diff --git a/distribution.pdf b/distribution.pdf index 8baf77e..02dddd5 100644 Binary files a/distribution.pdf and b/distribution.pdf differ diff --git a/distribution.tex b/distribution.tex index a32a9f7..62f6583 100644 --- a/distribution.tex +++ b/distribution.tex @@ -27,7 +27,7 @@ SODA23 paper also embeds $(X,d)$ into distribution. We call this kind of embeddi Consider the problem of embedding some finite metric into a tree metric. We can get an $O(n)$ lowerbound via the unit edge length cycle $C_n$. However, if embedding into distortions is allowed, we can do $O(\log n)$. \begin{theorem}[Bartal] -Let $(X,d)$ be a metric space on $n$ points with diameter $\Delta$, let $\mathcal D T$ be the set of tree metrics that dominate $d$, there is a distribution $\pi$ on $\mathcal D T$ such that $(X,d)$ embeds into $pi$ with distortion $O(\log n)$. +Let $(X,d)$ be a metric space on $n$ points with diameter $\Delta$, let $\mathcal D T$ be the set of tree metrics that dominate $d$, there is a distribution $\pi$ on $\mathcal D T$ such that $(X,d)$ embeds into $\pi$ with distortion $O(\log n)$. \end{theorem} % A kind of embedding problems which are closely related to outlier embeddings is Ramsey type embedding. Let $(X,d_X)$ be the original metric space and let $(Y,d_Y)$ be the target space. Given a fixed distortion $c$, Ramsey type embedding asks for the largest subset $Z$ of $X$ such that $(Z,d_X)$ embeds into $(Y,d_Y)$ with distortion at most $c$. This is the same as computing the smallest outlier set. @@ -39,6 +39,25 @@ We first ignore the outlier condition and see if stochastic embeddings break the For any metric space $(X,d)$ and for any $p$, there is an embedding of $(X,d)$ into $\ell_p^{O(\log^2 n)}$ with distortion $O(\log n)$. \end{theorem} -We want to beat Bourgain's in terms of the expected distortion. +Bourgain develops an algorithm that finds a desired embedding with probability at least $1/2$.\footnote{\url{https://home.ttic.edu/~harry/teaching/pdf/lecture3.pdf}} For the $\ell_2$ case, the embedding has the following bounds: +\begin{itemize} +\item[Expansion] $\|f(x)-f(y)\|_2\leq O(\log n) d(x,y)$ +\item[Contraction] $\|f(x)-f(y)\|_2 \geq \frac{d(x,y)}{O(1)}$ +\end{itemize} + +The contraction bound is almost tight. Let $K$ be the dimension of the target space. For the expansion bound, we have + +\begin{equation*} +\begin{aligned} +\|f(x)-f(y)\|_2 &= \left( \sum_{i=1}^{K} |f_i(x)-f_i(y)|^2\right)^{1/2}\\ + &\leq \left( \sum_{i=1}^{K} d(x,y)^2\right)^{1/2}\\ + &=\sqrt{K} d(x,y)\\ + &=O(\log n) d(x,y) +\end{aligned} +\end{equation*} + +One thing we can try is to tighten the second line. +Recall that for each dimension $i$ a random subset $S_i\subset X$ is selected and the value of $f_i(x)$ is $\min_{s\in S_i} d(x,s)$. +We want to show that for any fixed $x,y\in X$ and any dimension $i$ the event that distance $|f_i(x)-f_i(y)|^2$ is much smaller than $d(x,y)^2$ happends with high probability. \end{document} \ No newline at end of file