Changeset - be633a788363
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András Gilyén - 8 years ago 2017-09-14 00:25:36
gilyen@cwi.nl
General distance-degree lemma
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@@ -393,28 +393,28 @@ The following Lemma says that if a set $S$ splits the graph in two, then those t
 
           \sum_{\xi^{G\setminus X} \in \NZ{S}\cap A^Y}
 
            \P^{G\setminus Y}_S(\xi^{G\setminus Y}) \cdot
 
            \P^{G\setminus X}_S(\xi^{G\setminus X}) \\
 
        &= \P^{G\setminus Y}_S(\NZ{S} \cap A^X) \; \cdot \; \P^{G\setminus X}_S(\NZ{S} \cap A^Y)
 
    \end{align*}
 
\end{proof}
 

	
 
The intuition of the following lemma is that if two sites have distance $d$ in the graph, then the only way they can affect each other is that an interaction chain is forming between them, meaning that every vertex should get resampled to $0$ at least once in between them.
 

	
 
\begin{lemma}\label{lemma:distancePower}
 
	Suppose $G=(V,E)$ is a graph, $X,Y\subseteq V$ and $A^X$ is a local event on $X$. Then
 
	$$\P^{G}(A^X)-\P^{G\setminus Y}(A^X)=\bigO{p^{d(X,Y)}}.$$
 
	(Should be true with $+1$ in the degree!)
 
	(Should be true with $+1$ in the degree, when $d(X,Y)>0$!)
 
\end{lemma}
 
\begin{proof}
 
	We can assume without loss of generality, that $X\neq \emptyset\neq Y$, otherwise the statement is trivial.
 
	We can assume without loss of generality, that $X\neq \emptyset\neq Y$, otherwise the statement is trivial. Also we can assume without loss of generality that $d(X,Y)\leq \infty$, i.e., $X,Y$ are in the same connected component of $G$, otherwise we can use Lemma~\ref{lemma:splitting} with $S=\emptyset$.
 
	
 
	The proof goes by induction on $d(X,Y)$. The statement is trivial for $d(X,Y)=0$, and is easy to check for $d(X,Y)=1$, by looking at resample sequences that reach the all $1$ state in at most $0$ step (which is simply the case when everything is sampled to $1$ initially).
 
	
 
	Now we show the inductive step, assuming we know the statement for $d$, and that $d(X,Y)=d+1$.
 
	First we assume, that $\NZ{X}\subseteq\overline{A^X}$, i.e., $A^X\subseteq \Z{X}$.
 
	
 
	For $i\in[d]$ we define $A_i^X:=A^X\cap{\NZ{\overline{\partial}(X,i)}}\cap\bigcap_{j\in[i-1]}\Z{\overline{\partial}(X,j)}$, 
 
	and define $A_{d+1}^X:=A^X\cap\bigcap_{j\in[d]}\Z{\overline{\partial}(X,j)}$,
 
	so that they form a partition $A^X=\dot\bigcup_{i\in [d+1]}A_i^X$. 
 
	It is easy to see that for all $i\in[d+1]$ we have $A_{i}^X\subseteq\Z{X}\cap\bigcap_{j\in[i-1]}\Z{\overline{\partial}(X,j)}$, and therefore 
 
	\begin{equation}\label{eq:AXorder}
 
		\P^G(A_{i}^X)=\bigO{p^{i}}.
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