset{Lemma~\ref{claim:eventindependence}}{=}\sum_{\underset{P_l^{\max}+1<P_r^{\min}}{P_l,P_r\text{ patches}\,:\,0\in P_l,k\in P_r}}
 
    \P_{I}(P_l\in\mathcal{P})
 
    \P_{I}(P_r\in\mathcal{P})
 
    +\mathcal{O}(p^{k-|I|+1})\\
 
    &=\left(\sum_{P_l\text{ patch}\,:\,0\in P_l}
 
    \P_{I}(P_l\in\mathcal{P})\right)\left(\sum_{P_l\text{ patch}\,:\,0\in P_l}
 
    \P_{I}(P_l\in\mathcal{P})\right)
 
    +\mathcal{O}(p^{k-|I|+1})\\
 
    &=\P_I(Z^{0})\P_I(Z^{k})
 
    +\mathcal{O}(p^{k-|I|+1}).	
 
    \end{align*}
 
    \end{proof}
 

	
 
	Final observation: Suppose $S={a,b}$ 
 
	    \begin{align*}
 
	    &\sum_{f_{\overline{P}}\in\{0,1'\}^{2}} \rho_{S(f_{\overline{P}})} \mathbb{P}_{S(f_{\overline{P}})}(NZ^{(P_{\min}-1)}\cap NZ^{(P_{\max}+1)})  \\
 
	    &= \sum_{f_{\overline{P}}\in\{0,1'\}^{2}} \rho_{S(f_{\overline{P}})} \mathbb{P}_{S(f_{\overline{P}})}(NZ^{(P_{\min}-1)}) \mathbb{P}_{S(f_{\overline{P}})}(NZ^{(P_{\max}+1)})   +\mathcal{O}(p^{|\overline{P}|-|S|})\\
 
 	    &= \sum_{f_{\overline{P}}\in\{0,1'\}^{2}}  \rho_{a_f}\mathbb{P}_{S(f_{\overline{P}})}(NZ^{(P_{\min}-1)})  \rho_{b_f}\mathbb{P}_{S(f_{\overline{P}})}(NZ^{(P_{\max}+1)})   +\mathcal{O}(p^{|\overline{P}|-|S|})\\
 
	    \end{align*}
 

	
 

	
 
    
 
\begin{comment}
 
    \subsection{Sketch of the (false) proof of the linear bound \ref{it:const}}
 
    Let us interpret $[n]$ as the vertices of a length-$n$ cycle, and interpret operations on vertices mod $n$ s.t. $n+1\equiv 1$ and $1-1\equiv n$.
 
    %\begin{definition}[Resample sequences]
 
    %	A sequence of indices $(r_\ell)=(r_1,r_2,\ldots,r_k)\in[n]^k$ is called resample sequence if our procedure performs $k$ consequtive resampling, where the first resampling of the procedure resamples around the mid point $r_1$ the second around $r_2$ and so on. Let $RS(k)$ the denote the set of length $k$ resample sequences, and let $RS=\cup_{k\in\mathbb{N}}RS(k)$.
 
    %\end{definition}
 
    %\begin{definition}[Constrained resample sequence]\label{def:constrainedRes}
 
    %	Let $C\subseteq[n]$ denote a slot configuration, and let $a\in\{\text{res},\neg\text{res}\}^{n-|C|}$, where the elements correspond to labels ``resampled" vs. ``not resampled" respectively. 
 
    %	For $j\in[n-|C|]$ let $i_j$ denote the $j$-th index in $[n]\setminus C$.
 
    %	We define the set $A^{(C,a)}\subseteq RS$ as the set of resample sequences $(r_\ell)$ such that for all $j$ which has $a_j=\text{res}$ we have that $i_j$ appears in $(r_\ell)$ but for $j'$-s which have $a_{j'}=\neg\text{res}$ we have that $i_{j'}$ never appears in $(r_\ell)$. 
 
    %\end{definition}    
 
    \begin{definition}[Conditional expected number of resamples]
 
    	For a slot configuration $C\subseteq[n]$ and $a\in\{\!\text{ever},\text{ never}\}^{n-|C|}$ we define the event $A^{(C,a)}:=\bigwedge_{j\in[n-|C|]}\{i_j\text{ has }a_j\text{ become }0\text{ before reaching }\mathbf{1}\}$,
 
    	where $i_j$ is the $j$-th vertex of $[n]\setminus C$.
 
    	Then we also define
 
    	$$R^{(C,a)}_b:=\mathbb{E}[\#\{\text{resamplings when started from inital state }b\}|A^{(C,a)}].$$
 
    \end{definition}     
 
    
 
    As in Mario's proof I use the observation that 
 
    \begin{align*}
 
    R^{(n)}(p) &= \frac{1}{n}\sum_{b\in\{0,1,1'\}^{n}} \rho_b \; R_{\bar{b}}(p)\\
 
    &= \frac{1}{n}\sum_{C\subseteq [n]}\sum_{f\in\{0,1'\}^{|C|}} \rho_{C(f)} R_{C(f)}(p)\\
 
    &= \frac{1}{n}\sum_{C\subseteq [n]}\sum_{f\in\{0,1'\}^{|C|}}\sum_{a\in\{\!\text{ever},\text{ never}\}^{n-|C|}} \rho_{C(f)} R^{{(C,a)}}_{C(f)}(p)P_{C(f)}(A^{(C,a)})\\
 
    &= \frac{1}{n}\sum_{C\subseteq [n]}\sum_{a\in\{\!\text{ever},\text{ never}\}^{n-|C|}} \sum_{f\in\{0,1'\}^{|C|}} \rho_{C(f)} R^{{(C,a)}}_{C(f)}(p)P_{C(f)}(A^{(C,a)}), 
 
    \end{align*}
 
    where we denote by $C\subseteq[n]$ a slot configuration, whereas $C(f)$ denotes the slots of $C$ filled with the particles described by $f$, while all other location in $[n]\setminus C$ are set to $1$. 
 
    When we write $R_{C(f)}$ we mean $R_{C(\bar{f})}$, i.e., replace $1'$-s with $1$-s. Since the notation is already heavy we dropped the bar from $f$, as it is clear from the context. Finally by $P_{C(f)}(A^{(C,a)})$ we denote the probability that the event $A^{(C,a)}$ holds.
 
    
 
    As in Definition for $j\in[n-|C|]$ let $i_j$ denote the $j$-th index in $[n]\setminus C$.
 
    Suppose that $a$ is such that there are two indices $j_1\neq j_2$ such that 
 
    $a_{j_1}=\text{never}=a_{j_2}$, moreover the sets $\{i_{j_1}+1,\ldots, i_{j_2}-1\}$ and $\{i_{j_2}+1,\ldots, i_{j_1}-1\}$ partition $C$ non-trivially, and we denote by $C_l$,$C_r$ the corresponding partitions. 
 
    I wanted to prove that
 
    \begin{equation}\label{eq:conditionalCancellation}
 
		\sum_{f\in\{0,1'\}^{|C|}} \rho_{C(f)} R^{{(C,a)}}_{C(f)}(p)=0,
 
    \end{equation}    
 
    based on the observation that for all $f\in\{0,1'\}^{|C|}$ we have 
 
    that 
 
    \begin{equation}\label{eq:keyIndependce}
 
    R^{{(C,a)}}_{C(f)}(p)=R^{{(C_l,a_l)}}_{C_l(f_l)}(p)+R^{{(C_r,a_r)}}_{C_r(f_r)}(p),
 
    \end{equation}
 
    where $f_l\in\{0,1'\}^{|C_l|}$ is defined as taking only the indices (and values) of $f$ corresponding to vertices of $C_l$, also $a_l\in[n-|C_l|]$ is defined such that $a$ and $a_l$ agree on vertices where $a$ is defined, and on the vertices where $a$ is not defined, i.e., the vertices of $C_r$ we define $a_l$ to contain ``never". We define things analogously for $f_r$ and $a_r$. 
 
    
 
    The reason why \eqref{eq:keyIndependce} holds is that as before the two halves of the cycle are conditionally independent because neither $i_{j_1}$ nor $i_{j_2}$ can become $0$. To be more precise each resample sequence $\left(C(f)\rightarrow \mathbf{1} \right)\in A^{(C,a)}$ can be uniquely decomposed to resample sequences $\left(C_l(f_l)\rightarrow \mathbf{1}\right)\in A^{(C_l,a_l)}$ and $\left(C_r(f_r)\rightarrow \mathbf{1}\right)\in A^{(C_r,a_r)}$. The sum of probabilities of the set of resample sequences $\{r\}$ which have decomposition $(r_l,r_r)$ have probability which is the product of the probabilities of $r_l$ and $r_r$ as shown in the proof of Claim~\ref{claim:expectationsum}. This proves that the set of all resample sequences $\left(C(f)\rightarrow \mathbf{1}\right)\in A^{(C,a)}$ for our purposes can be viewed as a product set with product probability distribution. Therefore the halves can be treated independently and so the expectation values just add up. 
 
    
 
    From here I wanted to mimic Mario's proof:
 
    \begin{align*}
 
    \sum_{f\in\{0,1'\}^{|C|}} \rho_{C(f)} R^{{(C,a)}}_{C(f)}(p)&=
 
    \sum_{f_l\in\{0,1'\}^{|C_l|}} \sum_{f_r\in\{0,1'\}^{|C_r|}}  (-1)^{|f_l|+|f_r|}p^{|C_l|+|C_r|} \left( R^{{(C_l,a_l)}}_{C_l(f_l)}(p) + R^{{(C_r,a_r)}}_{C_r(f_l)}(p) \right)\\
 
    &= p^{|C|}\sum_{f_l\in\{0,1'\}^{|C_l|}} (-1)^{|f_l|} R^{{(C_l,a_l)}}_{C_l(f_l)}(p) \sum_{f_r\in\{0,1'\}^{|C_r|}} (-1)^{|f_r|} \\
 
    &\quad + p^{|C|}\sum_{f_r\in\{0,1'\}^{|C_r|}} (-1)^{|f_r|} R^{{(C_r,a_r)}}_{C_r(f_r)}(p) \sum_{f_l\in\{0,1'\}^{|C_l|}} (-1)^{|f_l|} \\
 
    &= 0.
 
    \end{align*}
 
    The nasty issue which I did not realise that the missing term $P_{C(f)}(A^{(C,a)})$ is non-constant: even though the event $A^{(C,a)}$ is independent of $f$ the probability $P_{C(f)}(A^{(C,a)})=P_{C(f_l)}(A^{(C_l,a_l)})\cdot P_{C(f_r)}(A^{(C_r,a_r)})$ is not and so the above breaks down.
 
    
 
    Observe that if \eqref{eq:conditionalCancellation} would hold for configurations that cut the slot configuration to two halves it would imply that the only non-zero contribution comes from pairs $(C,a)$ such that $C\cup\{i_j:a_j=\text{ever}\}$ is connected. This is because if this set is not connected, then either we can cut $C$ to two halves non-trivially along ``never" vertices, or there is an island of $\text{ever}$ vertices separated from any slots, and therefore from any $0$-s. This latter case has zero contribution since we cannot set these indices to $0$, without reaching them by some resamplings, and thereby building a path of $0$-s leading there.
 
    
 
    If $|C\cup\{i_j:a_j=\text{ever}\}|\geq k+1$ then all contribution has a power at least $k+1$ in $p$ since $(C,a)$ requires the prior appearance of at least $k+1$ particles. If $n\geq k+1$ than all $(C,a)$ such that $|C\cup\{i_j:a_j=\text{ever}\}|\leq k$ appears exactly $n$ times, since $(C,a)$ cannot be translationally invariant. Moreover the quantity $R^{{(C,a)}}_{C(f)}(p)$ is independent of $n$ due to the conditioning that every resampling happens on a connected component of length at most $k<n$. This would prove that $a_k^{(n)}$ is constant for $n\geq k+1$. The same arguments would directly translate to the torus and other translationally invariant objects, so we could go higher dimensional as Mario suggested.
 
    
 
    Questions:
 
    \begin{itemize}
 
    	\item Is it possible to somehow fix this proof?
 
    	\item In view of this (false) proof, can we better characterise $a_k^{(k+1)}$?
 
    	\item Why did Mario's and Tom's simulation show that for fixed $C$ the contribution coefficients have constant sign? Is it relevant for proving \ref{it:pos}-\ref{it:geq}?
 
    	\item Can we prove the conjectured formula for $a_k^{(3)}$?		
 
    \end{itemize} 
 

	
 
\begin{comment}
 
    \subsection{Sketch of the proof of the linear bound \ref{it:const}}
 
    Let us interpret $[n]$ as the vertices of a length-$n$ cycle, and interpret operations on vertices mod $n$ s.t. $n+1\equiv 1$ and $1-1\equiv n$.
 
    \begin{definition}[Resample sequences]
 
		A sequence of indices $(r_\ell)=(r_1,r_2,\ldots,r_k)\in[n]^k$ is called resample sequence if our procedure performs $k$ consequtive resampling, where the first resampling of the procedure resamples around the mid point $r_1$ the second around $r_2$ and so on. Let $RS(k)$ the denote the set of length $k$ resample sequences, and let $RS=\cup_{k\in\mathbb{N}}RS(k)$.
 
    \end{definition}
 
    \begin{definition}[Constrained resample sequence]\label{def:constrainedRes}
 
    	Let $C\subseteq[n]$ denote a slot configuration, and let $a\in\{\text{res},\neg\text{res}\}^{n-|C|}$, where the elements correspond to labels ``resampled" vs. ``not resampled" respectively. 
 
    	For $j\in[n-|C|]$ let $i_j$ denote the $j$-th index in $[n]\setminus C$.
 
		We define the set $A^{(C,a)}\subseteq RS$ as the set of resample sequences $(r_\ell)$ such that for all $j$ which has $a_j=\text{res}$ we have that $i_j$ appears in $(r_\ell)$ but for $j'$-s which have $a_{j'}=\neg\text{res}$ we have that $i_{j'}$ never appears in $(r_\ell)$. 
 
    \end{definition}    
 
    \begin{definition}[Expected number of resamples]
 
		For $b\in\{0,1\}^n$ we define 
 
		$$R_b=\mathbb{E}[\#\{\text{resamplings when started from inital state }b\}],$$
 
		and for $(C,a)$ as in the previous definition we also define
 
		$$R^{(C,a)}_b=\mathbb{E}[\#\{\text{resamplings }\in A^{(C,a)} \text{ when started from inital state }b\}].$$
 
		Here we mean by the latter that after each resampling we check whether the sequence of resamplings so far is in $A^{(C,a)}$, if yes we count it, otherwise we do not count.
 
    \end{definition}     
 
    
 
    As in Mario's proof I use the observation that 
 
    \begin{align*}
 
    R^{(n)}(p) &= \frac{1}{n}\sum_{b\in\{0,1,1'\}^{n}} \rho_b \; R_{\bar{b}}(p)\\
 
    &= \frac{1}{n}\sum_{C\subseteq [n]}\sum_{f\in\{0,1'\}^{|C|}} \rho_{C(f)} R_{C(f)}(p)\\
 
    &= \frac{1}{n}\sum_{C\subseteq [n]}\sum_{f\in\{0,1'\}^{|C|}}\sum_{a\in\{\text{res},\neg\text{res}\}^{n-|C|}} \rho_{C(f)} R^{{(C,a)}}_{C(f)}(p)\\
 
    &= \frac{1}{n}\sum_{C\subseteq [n]}\sum_{a\in\{\text{res},\neg\text{res}\}^{n-|C|}} \sum_{f\in\{0,1'\}^{|C|}} \rho_{C(f)} R^{{(C,a)}}_{C(f)}(p), 
 
    \end{align*}
 
    where we denote by $C\subseteq[n]$ a slot configuration, whereas $C(f)$ denotes the slots of $C$ filled with the particles described by $f$, while all other location in $[n]\setminus C$ are set to $1$. 
 
	When we write $R_{C(f)}$ we mean $R_{C(\bar{f})}$, i.e., replace $1'$-s with $1$-s. Since the notation is already heavy we dropped the bar from $f$, as it is clear from the context.
 
    
 
    As in Definition~\ref{def:constrainedRes} for $j\in[n-|C|]$ let $i_j$ denote the $j$-th index in $[n]\setminus C$.
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