diff --git a/main.tex b/main.tex index 98b13e8..db20452 100644 --- a/main.tex +++ b/main.tex @@ -16,9 +16,13 @@ \section{Ideal base packing} Try to generalize Thorup's ideal tree packing \cite{Thorup_2008} to matroids. -Certainly it won't work on all matroids. +We cannot expect it to work on all matroids. The goal is to figure out some sufficient conditions and their relations with basepacking($\lambda\leq c \sigma$) and random contraction($\lambda \leq c \frac{|E|}{r(E)}$). +The idea is that we want to find a small set of bases such that for any minimum $k$-cocycle there is a base that the cocycle hits the base $O(k)$ times. +The number of bases should be as small as possible. +One can select all bases and see that for every $k$-cocycle there is a base that only gets hit exactly $k$ times. + Let $M=(E,\mathcal I)$ be a matroid with capacity $c:E\to \R_{\geq 0}$ on elements and let $\sigma=\min_{F\subset E} \frac{c(E-F)}{r(E)-r(F)}$ be its weighted strength. \begin{algo} @@ -90,6 +94,8 @@ Then the lemma follows by induction. \end{enumerate} \end{proof} +The following lemma does not seem related to the ideal tree packing. + \begin{lemma} If we decrease the capacity of an edge, no ideal edge utilization decreases. \end{lemma} @@ -116,7 +122,14 @@ If we increase the capacity, no ideal edge utilization increases. The proof is s Removing (contracting) edges has the same effect on ideal utilization as setting the capacity to $0$ ($\infty$). \end{remark} -\subsection{Hard part} +\subsection{Counting} + +Ideal base packing is a distribution on some bases. Given a subset $D\subset E$, consider the expected size of intersection with a base sampled from the ideal distribution. +The expectation is exactly $\sum_{e\in D} c(e)u^*(e)$. + +Recall that our goal is to show that for any minimum $k$-cocycle a random base in the ideal base packing uses $O(k)$ elements in the cocycle in expectation. +Thorup proves that there is a special $k$-cocycle that for any + Lemma~7 in \cite{Thorup_2008} does not generalize to all matroids. Let $F^*$ be the optimal flat. @@ -129,7 +142,10 @@ We want to upperbound the expected size of $X\setminus F$ using $c(r(E)-r(F))$. In graphs this seems like a random contraction on $G/\mathcal F^*$. However, instead of the probability certain mincut is preserved, we are interested the expected number of remaining edges of a spanning tree. -\subsection{Rigidity matroids} +\subsection{Support size} +... + +\subsection*{Rigidity matroids} \begin{conjecture}\label{conj:idealrigidbase} Let $M$ be a connected 2D rigidity matroid on graph $G=(V,E)$. Let $F^*$ be the optimal flat for strength $F^*=\argmin_{F\subset E}\frac{c(E\setminus F)}{r(E)-r(F)}$. Let $X\subset E\setminus F^*$ be a independent set with rank $r(E)-r(F^*)$. Then for any maximal independent set $B_{F^*}\subset F^*$, $X\cup B_{F^*}$ is a base of $M$. @@ -143,9 +159,12 @@ One can see that in this process we do not care the actual base $B_{F^*}$ and on \begin{conjecture} Assume \autoref{conj:idealrigidbase} is true. -??? +Then there is a hyperplane $H$ such that $|X\setminus H|\leq c$ for some constant $c$. \end{conjecture} +If what we need is only ``existance'' of such a hyperplane, sure there is one. +Need to really understand ideal tree packing. How to find a tree that is hit by certain minimum cocircuit constant times? + \section{Greedy base packing}