gap 4 conj doesn't seem correct... fixed some setminus

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Yu Cong 2025-05-05 22:46:29 +08:00
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commit 0d883a033e

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@ -121,7 +121,7 @@ s.t.& & \sum_{e\in T} x_e+y_e&\geq 1 & &\forall T & &\text{($x+y$ is
\begin{frame}{Normalized Mincut from LP} \begin{frame}{Normalized Mincut from LP}
One standard trick for dealing LPs with knapsack constraints is to consider its Lagrangian dual. One standard trick for dealing LPs with knapsack constraints is to consider its Lagrangian dual.
\begin{equation*} \begin{equation*}
\max_{\mu\geq 0} L(\mu)= \max_{\mu\geq 0} \min \left\{ w(C-F)-\mu(b-c(F)) | \forall \text{cut $C$}\;\forall F\subset C \max_{\mu\geq 0} L(\mu)= \max_{\mu\geq 0} \min \left\{ w(C\setminus F)-\mu(b-c(F)) | \forall \text{cut $C$}\;\forall F\subset C
% \land c(F)\leq b % \land c(F)\leq b
\right\} \right\}
\end{equation*} \end{equation*}
@ -130,11 +130,11 @@ One standard trick for dealing LPs with knapsack constraints is to consider its
$L(\mu)$ is piecewise linear and concave. $L(\mu)$ is piecewise linear and concave.
\end{lemma} \end{lemma}
For fixed $\mu$, how to solve $\min\limits_{C,F} \left\{ w(C-F)-\mu(b-c(F)) For fixed $\mu$, how to solve $\min\limits_{C,F} \left\{ w(C\setminus F)-\mu(b-c(F))
\right\}$? \right\}$?
For small enough $\mu\geq 0$, $w(C-F)$ term is dominanting. For small enough $\mu\geq 0$, $w(C\setminus F)$ term is dominanting.
Thus the optimal solution must be $C=F=\text{mincut with weight $c$}$. Thus the optimal solution must be $C=F=\text{mincut with capacity $c$}$.
\end{frame} \end{frame}
@ -144,7 +144,7 @@ We have see that the first line segment is $L(\mu)=(\lambda_c-b)\mu$ where $\lam
What is the first breakpoint on $L(\mu)$? What is the first breakpoint on $L(\mu)$?
\newline \newline
We have $-\mu(b-\lambda_c)\leq w(C- F)-\mu(b-c(F))$ for any cut $C$ and $F\subsetneq C$. Thus the first breakpoint is $\mu=\min \frac{w(C- F)}{\lambda_c-c(F)}$, which is the value of normalized mincut. We have $-\mu(b-\lambda_c)\leq w(C\setminus F)-\mu(b-c(F))$ for any cut $C$ and $F\subsetneq C$. Thus the first breakpoint is $\mu=\min \frac{w(C\setminus F)}{\lambda_c-c(F)}$, which is the value of normalized mincut.
\newline \newline
What about other breakpoints? What about other breakpoints?
@ -173,19 +173,19 @@ Again we first assume the $\mu$ is fixed. Then for each pair of constraints $\su
\end{frame} \end{frame}
\begin{frame}{Integrality Gap} \begin{frame}{Integrality Gap}
\begin{conjecture} % \begin{conjecture}
ILP\ref{IP} has an integrality gap of 4. % ILP\ref{IP} has an integrality gap of 4.
\end{conjecture} % \end{conjecture}
Suppose $\mu^*$ is the optimal solution to LP\ref{lp:dualcutint}. Let $\lambda^{fr}$ be the fractional mincut with capacity $w_\mu$. % Suppose $\mu^*$ is the optimal solution to LP\ref{lp:dualcutint}. Let $\lambda^{fr}$ be the fractional mincut with capacity $w_\mu$.
we have % we have
\begin{align*} % \begin{align*}
4\opt(LP) &=4\lambda^{fr}-4b\mu^*\\ % 4\opt(LP) &=4\lambda^{fr}-4b\mu^*\\
&\geq 2\lambda^{int} - 4b\mu^*\\ % &\geq 2\lambda^{int} - 4b\mu^*\\
&\geq w_{\mu^*}(C^*)-b\mu^*, % &\geq w_{\mu^*}(C^*)-b\mu^*,
\end{align*} % \end{align*}
which implies Theorem \autoref{thm:2approx}. % which implies Theorem \autoref{thm:2approx}.
\end{frame} \end{frame}
\begin{frame}{References} \begin{frame}{References}
\bibliographystyle{plainnat} \bibliographystyle{plainnat}