diff --git a/main.pdf b/main.pdf index febdecb..89a64e9 100644 Binary files a/main.pdf and b/main.pdf differ diff --git a/main.tex b/main.tex index ed1f4ed..415393b 100644 --- a/main.tex +++ b/main.tex @@ -233,33 +233,6 @@ We have $-\lambda(b-B)\leq w(C\setminus F)-\lambda(b-c(F))$ for any cut $C$ and \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{normalized mincut} -I read this trick in sparsest cut notes\footnote{\url{https://courses.grainger.illinois.edu/cs598csc/fa2024/Notes/lec-sparsest-cut.pdf}}. First we write a IP for normalized mincut. - -\begin{equation} -\begin{aligned}\label{ip:normalized} -\min& & \frac{\sum_e w(e)x_e}{b+1-\sum_e c(e)y_e}& & &\\ -s.t.& & \sum_{e\in T} x_e+y_e&\geq 1 & &\forall T\\ -& & \sum_{e} y_e c(e) &\leq b & &\\ -% & & x_e&\geq y_e & &\forall e\quad(F\subset C)\\ -& & y_e,x_e&\in\{0,1\} & &\forall e -\end{aligned} -\end{equation} - -A lp relaxation would be the following, -\begin{equation} -\begin{aligned}\label{lp:normalized} -\min& & \sum_e w(e)x_e& & &\\ -s.t.& & \sum_{e\in T} x_e+y_e&\geq 1 & &\forall T\\ -& & \sum_{e} y_e c(e) &= b & &\\ -% & & x_e&\geq y_e & &\forall e\quad(F\subset C)\\ -& & y_e,x_e&\geq 0 & &\forall e -\end{aligned} -\end{equation} -Note that we are forcing $b+1-\sum_e c(e)y_e=1$. -\autoref{lp:normalized} is exactly the fractional optimum of \autoref{lp:conninterdict} since we have shown that to satisfy the complementary slackness condition the budget constraint has to be tight. - -How is this related to the optimal $\lambda$ in \autoref{lp:dualcutint}? The integrality gap of \autoref{ip:normalized} is also not clear. \subsection{integrality gap} I guess the 2-approximate min-cut enumeration algorithm implies an integrality gap of 2 for cut interdiction problem.