diff --git a/main.pdf b/main.pdf index 86aeacd..7178a3c 100644 Binary files a/main.pdf and b/main.pdf differ diff --git a/main.tex b/main.tex index 8655c9a..9f73cac 100644 --- a/main.tex +++ b/main.tex @@ -1,5 +1,8 @@ \documentclass[12pt]{article} \usepackage{chao} + + +\geometry{a4paper,margin=2cm} \usepackage[breakable, theorems, skins]{tcolorbox} \tcbset{enhanced} \DeclareRobustCommand{\note}[2][blue]{% @@ -174,7 +177,7 @@ Now this LP looks similar to the normalized min-cut problem. A further reformulation (the new $x$ is $x-y$) gives us the following, -\begin{equation}\label{lp:cutinterdict} +\begin{equation}\label{lp:conninterdict} \begin{aligned} \min& & \sum_{e} x_e w(e) & & \\ s.t.& & \sum_{e\in T} x_e+y_e&\geq 1 & &\forall T\quad \text{($x+y$ is a cut)}\\ @@ -227,13 +230,13 @@ We have $-\lambda(b-B)\leq w(C\setminus F)-\lambda(b-c(F))$ for any cut $C$ and \note{It remains to show that the optimal solution at $\e$ guarantees $c(F)\leq b$? or maybe we don't need this for normalized mincut. I think normalized min-cut should not require $c(F)\leq b$. Further checks are needed. What we can guarantee is that $c(F)\leq B$.} -\subsection{a fundamental difference} -Consider $L(\lambda)$ for cut problem. One can see that the optimal $\lambda$ is clearly 0 since $L(\lambda)$ is pwl concave and the slope is negative at $\lambda=0$. What we are really interested in is the first segment on $L$. At the left end, $L(0)$ is exactly the weight of minimum cut. (the complementary slackness condition is satisfied.) At the right end, as we have shown in the previous paragraph, $\lambda$ equals to the value of the strength (which is the optimum of the linear relaxation of the cut IP). However, for cut interdiction problems $L(0)$ is not the optimum. Need to understand this better... +\subsection{differences} +Consider $L(\lambda)$ for cut problem. One can see that the optimal $\lambda$ is clearly 0 since $L(\lambda)$ is pwl concave and the slope is negative at $\lambda=0$. What we are really interested in is the first segment on $L$. At the left end, $L(0)$ is exactly the weight of minimum cut. (the complementary slackness condition is satisfied.) At the right end, as we have shown in the previous paragraph, $\lambda$ equals to the value of the strength (which is the optimum of the linear relaxation of the cut IP). However, for cut interdiction problems $L(0)$ is not the optimum. \subsection{integrality gap} I guess the 2-approximate min-cut enumeration algorithm implies an integrality gap of 2 for cut interdiction problem. -First consider the dual of linear relaxation of \autoref{lp:cutinterdict}. +First consider the dual of linear relaxation of \autoref{lp:conninterdict}. \begin{equation}\label{lp:dualcutint} \begin{aligned} @@ -252,10 +255,10 @@ I believe the previous conjecture is not likely to be true. \paragraph{Weight truncation} Assuming we know the optimal $\lambda$ to the LP dual, \autoref{lp:dualcutint} in fact gives the idea of weight truncation. The capacity of each edge $e$ in the ``tree packing'' is $\min\{c(e)\lambda,w(e)\}$. -\paragraph{The optimal $\lambda$} Denote by $\lambda^*$ the optimal $\lambda$ that maximizes $L(\lambda)$. From the previous argument on the first segment of $L(\lambda)$ we know that $\lambda^* \geq \min \frac{w(C\setminus F)}{B-c(F)}$. Now assume $\lambda^* > \min_{c(F)\leq b} \frac{w(C\setminus F)}{b-c(F)}$. We have $\min w(C\setminus F)-\lambda^*(b-c(F)) \min_{c(F)\leq b} \frac{w(C\setminus F)}{b-c(F)}$. We have $\min w(C\setminus F)-\lambda^*(b-c(F))